Gaia on the Move: the Rise of the Savanna Monkeys

This text had already been published as an appendix to a longer post on the evolution of forests. It is republished here as a stand-alone post on the role of humans in the evolution of the world's forests (link to the image above)

Primates are arboreal creatures that evolved in the warm environment of the Eocene forests. They used tree branches as a refuge, and they could adapt to various kinds of food. Modern primates do not shy from hunting other species, maybe even ancient primates did the same. From the viewpoint of these ancient primates, the shrinking of the area occupied by tropical forests that started with the "Grande Coupure," some 30 million years ago, was a disaster. They were not equipped to live in savannas: they were slow on the ground: an easy lunch for the powerful predators of the time. Primates also never colonized the northern taiga. Most likely, it was not because they couldn't live in cold environments (some modern monkeys can do that), but because they couldn't cross the "mammoth steppe" that separated the Tropical forests from the Northern forests. If some of them tried, the local carnivores made sure that they didn't succeed. So, "boreal monkeys" do not exist (actually, there is one, shown in the picture, but it is not exactly a monkey!).  

Eventually, monkeys were forced to move into the savanna. During the Pleistocene, about 4 million years ago, the Australopithecines appeared in Africa, (image source). We may call them the first "savanna monkeys." In parallel, perhaps a little later, another kind of savanna monkey, the baboon, also evolved in Africa. In the beginning, australopithecines and baboons were probably practicing similar living techniques, but in time they developed into very different species. The baboons still exist today as a rugged and adaptable species that, however, never developed the special characteristics of australopithecines that turned them into humans. The first creatures that we classify as belonging to the genus Homo, the homo habilis, appeared some 2.8 million years ago. They were also savanna dwellers. 

This branch of savanna monkeys won the game of survival by means of a series of evolutionary innovations. They increased their body size for better defense, they developed an erect stance to have a longer field of view, they super-charged their metabolism by getting rid of their body hair and using profuse sweating for cooling, they developed complex languages to create social groups for defense against predators, and they learned how to make stone tools adaptable to different situations. Finally, they developed a tool that no animal on Earth had mastered before: fire. Over a few hundred thousand years, they spread all over the world from their initial base in a small area of Central Africa. The savanna monkeys, now called "Homo sapiens," were a stunning evolutionary success. The consequences on the ecosystem were enormous.

First, the savanna monkeys exterminated most of the megafauna. The only large mammals that survived the onslaught were those living in Africa, perhaps because they evolved together with the australopithecines and developed specific defense techniques. For instance, the large ears of the African elephant are a cooling system destined to make elephants able to cope with the incredible stamina of human hunters. But in Eurasia, North America, and Australia, the arrival of the newcomers was so fast and so unexpected that most of the large animals were wiped out. 

By eliminating the megaherbivores, the monkeys had, theoretically, given a hand to the competitors of grass, forests, which now had an easier time encroaching on grassland without seeing their saplings trampled. But the savanna monkeys had also taken the role of megaherbivores. They used fires with great efficiency to clear forests to make space for the game they hunted. Later, as they developed metallurgy, the monkeys were able to cut down entire forests to make space for the cultivation of the grass species that they had domesticated meanwhile: wheat, rice, maize, oath, and many others. 

But the savanna monkeys were not necessarily enemies of the forests. In parallel to agriculture, they also managed entire forests as food sources. The story of the chestnut forests of North America is nearly forgotten today but, about one century ago, the forests of the region were largely formed of chestnut trees planted by Native Americans as a source of food (image source). By the start of the 20th century, the chestnut forest was devastated by the "chestnut blight," a fungal disease that came from China. It is said that some 3-4 billion chestnut trees were destroyed and, now, the chestnut forest doesn't exist anymore. The American chestnut forest is not the only example of a forest managed, or even created, by humans. Even the Amazon rainforest, sometimes considered an example of a "natural" forest, shows evidence of having been managed by the Amazonian Natives in the past as a source of food and other products. 

The action of the savanna monkeys was always massive and, in most cases, it ended in disaster. Even the oceans were not safe from the monkeys: they nearly managed to exterminate the baleen whales, turning large areas of the oceans into deserts. On land, entire forests were razed to the ground. Desertification ensued, brought upon by "megadroughts" when the rain cycle was no more controlled by the forests. Even when the monkeys spared a forest, they often turned it into a monoculture, subjected to be destroyed by pests, as the case of the American chestnuts shows. Yet, in a certain sense, the monkeys were making a favor to forests. Despite the huge losses to saws and hatchets, they never succeeded in completely exterminating a tree species, although some are critically endangered nowadays. 

The most important action of the monkeys was their habit of burning sedimented carbon species that had been removed from the ecosphere long before. The monkeys call these carbon species "fossil fuels" and they have been going on an incredible burning bonanza using the energy stored in this ancient carbon without the need of going through the need of the slow and laborious photosynthesis process. In so doing, they raised the concentration of CO2 in the atmosphere to levels that had not been seen for tens of millions of years before. That was welcome food for the trees, which are now rebounding from their former distressful situation during the Pleistocene, reconquering some of the lands they had lost to grass. In the North of Eurasia, the Taiga is expanding and gradually eliminating the old mammoth steppe. Areas that today are deserts are likely to become green. We are already seeing the trend in the Sahara desert. 

What the savanna monkeys could do was probably a surprise for Gaia herself, who must be now scratching her head and wondering what has happened to her beloved Earth. And what's going to happen, now?  There are several possibilities, including a cataclysmic extinction of most vertebrates, or perhaps all of them. Or, perhaps, a new burst of evolution could replace them with completely new life forms. What we can say is that evolution is turbo-charged in this phase of the existence of planet Earth. Changes will be many and very rapid. Not necessarily pleasant for the existing species but, as always, Gaia knows best. 

Survival of the fittest or non-survival of the unfit? How the theory of evolution has evolved.

Lynn Margulis, one of the brightest minds of the 20th century, developer or co-developer of concepts such as "Gaia", "endosymbiosis," and "holobiont." Image from Gabriela Govantes Morales

Life did not take over the world by combat, but by networking.

Lynn Margulis

The first study in the field we call today the “science of complex systems” was Darwin’s synthesis of evolution by natural selection, in 1859 1. Initially, Darwin’s ideas were often understood as implying mainly competition (“Nature in red tooth and clay”). Nowadays, we tend to emphasize more the concept of collaboration, often in terms of the concept of “symbiosis” and of the paradigm that goes under the name of “holobiont,” an ensemble of creature that live together in a condition of symbiosis.

“Holobiont” is a term known from the 1930s 2, but the idea of collaboration among living beings, microorganisms in particular, is much older. It may go back to the concept of “consortia” developed in mid 19th century 3. Nowadays, it is mostly associated to the work of Lynn Margulis, who used it starting with the 1990 4 5, although she had been studying the subject already in the 1960s 6. The interest in holobionts has been rapidly growing with the 2000s 2 and that led to a variety of different interpretations of what exactly a holobiont is. Here, I am not even trying to provide a comprehensive discussion of the many ways in which the holobiont concept is interpreted. Let’s say that the holobiont term can be seen as a meme growing in the memesphere 7 and, true to its evolutionary background, has been following a memetic evolutionary path, changing and adapting as its memetic environment changes.

The most common interpretation sees holobionts in a relatively narrow sense, as systems formed by a host organism and its associated symbiotic micro organisms, e.g. the gut bacteria of human beings (see, e.g. the work by Rosenberg 8. We can call this the “classical” view that focus on host-microbe interactions. A more general definition is provided by Castell and others 9, 10 in terms of the concept of “complex adaptive systems” (CAS) 11. CAS systems are formed of a network of elements (“nodes”) which, in this case, may consist of individual organisms or smaller holobionts, thus making the definition “fractal” or “self-similar.” The highest rank holobiont of the ecosystem is the ecosystem itself, that some call Gaia 12), 13.

The nodes (the creatures) of the holobiont network are connected to each other by flows of matter, energy, or information and are affected by “feedback” phenomena. The term “feedback“ indicates a condition that makes the flow proportional (or inversely proportional) to the status of the nodes they are connected to. It is the interplay of feedbacks that stabilizes the system and makes it tend to reach homeostasis. CAS systems have several typical characteristics, one is of reacting to perturbations by damping them and trying to maintain their state and reach the condition called “homeostasis.”

Castell et al. use the term “Holobiont-like system” (HLS) for these CAS systems that they describe as biological systems. In this paper, I’ll use the term “holobiont” as a general term that includes HLSs and I will argue that the “network” characteristics of these systems makes the definition applicable to non-biological systems, e.g. social and economic networks.

One characteristic of CAS is their capability to evolve. It is embedded in their tendency to react to external perturbations (called “forcings” in the jargon of system science) in order to attain homeostasis. Those systems which can do that tend to persist for a long time, so they are winners at the evolutionary game. But there is more to the concept of holobiont: it can be used to explain how evolution moves in steps by co-opting of different creatures in a single organism 6. According to Margulis 5, the unit of evolution is not the genome of individual organisms, but the “hologenome,” the ensemble of the genomes of the creatures that compose a holobiont. This concept is not without problems, in particular about what are the boundaries that define a specific holobiont, but it is gaining acceptance in the scientific community 14, 15.

The present paper is a review of the concept of “holobiont” aimed at developing mental tools helping us to understand how and why many complex systems around us exist and operate. The “holobiont paradigm” can help redress our views of the world and lead us to take a more collaborative attitude toward nature and our fellow humans. It is a way to change our current way of thinking and transform it into a gentler and more balanced view of the world, where we take what we need from Nature and give back to Nature what Nature needs. And the same concept holds for human life in human society.

Holobiont: how evolution has evolved


A well-known poem by Tennyson mentions “Nature in Red Tooth and Claw.” It was written in 1850, before Darwin’s book, “The Origin of Species”(1859), appeared in print. But, in time, Tennyson’s sentence started to be seen as embodying the very essence of Darwin’s ideas. It implied that the natural world is a continuous fight where the best individuals prevail and reproduce, while the others leave no descendants and disappear. During the 19th and 20th centuries, Darwin’s ideas were often coerced into a version that considered evolution as not a purely random phenomenon, but a purposeful motion toward creating species of higher and higher perfection 16. This distorted interpretation of Darwin’s ideas percolated into fields other than biology. Competition is often seen as the key element that makes it possible for people to become better at their jobs, for technology to improve, for companies to produce better products, and more. Even sports are always supposed to be competitive: from the time of Baron De Coubertin, we accept that we should let the stronger win the battle and the swifter the race (even though the Ecclesiastes book says otherwise). During the 20th century, distorted versions of Darwin’s theory were used to justify all sorts of crimes against humankind. Darwin himself would have been horrified if he had known that his ideas would have been used in this way.

The idea of the “survival of the fittest” is not wrong, but it has created a lot of confusion about how exactly natural selection works. Darwin emphasized the survival of the fittest because of his reliance on human breeding of domesticated animals and plants as examples. Human-led selection is focused on forcing species to acquire one or a few specific characteristics that humans find useful. The results are optimized, but only for that characteristic, often at the expense of many others that are needed for survival in the wild. Horses can be optimized for their ability to run, donkeys for their ability to carry weights, grains for the size of their eaves, apples for their taste and size, and there are many more similar cases. But most of the creatures optimized by humans would not survive for long in the wild. It is similar to the mitochondria in eukaryotes, which could not survive outside the larger cells they populate. Individual organisms rarely survive alone: mostly they are part of groups (herds, flocks, prides, etc.) that provide shelter, protection, and more. A fitter animal still needs to be part of a group, and that limits the advantage of being special. Rather than for the fittest, evolution strives for the not so bad.

An example. Think of a herd of wild zebras. The paradigm of “survival of the fittest” implies that zebras continuously evolve toward fitter and fitter creatures. Predators which attack zebras do the same. The result is called, sometimes, the “evolutionary arms race” between prey and predators 17. If we understand evolution at the individual level, we can imagine that it would result in creating a "super-zebra" that can run faster than the other zebras. Would that give it an evolutionary advantage?

We may not be so good a thinking like zebras, but it seems obvious that, for a zebra, survival is mostly guaranteed by being part of the herd. The herd will not directly fight back against predators (although in some cases it will, it is called "mobbing"), but it will provide early warning of the presence of predators, its movement will confuse them and single zebras will have a statistically better chance of survival. An isolated zebra has no chances to escape once it is the target of a group of predators, so being able to run very fast is probably a negative characteristic, especially considering that it could only be obtained at the expense of other useful characteristics, say, endurance, metabolic efficiency, or others. On the opposite side of fitness, instead, evolution works by removing the unfit. Clearly, zebras that cannot follow the herd are a preferred target for predators and their genes are removed from the gene pool. It is natural selection at work

Overall, therefore, the herd will be formed of individuals of similar characteristics. It is a typical holobiont formed by a group of individual organisms linked to each other, in this case mostly by visual signals. No zebra aims at helping its companions escape predators, but once they get together, their chance to be killed is reduced. A typical example of symbiosis. There is no altruism in symbiosis, only individual advantage. But that individual advantage benefits the whole group. The “hologenome” of the herd, the set of individual genomes, is the unit of evolution.

Once we define evolution in terms of holobionts, we can take a different look at the mechanism of selection. Darwin used the terms “survival of the fittest” and “natural selection” as nearly interchangeable ones, but that’s not the case. “Survival of the fittest” means that Nature selects in favor of the fitter creatures. “Natural selection,” instead, can be seen as meaning that nature selects against unfit creatures. The first interpretation (survival of the fittest) implies a continues evolution toward better and better fitness. The second (natural selection) implies a tendency toward stability or, using a more specific term, homeostasis.

The "removal of the unfit" is a fundamental concept. The biosphere, like any complex system, is subjected to an increase in disorder according to the 2nd law of thermodynamics. But the law applies only to isolated systems. The biosphere is not one, and it can keep entropy growth in check using natural selection to maintain homeostasis. The concept of evolution as the removal of the unfit was developed in particular by Gorshkov et al., 18 who put together a synthesis of how the ensemble of the living creatures on Earth (the biosphere) control the wider entity that we call the “ecosphere” generating the condition of dynamic stability we call “homeostasis.” This view was described in terms of the concept of “biotic regulation of the environment.” So, the biosphere does not normally aim at increasing the degree of fitness of individuals. The system strives for stability and the winners of the evolutionary game are those organisms which can best maintain it.

If stability is the aim of evolution, then, how does anything evolve at all? That is, how is it that Earth isn’t still populated only by marine prokaryotes, as it was during the Archean, billions of years ago? The holobiont paradigm helps resolve this problem, too, taking into account the phenomenon of “endosymbiosis,” the true motor of evolution.

Darwin’s ideas about evolution involved gradual and continuous change. The modern paradigm, called “neodarwinism” is based on molecular biology and assumes that evolution occurs as the result of modification of the genetic code of organisms, the result of either random mutation or sexual mixing (meiosis). In other words, evolution is a continuous “probing” of the fitness of organisms, where those better adapted at survival pass their genes to their descendants.

Yet, this interpretation has a problem. The gradual change of the genetic code can hardly account for the discrete evolutionary “jumps” needed to go from one species to another. This question was termed the “irreducible complexity problem” 19, and sometimes it was understood as implying the need to admit a supernatural intervention (“intelligent design”) to explain some features of living beings. A classic example is that of eyes. How could ancient, blind animals, develop eyes?

It is hard to find for eyes the same evidence of a gradual evolution that Darwin had observed for the beaks of finches in the Galápagos Islands. The first multicellular animals appeared at the time of the “Ediacaran Fauna” (also known as “Vendobionts”), some 600 million years ago. There is no evidence that these creatures had eyes. Instead, fully formed eyes clearly appear in the early Cambrian biota, starting from about 520 million years ago 20. So, we have this new evolutionary invention, eyes, that appear relatively suddenly, already in a functional form. How could it happen?

Attempts to solve the problem by postulating gradual steps were only partially successful. For instance, in 1994, Nilson and Pelger 21 proposed that a flat, light-sensitive surface could naturally turn into an eye-like optical structure in a few hundred thousand years of gradual evolution. It may be a correct estimate, but where did the light-sensitive surface come from? What benefit could early animals have from such a capability?

The concept of endosymbiosis gives us a way to explain this and many more evolutionary rapid changes. Endosymbiosis assumes that multicellular creatures are collections of formerly independent unicellular organisms. We may assume that early multicellular organisms co-opted microorganisms that had developed the stigma, a feature still present in some modern microorganisms. The stigma is a pigmented area in the cell connected to photoreceptor molecules. These molecules directly act on the flagellum (the “tail” of the creature). In this way, the microorganism can detect light and move toward it. It is typical of a creature called euglena, a protist that is both plant and animal. It uses photosynthesis – hence it is a plant – but it also moves, a characteristic of animals. There is no doubt that having a photosensitive receptor gives to the euglena an evolutionary advantage: it can move toward the light, which it needs for its photosynthetic activity. Light-sensitive microorganisms (not necessarily the euglena, which is a modern organism) give us the key to solve the “irreducible complexity” problem. Animals didn’t need to develop eyes from scratch. They borrowed light-sensitive organs by entering into a symbiotic relationship with unicellular creatures that had developed them. Later, they gradually turned these light-sensitive layers into organs able to form an image of the external world -- the organs we call “eyes.” This point is very general and another example of the usefulness of the concept of “holobiont”. There are many issues in evolution that cannot be solved unless we recur to the concept of endosymbiosis.

 
This point is very general. There are many issues in evolution that cannot be solved unless we recur to the concept of endosymbiosis. For another example, note how there are no intermediate species between prokaryotes (simple cells with no organelles) and eukaryotes (complex cells, with internal organelles) Nick Lane wrote a fascinating book in 2015 titled “The Vital Question,” (Lane 2015) dedicated to the attempt to explain why it is so. Clearly, we have another problem of irreducible complexity that can only be explained if we assume that what we call “organelles” are formerly independent organisms that were fagocitated by a larger cell and became part of it. This process is the basis of the concept of holobiont as proposed by Lynn Margulis (Sagan 1967), (L. Margulis 1975).

Once we define evolution in terms of holobionts, we can take a different look at the mechanism of selection. Darwin used the terms “survival of the fittest” and “natural selection” as nearly interchangeable ones, but that’s not the case. “Survival of the fittest” means that Nature selects in favor of the fitter creatures. “Natural selection,” instead, can be seen as meaning that nature selects against unfit creatures. The first interpretation (survival of the fittest) implies a continuous evolution toward better and better fitness. The second (natural selection) implies a tendency toward stability or, using a more specific term, homeostasis. It means that individuals seek for collaboration rather than competition -- a basic characteristic of holobionts. The winners of the evolutionary game are always good holobionts!




References

1. Charles Darwin. The Origin of Species. (John Murray, 1859).
2. Baedke, J., Fábregas‐Tejeda, A. & Delgado, A. N. The holobiont concept before Margulis. J. Exp. Zoolog. B Mol. Dev. Evol. 334, 149–155 (2020).
3. Kull, K. Ecosystems are Made of Semiosic Bonds: Consortia, Umwelten, Biophony and Ecological Codes. Biosemiotics 3, 347–357 (2010).
4. Margulis, L. Words as battle cries—symbiogenesis and the new field of endocytobiology. BioScience 40, 673–677 (1990).
5. Margulis, L. Symbiotic Planet: A New Look At Evolution. (Basic Books, 2008).
6. Sagan, L. On the origin of mitosing cells. J. Theor. Biol. 14, 225-IN6 (1967).
7. Dawkins, R. The selfish gene. (Oxford ; New York : Oxford University Press, 1976).
8. Rosenberg, E., Koren, O., Reshef, L., Efrony, R. & Zilber-Rosenberg, I. The role of microorganisms in coral health, disease and evolution. Nat. Rev. Microbiol. 5, 355–362 (2007).
9. Castell, W., Fleischmann, F., Heger, T. & Matyssek, R. Shaping Theoretic Foundations of Holobiont-Like Systems. in Progress in Botany 77 (eds. Lüttge, U., Cánovas, F. M. & Matyssek, R.) 219–244 (Springer International Publishing, 2016). doi:10.1007/978-3-319-25688-7_7.
10. Matyssek, R., Lüttge, U. & Castell, W. Evolution of Holobiont-Like Systems: From Individual to Composed Ecological and Global Units. in Progress in Botany 1–46 (Springer, 2022). doi:10.1007/124_2022_57.
11. Mobus, G. E. & Kalton, M. C. Principles of System Science. (Springer, 2015). doi:10.1007/978-I-4939-1920-8.
12. Matyssek, R. & Luttge, U. Gaia: The Planet Holobiont. Nova Acta Leopoldina 114, 325–344 (2013).
13. Castell, W., Lüttge, U. & Matyssek, R. Gaia—A Holobiont-like System Emerging From Interaction. in Emergence and Modularity in Life Sciences (eds. Wegner, L. H. & Lüttge, U.) 255–279 (Springer International Publishing, 2019). doi:10.1007/978-3-030-06128-9_12.
14. Gray, M. W. Lynn Margulis and the endosymbiont hypothesis: 50 years later. Mol. Biol. Cell 28, 1285–1287 (2017).
15. Morris, J. J. What is the hologenome concept of evolution? F1000Research 7, F1000 Faculty Rev-1664 (2018).
16. Bowler, P. J. The Non-Darwinian Revolution: Reinterpreting a Historical Myth. (Johns Hopkins Univ Pr, 1992).
17. Dawkins, R. The Blind Watchmaker. (Norton & Company, 1986).
18. Gorshkov, V. G., Makarʹeva, A. Mikhaĭlovna. & Gorshkov, V. V. Biotic regulation of the environment : key issue of global change. (Springer-Verlag, 2000).
19. Behe, M. J. Darwin’s Black Box: The Biochemical Challenge to Evolution. (Simon and Schuster, 2001).
20. Zhao, F., Bottjer, D. J., Hu, S., Yin, Z. & Zhu, M. Complexity and diversity of eyes in Early Cambrian ecosystems. Sci. Rep. 3, (2013).
21. Nilsson, D. E. & Pelger, S. A pessimistic estimate of the time required for an eye to evolve. Proc. Biol. Sci. 256, 53–58 (1994).
22. Jensen, K. H. & Zwieniecki, M. A. Physical Limits to Leaf Size in Tall Trees. Phys. Rev. Lett. 110, 018104 (2013).

The Great Cycle of Earth's Forests

 

 Forests appeared on Earth some 400 million years ago, and they have been thriving over that long period. But, during the past 150 million years, they started to show signs of distress, reacting to the decline in atmospheric CO2 concentrations and to the competition with grasslands. As Earth changes, will forests be able to cope and survive? It is an extremely slow trend, but we cannot rule out that forests will conclude their cycle and disappear in a geologically short time. This text is an attempt to reconstruct the story of forests and to imagine what their future could be in deep time. (image courtesy of Chuck Pezeshky) 

A forest is a magnificent, structured, and functional entity where the individual elements -- trees -- work together to ensure the survival of the ensemble. Each tree pumps water and nutrients all the way to the crown by the mechanism called evapotranspiration. The condensation of the evaporated water triggers the phenomenon called the "biotic pump" that benefits all the trees by pumping water from the sea. Each tree pumps down the carbohydrates it manufactures using photosynthesis to its mycorrhizal space, the underground system of roots and fungi that extracts mineral nutrients for the tree. The whole "rhizosphere" -- the root space -- forms a giant brain-like network that connects the trees to each other, sometimes termed "the Wood Wide Web." It is an optimized environment where almost everything is recycled. We can see it as similar to the concept of "just in time manufacturing" in the human economy. 

Forests are wonderful biological machines, but they are also easily destroyed by fires and attacks by parasites. And forests have a competitor: grass, a plant that tends to replace them whenever it has a chance to. Areas called savannas are mainly grass, although they host some trees. But they don't have a closed canopy, they don't evapotranspirate so much as forests, and they tend to exist in much drier climate conditions. Forests and grasslands are engaged in a struggle that may have started about 150 million years ago when grass appeared for the first time. During the past few million years, grasses seem to have gained an edge in the competition, in large part exploiting their higher efficiency in photosynthesis (the "C4" pathway) in a system where plants are starved for CO2.

Another competitor of forests is a primate that left its ancestral forest home just a couple of million years ago to become a savanna dweller -- we may call it the "savanna monkey," although it is also known as "Homo," or "Homo sapiens." These monkeys are clever creatures that seem to be engaged mainly in razing forests to the ground. Yet, in the long run, they may be doing forests a favor by returning the atmospheric CO2 concentration to values more congenial to the old "C3" photosynthetic mechanism still used by trees. 

Seen along the eons, we have an extremely complex and fascinating story. If forests have dominated Earth's landscape for hundreds of millions of years, one day they may disappear as Gaia gets old. In this post, I am describing this story from a "systemic" viewpoint -- that is, emphasizing the interactions of the elements of the system in a long-term view (it is called also "deep time"). The post is written in a light mood, as I hope to be able to convey the fascination of the story also to people who are not scientists. I tried to do my best to interpret the current knowledge, I apologize in advance for the unavoidable omissions and mistakes in such a complex matter, and I hope you'll enjoy this post. 

The Origin of Forests: 400 million years ago

Life on Earth may be almost 4 billion years old but, since we are multicellular animals, we pay special attention to multicellular life. So, we tend to focus on the Cambrian period (542-488 million years ago), when multicellular creatures became common. But that spectacular explosion of life was all about marine animals. Plants started colonizing the land only during the period that followed the Cambrian, the Ordovician, (485 - 443 million years ago). 

To be sure, the Ordovician flora on land was far from impressive. As far as we know, it was formed only by moss (perhaps lichens, too, but it is not certain). Mosses are humble plants: they are not vascularized, they don't grow tall, and they surely can't compare with trees. Nevertheless, mosses could change the planetary albedo and perhaps contribute to the fertilization of the marine biota -- something that may be related to the spectacular ice ages of the Ordovician. It is a characteristic of the Earth system that the temperature of the atmosphere is related to the abundance of life. More life draws down atmospheric CO2, and that cools the planet. The Ordovician saw one of these periodic episodes of cooling with the start of the colonization of the land. (image from Wikipedia)

There followed another long period called the "Silurian" (444 – 419 My ago) when plants kept evolving but still remained of the size of small shrubs at most. Then, during the Devonian (419 -359 million years ago) we have evidence of the existence of wood. And not only that, the fossil record shows the kind of channels called "Xylem" that connect the roots to the leaves in a tree. These plants were already tall and had a crown, a trunk, and roots. By the following geological period, the Carboniferous (359 - 299 My ago), forests seem to have been widespread.  

A major feature of these ancient trees was the development of an association with fungi. Their roots formed what we call a "mycorrhizal" symbiotic system. The fungi receive carbohydrates that the tree manufactures using photosynthesis, while the tree receives from the fungi essential minerals, including nitrogen and phosphorous. We don't know the details of how this symbiotic relationship evolved over hundreds of millions of years but, below, you can see a hypothesis of how it could have happened. (Source) (in the figure, "AM" stands for "arbuscular mycorrhiza" - the oldest form of symbiotic fungi).

Another major evolutionary innovation that may have been already operating in the Paleozoic forests is the "biotic pump." As an effect of the pressure drop created by the condensation of evapotranspirated water, forests can create pump water vapor from the ocean and create the "atmospheric rivers" that bring water inland. That, in turn, creates the land rivers that bring that water back to the sea. As forests create their own climate, they can expand nearly everywhere. The image shows clouds created by condensation over the modern Amazon rainforest (source).  

If we could walk in one of those ancient forests, we would find the place familiar, but also a little dreary. No birds and not even flying insects, they evolved only tens of million years later. No tree-climbing animals: no monkeys, no squirrels, nothing like that. Even in terms of herbivores, we have no evidence of the kind of creatures we are used to, nowadays. Grass didn't exist, so grazers couldn't exist either. Herbivores were browsers surviving on leaves or on decaying plant matter. Lots of greenery but no flowers, they had not evolved yet. You see in the image (source) an impression of what an ancient forest of Cladoxylopsida could have looked like during the Paleozoic era.

The Paleozoic forests already had one of the characteristics of modern forests: fires. There had never been fires on Earth before for at least two good reasons: one was that there was not enough oxygen, and the other was that there was nothing flammable. But now, with the oxygen concentration increasing and plants colonizing the land, fires appeared, lighting up the night. They would remain a characteristic of the land biosphere for hundreds of millions of years.

Image Source. The "fire window" is the region of concentrations in atmospheric oxygen in which fires can occur. Note how during the Paleozoic, the concentration could be considerably larger than it is now. Fireworks aplenty, probably. Note also how there exist traces of fires even before the development of full-fledge trees, in the Devonian. Wood didn't exist at that time, but the concentration of oxygen may have been high enough to set other kinds of dry organic matter on fire. 

Wildfires are a classic case of a self-regulating system. The oxygen stock in the atmosphere is replenished by plant photosynthesis but is removed by burning wood. So, fires tend to reduce the oxygen concentration and that makes fires more difficult. But the story is more complicated than that. Fires also tend to create "recalcitrant" carbon compounds, charcoal for instance, that are not recycled by the biosphere and tend to remain buried for long times -- almost forever. So, over very long periods, fires tend to increase the oxygen concentration in the atmosphere by removing CO2 from it. The conclusion is that fires both decrease and increase the oxygen concentration. How about that for a taste of how complicated the biosphere processes are? 

 The Mesozoic: Forests and Dinosaurs

At the end of the Paleozoic, some 252 million years ago, there came the great destruction. A gigantic volcanic eruption of the kind we call "large igneous province" (sometimes affectionately "LIP") took place in the region we call Siberia today. It was huge beyond imagination: think of an area as large as modern Europe becoming a lake of molten lava. (image source)

It spewed enormous amounts of carbon into the atmosphere in the form of greenhouse gases. That warmed the planet, so much that it almost sterilized the biosphere. It was not the first, but it was the largest mass extinction of the Phanerozoic age. Gaia is normally busy keeping Earth's climate stable, but sometimes she seems to be sleeping at the wheel -- or maybe she gets drunk or stoned. The result is one of these disasters.  

Yet, the ecosystem survived the great extinction and rebounded. It was now the turn of the Mesozoic era, with forests re-colonizing the land. Over time, the angiosperms ("flowering plants") become dominant over the earlier conifers. With flowers, forests may have been much noisier than before, with bees and all kinds of insects. Avian dinosaurs also appeared. They seem to have been living mostly on trees, just like modern birds. 

For a long period during the Mesozoic, the landscape must have been mainly forested. No evidence of grass being common, although smaller plants, ferns, for instance, were abundant. Nevertheless, the great evolution machine kept moving. During the Jurassic, a new kind of mycorrhiza system evolved, the "Ectomycorrhizae" which allowed better control of the mineral nutrients in the rhizosphere, avoiding losses when the plants were not active. This mechanism was typical of conifers that could colonize cold regions of the supercontinent of the time, the "Pangea."  

During the Mesozoic, the dinosaurs appeared and diffused all over the planet. You surely noted how the Jurassic dinosaurs were often bipedal (See the illustration showing an early form of Iguanodon). They are also called "ornithopods," it is a body plan that allows herbivorous creatures to browse on leaves on the high branches of trees. A bipedal stance makes the creature able to stand up, balancing on its tail, reaching higher heights. Some dinosaurs chose a different strategy, developing very long necks for the same purpose: the brontosaurus is iconic in this sense even though, traditionally, it was shown half-submerged in swamps (the illustration is from the New York Tribune of 1919). The idea that brontosaurs lived mainly in swamps is not so bright, if you think about that. Why should a semiaquatic creature need a long neck? Think of a hippo with the neck of a giraffe: it wouldn't work so well. 

A much better representation of long-necked dinosaurs came with the first episode of the "Jurassic Park" (1993) movie series when a gigantic diplodocus eats leaves. At some moment, the beast rises on its hind legs, using the tail as further support. 

If you are a dinosaur lover (and you probably are if you are reading this post) seeing this scene must have been a special moment in your life. And, after having seen it maybe a hundred times, it still moves me. But note how the diplodocus is shown in a grassy environment with sparse trees: a Savanna. That's not realistic because grass didn't exist yet when the creature went extinct at the end of the Jurassic period, about 145 million years ago. 

To see grass and grazers, animals specialized in eating it, we need to wait for the Cretaceous (145-66 million years ago). Evidence that some dinosaurs had started eating grass comes from the poop of long-necked dinosaurs. That's a little strange because, if you are a grazer, the last thing you need is a long neck. But new body plants rapidly evolved. The Ceratopsia were the first true grazers, also called "mega-herbivores". Heavy, four-legged beasts that lived their life keeping their head close to the ground. The Triceratopses gained a space in human fantasy as prototypical dinosaurs, and they are often shown in movies while fighting tyrannosauruses. You see that scene in Walt Disney's movie "Fantasia" (1940). It may have happened for real.

Note the heavy bone shield over the head. With so much weight on board, Trixie couldn't possibly rise on its hind legs to munch on leaves on tree branches. Note also the beak, it looks perfectly adapted for collecting grass. It means that the Cretaceous landscape was probably similar to our world. We don't know if there existed the kind of biome we call today "savanna" -- a mix of grass and trees, but surely the land was shared by forests and grass, each biome with its typical fauna. 

The Great Cooling and the Rise of C4 Grass

At the end of the Cretaceous period, 66 million years ago, a new large igneous province appeared in the Deccan region, in India. It generated another climate disaster with the associated mass extinction. Most dinosaurs were wiped out, except those we call "birds" today. A large meteorite also hit Earth at that time. It caused only minor damage but, millions of years later, it gave human filmmakers a subject to explore in many dramatic movies. 

In time, the Deccan LIP faded away, and the era that followed is called the "Cenozoic." The ecosystem recovered, forests re-colonized the land, and mammals and birds (the only survivors of the Dinosauria clade) fought to occupy the ecological niches left free by their old masters. The early Cenozoic was a warm period of lush forests that offered refuge to a variety of animals: birds made their nests in branches, while squirrels and other small mammals jumped from branch to branch, or lived at the bottom. It is during this period that primates evolved: the huge forests of those times offered refuge for a variety of species that had probably already developed sophisticated social behaviors.  

Grass also survived the end-Cretaceous catastrophe. As a result, some mammals evolved into new "megaherbivores" or "megafauna" that occupied the same ecological niche that the triceratopsides had colonized long before.  Here is a brontotherium, a large herbivorous mammal that lived some 37-35 million years ago, during the late Eocene period (image from BBC).

The megabeasts of the Cenozoic do not have the same fascination of the giant dinosaurs, but this creature has a nice-sounding name, and it looks a little like Shrek, the ogre of Spielberg's movie. Note how the beast is correctly shown walking on a grassy plain. The Eocene is supposed to have been mostly forested, but grass existed, too. The brontotherium was an opportunistic grazer, apparently able to subsist on various kinds of food, not just grass. 

During the warm phase of the Cenozoic, Earth reached a maximum temperature around 55 million years ago, some 8-12 deg C higher than today. The concentration of CO2, too, was large. That is called the "early Eocene climatic optimum". It doesn't mean that this period was better than other periods in terms of climate, but it seems that Earth was mainly covered with lush forests and that the biosphere thrived.  

Then, the atmosphere started cooling. It was a descent that culminated at the Eocene-Oligocene boundary, about 34 million years ago, with a new mass extinction. It was a relatively small event in comparison to other, more famous, mass extinctions, but still noticeable enough that the Swiss paleontologist Hans Georg Stehlin gave it the name of the "Grande Coupure" (the big break) in 1910. One of the victims was the Brontotherium -- too bad, it was a nice-looking beast. 

Unlike other cases, the extinction at the Grande Coupure was not correlated to the warming created by a LIP, but to rapid cooling. You see the "step" in temperature decline in the figure. 

Why the big cooling? The answer is not completely known. Surely, cooling was correlated to a decline in the CO2 content in the atmosphere and that, in turn, may have been generated by the collision of the Indian plate with Eurasia. It was a gigantic geological event that generated the Himalayan mountain belt. It exposed huge amounts of fresh rock to the atmosphere, and the result was the removal of CO2 because of silicate erosion and weathering. 

The Himalaya hypothesis is one of those explanations that seem to make a lot of sense, but it has big problems. Another possible explanation is that Earth just outgassed less CO2 than before. The CO2 that plants need for their photosynthesis is generated mainly at the mid-oceanic ridges where the hot mantle (the molten rock layer below Earth's crust) outgasses it, as it has been doing for billions of years. It may well be that the mantle is getting a little colder over the eons, so it outgasses less CO2 than before. It may be true, but it seems to be a weak effect -- not enough to explain the CO2 decline of the Cenozoic.

In my opinion, the most likely hypothesis is that the CO2 concentration declined because of higher biological productivity not just on land, but also in the sea (as seems to be implied in a recent study )

In other words, the early Cenozoic may have been so booming with life of all kinds that it absorbed more CO2 from the atmosphere than the mantle could replace by outgassing. The result was the cooling phase. The abrupt step at the "Grande Coupure" may be related to the evolution of a specific life form: baleen whales, which changed the equilibria of the whole marine ecosystem, drawing down even more CO2 from the atmosphere.

This interpretation agrees with the fact that ice ages are often observed after LIPs. It may be one of the many cycles of the ecosphere. When a major LIP appears, the rise of CO2 is disastrous at the beginning but, in the long run, it gives the biosphere a chance to rebound and expand in a CO2 rich system.  Then, the rebound generates its own doom: the abundant biological productivity draws down CO2 from the atmosphere, cools the planet, and the system finds itself CO2-starved again. In this interpretation, the Eocene cooling and the Grande Coupure were long-term consequences of the Deccan LIP that had destroyed the dinosaurs, millions of years before. I hasten to note that this is just one of the several possible interpretations but, in my opinion, it makes a lot of sense.    

The Eocene cooling had profound effects on forests. First, the CO2 decline gave an advantage to those plants which utilized a more efficient photosynthesis mechanism called the "C4" pathway. Earlier on, the standard photosynthesis mechanism (called "C3") had evolved in an atmosphere rich in CO2. The C3 mechanism is efficient in processing carbon dioxide, but it is hampered by the opposite process called "photorespiration," which becomes important when the CO2 concentration is low. Using the C4 mechanism, plants can concentrate CO2 in the cells where photosynthesis occurs and avoid the losses by photorespiration. 

C4 plants appeared shortly after the Grande Coupure and diffused mainly in grasses, Trees, instead, didn't adopt the new mechanism. The explanation is subtle: photosynthesis needs water, and the process that goes on in leaves is strongly connected to the evapotranspiration mechanism. The C4 mechanism needs less water than the C3 one, so evapotranspiration is hampered. The result is that C4 trees -- if they exist -- cannot be as tall as the ordinary C3 ones, and so they are not favored by natural selection in forests. In an atmosphere of very low CO2, forests are disadvantaged because of the higher photosynthesis efficiency of grasses. 

During the period that followed the Grande Coupure, temperatures and CO2 concentrations remained stable, but at relatively low levels. The result was that many forests disappeared, replaced by grasslands and savannas. Herbivorous species evolved teeth more specialized for grazing and became "mega-herbivorous" species. The landscape must have become similar to the modern one, with patches of forests alternating with savannas. 

A typical savanna ecosystem: the Tarangire national park in Tanzania. (Image From Wikipedia). Compare with the forest image at the beginning of this post. 

Despite the expansion of savannas, rainforests continued to exist in the tropical regions. Conifer forests kept a foothold in the Northern regions, helped by their Ectomycorrhiza system that avoided the runoff of nutrients in winter. The boreal forest is also called "Taiga." 

Then, a new cooling phase started, apparently a continuation of the previous trend: cooling begets more cooling. It was the beginning of the "Pleistocene," a period of unstable climate with ice ages and interglacials following each other, triggered by small oscillations in solar irradiation caused by the characteristics of Earth's orbit. These oscillations are called "Milankovich Cycles" -- they are not the cause of the ice ages, just triggers. (Image Source).

The oscillations are caused by ice having a built-in albedo feedback so that the more ice expands, the more sunlight is directly reflected into space. That causes the temperature to decline and ice to expand even more. Taken to its extreme consequences, this mechanism may lead to the "Snowball Earth" condition, with ice covering the whole planet's surface. It may have happened for real during the "Cryogenian" period, some 600 million years ago. Fortunately, there were mechanisms able to re-heat Earth and return it to the conditions we consider "normal." 

During the Pleistocene, the CO2 concentration in Earth's atmosphere plunged to very low levels, especially during the glacial periods, when it reached levels as low as around 150 parts per million (ppm). Earth may have inched close to a new snowball Earth but, fortunately, that didn't happen. In part, it may be because the sun, today, is about 5% hotter than it was during the Cryogenian. But we will never know how close Gaia got to freeze to death. 

During the Pleistocene, the advancing ice sheets swept away all plants, but even in non-glaciated areas, forests suffered badly.  Tropical rainforests didn't disappear, but they were much reduced in extension. In the North, most of the Eurasian boreal forests were replaced by the "mammoth steppe," a huge area that went from Spain to Kamchatka. where mammoths and other mega-herbivores roamed. 

It is not impossible that an ice age colder than the Pleistocene average could have led to the eventual extinction of the forests, completely replaced by grasses. But that didn't happen, and things were going to change again with the appearance of the Savanna Monkeys -- a completely new species that came to dominate the ecosystem. 

The rise of the Savanna Monkeys

Primates are arboreal creatures that evolved in the warm environment of the Eocene forests. They used tree branches as a refuge, and they could adapt to various kinds of food. Modern primates do not shy from hunting other species, maybe even ancient primates did the same. From the viewpoint of these ancient primates, the shrinking of the area occupied by tropical forests that started with the "Grande Coupure," some 30 million years ago, was a disaster. They were not equipped to live in savannas: they were slow on the ground: an easy lunch for the powerful predators of the time. Primates also never colonized the northern taiga. Most likely, it was not because they couldn't live in cold environments (some modern monkeys can do that), but because they couldn't cross the "mammoth steppe" that separated tropical forests from the Northern forests. If some of them tried, the local carnivores made sure that they didn't succeed. So, "boreal monkeys" do not exist (actually, there is one, shown in the picture, but it is not exactly a monkey!). 

Nevertheless, eventually, monkeys were forced to move into the savanna. During the Pleistocene, about 4 million years ago, the Australopithecines appeared in Africa, (image source). We may call them the first "savanna monkeys." In parallel, perhaps some time later, another kind of savanna monkey, the baboon, also evolved in Africa. In the beginning, australopithecines and baboons were probably practicing similar living techniques, but in time they developed into very different species. The baboons still exist today as a rugged and adaptable species that, however, never developed the special characteristics of australopithecines that turned them into a very different kind of animals. The first creatures that we classify as belonging to the genus Homo, the homo habilis, appeared some 2.8 million years ago. They were also savanna dwellers.

This branch of savanna monkeys won the game of survival by means of a series of evolutionary innovations. They increased their body size for better defense, they developed an erect stance to have a longer field of view, they super-charged their metabolism by getting rid of their body hair and using profuse sweating for cooling, they developed complex languages to create social groups for defense against predators, and they learned how to make stone tools adaptable to different situations. Finally, they developed a tool that no animal on Earth had mastered before: fire. Over a few hundred thousand years, they spread all over the world from their initial base in a small area of Central Africa. The savanna monkeys, now called "Homo sapiens," were a stunning evolutionary success. The consequences on the ecosystem were enormous.

First, the savanna monkeys exterminated most of the megafauna. The only large mammals that survived the onslaught were those living in Africa, where they had the time to adapt to the new predator. For instance, the large ears of the African elephant are a cooling system destined to make elephants able to cope with the incredible stamina of human hunters. But in Eurasia, North America, and Australia, the arrival of the newcomers was so fast and so unexpected that most of the large animals were wiped out. 

By eliminating the megaherbivores, the monkeys had, theoretically, given a hand to the competitors of grass, forests, which now had an easier time encroaching on grassland without seeing their saplings trampled. But the savanna monkeys had also taken the role of megaherbivores. They used fires with great efficiency to clear forests to make space for the game they hunted. In the book "1491" Charles Mann reports (. p 286) how "rather than domesticating animals for meat, Indians retooled ecosystems to encourage elk, deer, and bear. Constant burning of undergrowth increased the number of herbivores, the predators that fed on them, and the people who ate them both"  

Later, as they developed metallurgy, the monkeys were able to cut down entire forests to make space for the cultivation of the grass species that they had domesticated meanwhile: wheat, rice, maize, oath, and many others. 

But the savanna monkeys were not necessarily enemies of the forests. In parallel to agriculture, they also managed entire forests as food sources. The story of the chestnut forests of North America is nearly forgotten today but, about one century ago, the forests of the region were largely formed of chestnut trees planted by Native Americans as a source of food (image source). By the start of the 20th century, the chestnut forest was devastated by the "chestnut blight," a fungal disease that came from China. It is said that some 3-4 billion chestnut trees were destroyed and, now, the chestnut forest doesn't exist anymore. The American chestnut forest is not the only example of a forest managed, or even created, by humans. Even the Amazon rainforest, sometimes considered an example of a "natural" forest, shows evidence of having been managed by the Amazonian Natives in the past as a source of food and other products. 

The action of the savanna monkeys was always massive and, in most cases, it ended in disaster. Even the oceans were not safe from the monkeys: they nearly managed to exterminate the baleen whales, turning large areas of the oceans into deserts. On land, entire forests were razed to the ground. Desertification ensued, brought upon by "megadroughts" when the rain cycle was no more controlled by the forests. Even when the monkeys spared a forest, they often turned it into a monoculture, subjected to be destroyed by pests, as the case of the American chestnuts shows. Yet, in a certain sense, the monkeys were making a favor to forests. Despite the huge losses to saws and hatchets, they never succeeded in completely exterminating a tree species, although some are critically endangered nowadays. 

The most important action of the monkeys was their habit of burning sedimented carbon species that had been removed from the ecosphere long before. The monkeys call these carbon species "fossil fuels" and they have been going on an incredible burning bonanza using the energy stored in this ancient carbon without the need of going through the need of the slow and laborious photosynthesis process. In so doing, they raised the concentration of CO2 in the atmosphere to levels that had not been seen for tens of millions of years before. That was welcome food for the trees, which are now rebounding from their former distress during the Pleistocene and reconquering the lands they had lost to grass. In the North of Eurasia, the Taiga is expanding and gradually eliminating the old mammoth steppe. Areas that today are deserts are likely to become green. We are already seeing the trend in the Sahara desert. 

What the savanna monkeys could do was probably a surprise for Gaia herself, who must be now scratching her head and wondering what has happened to her beloved Earth. And what's going to happen, now?  

The New Large Igneous Province made by Monkeys

The giant volcanic eruptions called LIPs tend to appear with periodicities of the order of tens or hundreds of million years. But nobody can predict a LIP and, instead, the savanna monkeys engaged in the remarkable feat of creating a LIP-equivalent by burning huge amounts of organic ("fossil") carbon that had sedimented underground over tens or hundreds of millions of years of biological activity. 

It is remarkable how rapid the monkey LIP (MLIP) has been. Geological LIPS typically span millions of years. The MLIP went through its cycle in a few hundreds of years. It will be over when the concentration of fossil carbon stored in the crust will become too low to self-sustain the combustion with atmospheric oxygen. Just like all fires, the great fire of fossil carbon will end when it runs out of fuel, probably in less than a century from now. Even in such a short time, the concentration of CO2 is likely to reach, and perhaps exceed, levels never seen after the Eocene, some 50 million years ago. It is not impossible that it could reach more than 1000 parts per million. 

There is always the possibility that such a high carbon concentration in the atmosphere will push Earth over the edge of stability and kill Gaia by overheating the planet. But that's not a very interesting scenario, so let's examine the possibility that the biosphere will survive the carbon pulse. What's going to happen to the ecosphere?

The Savanna Monkeys are likely to be the first victims of the CO2 pulse that they themselves generated. Without the fossil fuels they had come to rely on, their numbers are going to decline very rapidly. From the incredible number of 8 billion individuals, they are going to return to levels typical of their early savanna ancestors: maybe just a few tens of thousands, quite possibly they'll go extinct. In any case, they will hardly be able to keep their habit of razing down entire forests. Without monkeys engaged in the cutting business and with high concentrations of CO2, forests are advantaged over savannas, and they are likely to recolonize the land, and we are going to see again a lush, forested planet (arboreal monkeys will probably survive and thrive). Nevertheless, savannas will not disappear. They are part of the ecosystem, and new megaherbivores will evolve in a few hundreds of thousands of years. 

Over deep time, the great cycle of warming and cooling may restart after the monkey LIP, just as it does for geological LIPs. In a few million years, Earth may be seeing a new cooling cycle that will lead again to a Pleistocene-like series of ice ages. At that point, new savanna monkeys may evolve. They may restart their habit of exterminating the megafauna, burning forests, and building things in stone. But they won't have the same abundance of fossil fuel that the monkeys called "Homo sapiens" found when they emerged into the savannas. So, their impact on the ecosystem will be smaller, and they won't be able to create a new monkey-LIP. 

And then what? In deep time, the destiny of Earth is determined by the slowly increasing solar irradiation that is going, eventually, to eliminate the oxygen from the atmosphere and sterilize the biosphere, maybe in less than a billion years from now. So, we may be seeing more cycles of warming and cooling before Earth's ecosystem collapses. At that point, there will be no more forests, no more animals, and only single-celled life may persist. It has to be. Gaia, poor lady, is doing what she can to keep the biosphere alive, but she is not all-powerful. And not immortal, either. 

Nevertheless, the future is always full of surprises, and you should never underestimate how clever and resourceful Gaia is. Think of how she reacted to the CO2 starvation of the past few tens of millions of years. She came up with not just one, but two brand-new photosynthesis mechanisms designed to operate at low CO2 concentrations: the C4 mechanism typical of grasses, and another one called crassulacean acid metabolism (CAM). To say nothing about how the fungal-plant symbiosis in the rhizosphere has been evolving with new tricks and new mechanisms. You can't imagine what the old lady may concoct in her garage together with her Elf scientists (those who also work part-time for Santa Claus). 

Now, what if Gaia invents something even more radical in terms of photosynthesis? One possibility would be for trees to adopt the C4 mechanism and create new forests that would be more resilient against low CO2 concentrations. But we may think of even more radical innovations. How about a light fixation pathway that doesn't just work with less CO2, but that doesn't even need CO2? That would be nearly miraculous but, remarkably, that pathway exists. And it has been developed exactly by those savanna monkeys who have been tinkering -- and mainly ruining -- the ecosphere. 

The new photosynthetic pathway doesn't even use carbon molecules but does the trick with solid silicon (the monkeys call it "photovoltaics"). It stores solar energy as excited electrons that can be kept for a long time in the form of reduced metals or other chemical species. The creatures using this mechanism don't need carbon dioxide in the atmosphere, don't need water, they may get along even without oxygen. What the new creatures can do is hard to imagine for us (although we may try). In any case, Gaia is a tough lady, and she may survive much longer than we may imagine, even to a sun hot enough to torch the biosphere to cinders. Forests, too, are Gaia's creatures, and she is benevolent and merciful (not always, though), so she may keep them with her for a long, long time. (and, who knows, she may even spare the Savanna Monkeys from her wrath!). 

We may be savanna monkeys, but we remain awed by the majesty of forests. The image of a fantasy forest from Hayao Miyazaki's movie, "Mononoke no Hime" resonates a lot with us. But can you see the mistake in this image? What makes this forest not a real forest? 

__________________________

Note: You always write what you would like to read, and that's why I wrote this post. But, of course, this is a work in progress. I am tackling a subject so vast that I can't possibly hope to be sufficiently expert in all its facets to avoid errors, omissions, and wrong interpretations. Corrections from readers who are more expert than me are welcome! I would also like to thank Anastassia Makarieva for all she taught me about the biotic pump and about forests in general, and Mihail Voytehov for his comments about the rhizosphere. Of course, all mistakes in this text here are mine, not theirs.

Searching for Our Ancestresses: A Travel to the Origin of Time

 

Emma Ardinghi, my great-grandmother, animated by deep-fake technologies. She was born in the 1860s and died in the 1930s. Not many of us have images of their ancestors of more than a century ago. But what if some ultra-advanced technology could show your ancestors all the way to back to the creation of the world? (image created on this site. To know more about Emma, see here.)

Let's imagine a magic trick, or maybe a time machine, or a DNA-reconstructing technology, or some unknown laws of physics. Something, anyway, that shows you your ancestresses, all of them, one by one in a long, long string of mothers that leads you far, far back in time, all the way to the beginning of life on Earth. You are looking maybe into a screen, maybe into a crystal ball, or maybe into the clear waters of a stream in the light of the Moon. Just imagine.

Ten Generations (250 years). This stream of ten generations lasts ten minutes, one minute for each ancestress. The first face is our mother, of course. You know her well. You see her as a young woman, as you saw her in many old photos. Then, there appears your Grandmother, again as a young woman. Maybe you met her, maybe you remember her only from some faded pictures. She looks a little like you, same skin color and same eyes. As new images appear, you see faces you had never seen. The parade stops with the face of a woman who is your ancestor of 10 generations ago. She was born around 250 years before you, in a century when almost everyone was a peasant and there existed no such things as electricity, cars, or radio and TV. She has the same skin color as you, the same shape of eyes and nose, and probably a similar hair color. Let's assume that she is from Europe or North America and, in this case, she is most likely white. She wears a long cotton skirt and a shirt under a wool corset. She also wears a head cap and wooden sandals, no makeup on her face, her only ornament is a hairpin that she uses to keep her hair in a bun. She looks strong, in good physical shape, not a trace of fat on her body. Her husband appears behind her, a peasant wearing simple clothes and wooden sandals. In the background, you see the brick walls of the house. The windows are small and have wooden shutters. In a corner, a large fireplace. Although she is linked to you by a series of ten generations, her genetic imprint on you has been so diluted that she doesn't look very much like you. Still, if you look into her eyes, you feel a certain kindness, a certain sensation of familiarity. From wherever the image comes from, she locks her eyes into yours and smiles before fading. 

100 Generations (2000 years). Now the images change on the screen every six seconds. In ten minutes you see a hundred ancestresses, one after the other, separated by about 20 years from each other. Tall and not so tall, with long hair, short hair, sturdy, thin, lean, smiling, or maybe sad. There is a certain continuity with these images, the skin color remains about the same as yours: if you are black, they are black, and if you are white, they are white. And if you have slant eyes, they have slant eyes, too. But if you are blond, or you have clear-colored eyes, you'll see this feature disappearing: the eyes of your ancestresses gradually becoming brown, and their hair turns dark brown or black. When you arrive at the end of the sequence, you see someone who lived around 2000 years ago, at the time of the highest glory of the Roman Empire. She looks sturdy, but a little worn out by work and fatigue. She may wear a linen tunic under a heavy woolen cape. She may be a citizen of a large city, or someone living in a small village.  Behind her, her husband shows up. He is wearing similar clothes, a tunic, and a woolen mantle. Behind them, a brick wall with no windows, a fireplace in a corner. You have a glimpse of a simple wooden bed and a door. She doesn't look very much like you but, as you look into her face, you note a certain fire in her dark eyes. She is proud to be a citizen of the city or of the village where she lives. She smiles with an air of satisfaction at seeing that remote descendant of hers. For a moment, you are lost in those black eyes of her, then she fades away, and the parade restarts.

1000 generations (15 thousand years). Now the faces flash even faster, less than one second after each other. You see flickering faces, giving you just a quick glance of a few details: a hairpin, earrings, and an especially bright smile. These women maintain the same skin color you have and the same eye shape. But they look robust and sturdy, in good physical shape. The flickering stops with a woman appearing in front of you, looking straight at you. Let's assume that she is white. She wears a jacket and a gown in tanned skin. Around her neck, a leather necklace with hanging ivory teeth, maybe of a shark. Her hair is black, kept together by a leather lace at the back. Around her, you perceive a hut made of animal skin, mammoth tusks planted vertically in the soil are holding the skins together and a fire is burning at the center. Behind her, you see her husband. He wears the same kind of leather clothes. You see a stone-tipped lance leaning against the leather wall, and you can almost smell the burned fat of the animals in the hut. You look at her face. She doesn't look like you, not at all. But she has a certain air of pride that you recognize as universal among humans. She smiles at you, she seems to be happy to see this descendant of hers that she sees for such a short time. Then, she fades away.

10,000 generations (150,000 years). It is a whirl of faces that you see dancing on the screen. What you notice most is how the skin of your ancestresses is becoming darker and darker. As the movement stops, you are looking into the face of a black woman. She has curly brown hair, black eyes looking straight into yours. She is tall, she looks athletic. She wears a necklace made with seashells and a leather belt around her waist as she stands in the bright sun. You know that she is living somewhere in Africa and, behind her, you see the blue of the Ocean. In the distance, the Savannah extends all the way to the horizon. Around her, you see stones arranged in circles, traces of a campground where she lives, together with her group. You see her man standing behind her. Like her, he is black, tall and athletic, wearing only a leather belt around his waist. He holds a stone-tipped spear in his hands. An abyss of time separates you from her, and yet, somehow you recognize each other. You look into her eyes, she looks into yours. She smiles, although she looks a little perplexed at seeing this weird descendant of hers, so remote from her. But she seems to know that her descendants will cross that vast desert, spreading in the Northern forests and everywhere on Earth. She smiles at you as she says something that you don't understand, but that may be a charm of good luck. You smile back as she fades away.

100,000 generations. (one million years). What you see now is a continuous transformation, a morphing. The faces of your ancestresses change, shrink, their bones shift, they develop a pronounced brow ridge. Their skin remains dark, and when the sequence stops, you are in front of the face of this remote ancestress of yours, separated from you by one million years.  You see her standing in the open, among rocks and animal bones. She is completely naked, her skin mostly hairless. She is not tall, but not so much shorter than you. Behind her, a flat Savannah's landscape with shrubs and isolated trees. She is different, alien, with her broad nose, her flat skull under her black, long hair. You know that she is a member of the species called "homo erectus," not the same species you belong to, and you are tempted to call her "female," rather than "woman."  And yet, she partakes something of the human nature, you can't miss that. She has round breasts, and rounded buttocks, a human trait that other primates do not share. Nearby, you see her man, holding a hand axe in his hands. She is linked to you by an incredibly long series of motherhoods, each one an improbable event, and yet that chain led exactly to you.  You think for a moment at how a hundred thousand times a man and a woman, your ancestors, mated to generate the unlikely series of creatures that led to what you are. You look at each other in the eyes. Across a huge chasm of millennia, she nods at you, smiling, a gesture unchanged over such a long time. Then, she fades away.

One million generations (10 million years). Now you are rushing to the remote origins of the creatures we call "hominina." As the face morphs in front of you, gradually it becomes something not fully human. The sequence stops as you are in front of a face. Still a face, yes, but not the face of a sapiens. You are looking at a female of a different species. She is hairy, small, and sitting in a forested area together with other members of her group. The male, behind her, is bigger than her, and he looks suspiciously in your direction. You know that these creatures are common ancestors not just to you, but also to chimpanzees and gorillas. She looks at you, tilting her head a little as if she were surprised. You smile at her, she does not respond in kind, but she smacks her lips in your direction. A kiss from an ancestress of you who lived 10 million years ago. Then, the image fades away.

Ten million generations (100 million years). Now the creatures you see morphing on the screen rapidly cease having a face, they now have a snout. You see furry creatures quivering on the screen. The sequence stops to give you a glimpse of a four-footed creature that looks at you, puzzled, for a moment, before scuttling away, disappearing among the vegetation. Was that your ancestress? Yes, she was.  

A hundred million generations (400 million years). Creatures smaller and smaller appear on the screen, they rapidly lose their fur, they become flat, lizard-like animals. Smaller and smaller, until they disappear and you are facing the seashore from an empty beach in the sun. You know that somewhere, under the surface, a female creature is swimming. An extremely remote ancestress of yours. 

A billion generations (one billion years ago). You are still looking at the sea. An ocean of time goes by and nothing happens. You only know that, down there, there is something alive, quivering, changing. Microscopic unicellular creatures are busy reproducing themselves by mitosis, splitting themselves in two. They are no more males and females, yet they are the origin of what you are, there, below the surface of that remote ocean. 

10 billion generations (5 billion years ago). Now the view moves underwater. In the darkness, you dimly perceive submarine mountains spewing gas bubbles everywhere. You know that those mounds are the factories where organic life is being created. It is strange to think that your so-remote ancestor is not anymore a living creature, but an organic molecule. As you watch, the sea boils in a great turmoil as you enter the Hadean, the age of hell. Then, everything fades into darkness. Before the scene goes away, you have a glimpse of large, bright eyes looking at you. The Goddess herself is there, dancing in the emptiness of space while she creates the world. Gaia, the mother of us all. 

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Some images. None of them refers to a specific description in the text of this post, but they can give us some idea of what our ancestresses may have been looking like. 

This woman lived in Tuscany probably during the 2nd century BC, more than 2000 years ago. We know her name, "Larthia Seianti." She was Etruscan, a rich woman (note the armilla bracelet on her arm) who could afford an elaborate sculpture over her sarcophagus. It is a realistic portrait, at least in part. And, curiously, she looks a lot like a Tuscan friend of mine, living in Tuscany nowadays. She is likely one of her ancestress, just as an ancestress of mine. 

The reconstruction of the face of a Neolithic woman who lived about 7,500 years ago in the area that is now called Gibraltar. She has been nicknamed "Calpeia" from the ancient name of the Gibraltar mountain, "Mons Calpe." DNA analysis shows that she, or her immediate ancestors, came to the Iberian Peninsula most likely from Anatolia by boat. (source)

The "Venus of Brassenpouy," discovered in 1894 in Southern France. It is a  portrait of someone who lived more than 20,000 years ago. We cannot say whether it is realistic or not, just as we cannot be completely sure that it is a woman's face. But it is perhaps the earliest recognizable portrait ever made in human history.  

An impression of how a Paleolithic woman could have looked like, living maybe 20,000-50,000 years ago in Europe. Note her dark skin: it is a remnant of her African ancestry. (source)

 

A reconstruction of the face of a Neanderthal woman (from Earth Archives). She might have lived a few hundred thousand years ago. Several things in this image are uncertain, but note her heavy brow ridges, a typical Neanderthal characteristic. Note her eyes and her skin: the light color is a characteristic of people living in Northern regions. Is she an ancestor of some of us? It is not completely certain but, yes, many of us have Neanderthal genes in their DNA.

The reconstruction of a female Australopithecus who could have been living in Africa some 2 million years ago. If she had the typical human metabolism, she must have been capable of cooling by sweating. It is a feature incompatible with a thick body hair. And, indeed, she is shown with sparse hair, although she may well have had none. Note also her human-like round breasts. It is a likely secondary sexual characteristics of hominins which tend to stand upright most of the time. An ancestress of ours? Maybe. (source)