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 (Charles Darwin 1859). 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 the paradigm that goes under the name of “holobiont,” an ensemble of creatures that live together in a condition of symbiosis.

“Holobiont” is a term known from the 1930s (Baedke, Fábregas‐Tejeda, and Delgado 2020), but the idea of collaboration among living beings, microorganisms in particular, is much older and it may go back to the concept of “consortia” developed in mid 19th century (Kull 2010). Nowadays, it is mostly associated to the work of Lynn Margulis, who used it starting in the 1990 (Lynn Margulis 1990) (Lynn Margulis 2008), although she had been studying the subject already in the 1960s (Sagan 1967).

The interest in the concept of holobiont has been rapidly growing with the 2000s (Baedke, Fábregas‐Tejeda, and Delgado 2020) and that led to a variety of different interpretations of what exactly a holobiont is. Let’s say that the holobiont term can be seen as a meme growing in the memesphere (Dawkins 1976) and, true to its evolutionary background, has been following a memetic evolutionary path, changing and adapting as its complex memetic environment has changed.

Here, I am not even trying to provide a comprehensive discussion of the many ways in which the holobiont concept is interpreted. Let’s just say that the most common interpretation sees holobionts in a relatively narrow sense, as the systems formed by a host organism and its associated symbiotic microorganisms, e.g. the gut bacteria of human beings (see, e.g. the work by Rosenberg (Rosenberg et al. 2007). We can call this the “classical” view that focuses on host-microbe interactions. A more general definition is provided by Castell and others (Castell et al. 2016), (Matyssek, Lüttge, and Castell 2022) in terms of the concept of “complex adaptive systems” (CAS) (Mobus and Kalton 2015). 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, which some call Gaia (Matyssek and Luttge 2013)), (Castell, Lüttge, and Matyssek 2019).

The nodes (the creatures) of the holobiont network are connected to each other by flows of matter, energy, or information 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 CASs 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 different creatures in a single organism (Sagan 1967). According to Margulis (Lynn Margulis 2008), 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 (Gray 2017), (Morris 2018).

The present paper is a review of the concept of “holobiont” aimed at finding a unifying concept that could give us the key to understanding how and why many complex systems around us exist and operate. The “holobiont revolution” 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.

How Evolution 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. 

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 the 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.

Apart from politics, Darwin’s theory became more and more a cornerstone of biology, especially with the birth of modern molecular biology. The molecular interpretation of evolution became known as “neodarwinism,” based on the well-known concept that the information (the “blueprint”) of a living being is contained in its genetic code in the form of DNA molecules. The concept that information flows from the DNA to proteins, and not the reverse, is known as the “central dogma” of neodarwinism. Within this paradigm, evolution is supposed to occur in the form of changes in the genetic code. So successful was molecular biology that the term DNA became synonymous in common parlance with concepts such as “destiny”, “blood,” or “karma.” Evolution, then, was the result of changes in the DNA molecules passed from one organism to another caused by random mutation or sexual mixing (meiosis).

Yet, there remained a basic problem in the theory of evolution: the gradual change of DNA molecules as the result of mutation or meiosis can hardly account for the discrete jumps needed to go from one species to another. This problem was sometimes described in terms of “missing links” supposed to relate modern humans to their remote ancestors. Of course, the missing link is just an invention of the media -- it does not have a scientific basis. Yet, there are features in living beings that are difficult to understand in terms of a gradual process of natural selection, as Darwin described it. A classic example is that of eyes. How could ancient, blind animals, develop eyes?

Indeed, 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. So, we have this new evolutionary invention, eyes, that appear relatively suddenly, already in a functional form. How could it happen?

This question was termed the “irreducible complexity problem” by Behe et al., and sometimes it was understood as implying the need to admit a supernatural intervention to explain some features of living beings. Attempts to solve the problem by postulating gradual steps were only partially successful. For instance, in 1994, Nilson and Pelger 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 evolution of eyes gives us another window on the concept of holobiont and the related concept of endosymbiosis. According to this view, multicellular creatures are multicellular collections of formerly independent unicellular organisms that were co-opted to form a holobiont. Following this line of reasoning, we may assume that the 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 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 solving 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. 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.

An example. Think of a herd of wild zebras. Imagine that the survival of the fittest manages to create a "super-zebra" that can run faster than the other zebras. Would that make it easier for it to survive? We may not be so good a thinking like zebras, but it should be obvious that, for a zebra, survival is a matter of 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 its movement will confuse them and provide 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 for survival, especially considering that it could only be obtained at the expense of other useful characteristics, such as endurance.  Overall, therefore, the herd will be formed of individuals of similar characteristics. A herd is an example of a 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.

Note also that, of course, a zebra that can't keep up with the herd will be the preferred target of predators. It is the removal of the unfit, a concept that was developed in particular by Gorshkov et al., (Gorshkov, Makarʹeva, and Gorshkov 2000) 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.” 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. 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 that can best maintain it. So, the winners are always good holobionts!

 

References

Baedke, Jan, Alejandro Fábregas‐Tejeda, and Abigail Nieves Delgado. 2020. “The Holobiont Concept before Margulis.” Journal of Experimental Zoology Part B: Molecular and Developmental Evolution 334 (3): 149–55. https://doi.org/10.1002/jez.b.22931.

Behe, Michael J. 2001. Darwin’s Black Box: The Biochemical Challenge to Evolution. Simon and Schuster.

Bowler, Peter J. 1992. The Non-Darwinian Revolution: Reinterpreting a Historical Myth. Reprint edizione. Baltimore: Johns Hopkins Univ Pr.

Castell, Wolfgang, Frank Fleischmann, Tina Heger, and Rainer Matyssek. 2016. “Shaping Theoretic Foundations of Holobiont-Like Systems.” In Progress in Botany 77, edited by Ulrich Lüttge, Francisco M. Cánovas, and Rainer Matyssek, 219–44. Progress in Botany. Cham: Springer International Publishing. https://doi.org/10.1007/978-3-319-25688-7_7.

Castell, Wolfgang, Ulrich Lüttge, and Rainer Matyssek. 2019. “Gaia—A Holobiont-like System Emerging From Interaction.” In Emergence and Modularity in Life Sciences, edited by Lars H. Wegner and Ulrich Lüttge, 255–79. Cham: Springer International Publishing. https://doi.org/10.1007/978-3-030-06128-9_12.

Charles Darwin. 1859. The Origin of Species. John Murray.

Dawkins, Richard. 1976. The Selfish Gene. Oxford ; New York : Oxford University Press. http://archive.org/details/selfishgene00dawk_669.

Gorshkov, V. G., Anastasii︠a︡ Mikhaĭlovna. Makarʹeva, and Vadim V. Gorshkov. 2000. Biotic Regulation of the Environment : Key Issue of Global Change. Springer-Praxis Series in Environmental Sciences. Springer-Verlag. http://www.springer.com/it/book/9781852331818.

Gray, Michael W. 2017. “Lynn Margulis and the Endosymbiont Hypothesis: 50 Years Later.” Molecular Biology of the Cell 28 (10): 1285–87. https://doi.org/10.1091/mbc.e16-07-0509.

Kull, Kalevi. 2010. “Ecosystems Are Made of Semiosic Bonds: Consortia, Umwelten, Biophony and Ecological Codes.” Biosemiotics 3 (December): 347–57. https://doi.org/10.1007/s12304-010-9081-1.

Lane, Nick. 2015. The Vital Question. London: W W Norton & Co Inc. https://profilebooks.com/the-vital-question.html.

Margulis, L. 1975. “Symbiotic Theory of the Origin of Eukaryotic Organelles; Criteria for Proof.” Symposia of the Society for Experimental Biology, no. 29 (January): 21–38.

Margulis, Lynn. 1990. “Words as Battle Cries—Symbiogenesis and the New Field of Endocytobiology.” BioScience 40 (9): 673–77. https://doi.org/10.2307/1311435.

———. 2008. Symbiotic Planet: A New Look At Evolution. 1st edition. Basic Books.

Matyssek, Rainer, and Ulrich Luttge. 2013. “Gaia: The Planet Holobiont.” Nova Acta Leopoldina 114 (391): 325–44.

Matyssek, Rainer, Ulrich Lüttge, and Wolfgang Castell. 2022. “Evolution of Holobiont-Like Systems: From Individual to Composed Ecological and Global Units.” In Progress in Botany, 1–46. Progress in Botany. Berlin, Heidelberg: Springer. https://doi.org/10.1007/124_2022_57.

Mobus, G.E., and M. C. Kalton. 2015. Principles of System Science. New York, Heidelberg, Dordrecht, London: Springer. https://doi.org/10.1007/978-I-4939-1920-8.

Morris, J. Jeffrey. 2018. “What Is the Hologenome Concept of Evolution?” F1000Research 7 (October): F1000 Faculty Rev-1664. https://doi.org/10.12688/f1000research.14385.1.

Nilsson, D. E., and S. Pelger. 1994. “A Pessimistic Estimate of the Time Required for an Eye to Evolve.” Proceedings. Biological Sciences 256 (1345): 53–58. https://doi.org/10.1098/rspb.1994.0048.

Rosenberg, Eugene, Omry Koren, Leah Reshef, Rotem Efrony, and Ilana Zilber-Rosenberg. 2007. “The Role of Microorganisms in Coral Health, Disease and Evolution.” Nature Reviews. Microbiology 5 (5): 355–62. https://doi.org/10.1038/nrmicro1635.

Sagan, Lynn. 1967. “On the Origin of Mitosing Cells.” Journal of Theoretical Biology 14 (3): 225-IN6. https://doi.org/10.1016/0022-5193(67)90079-3.

Zhao, Fangchen, David J. Bottjer, Shixue Hu, Zongjun Yin, and Maoyan Zhu. 2013. “Complexity and Diversity of Eyes in Early Cambrian Ecosystems.” Scientific Reports 3 (September). https://doi.org/10.1038/srep02751.

Do we focus too much on CO2 alone? An appeal for the conservation of natural ecosystems

 

Image from the University of Toronto. 

Have we exaggerated with the idea that CO2 -- carbon dioxide -- is the arch villain of the story? Aren't we overemphasizing solutions that imply CO2 removal? How about geoengineering, sometimes touted as "the" solution that will allow us to keep going on burning fossil fuels? 

There is no doubt that the emissions of carbon dioxide are returning the ecosystem to a condition that was never seen before at least one million years ago. There is no doubt that CO2 is warming the planet and that none of our Sapiens ancestors ever breathed in an atmosphere that contains a concentration of CO2 of 420 parts per million -- as we are doing. 

But by focussing so much on CO2 alone is easy to forget what humans have been doing to the ecosystems that keep the biosphere alive (and with it, humankind). The ecosystem is a giant holobiont that strives for stability: a fundamental element to stabilize Earth's climate. It is a dangerous illusion to think that we, humans, can replace the work of Gaia with our fancy carbon capture machinery, or whatever other tricks we may concoct. 

Here is a reminder by a group of people from Eastern Europe who managed to maintain a certain degree of mental sanity. They remind us of the damage we are doing. Will anyone listen to them? (UB)

Appeal to the international community, governments, scientific, public organizations and business

https://www.es-partnership.org/wp-content/uploads/2021/06/Appeal_Protect-Ecosystems.pdf

RECOGNIZE THE VALUE AND ROLE OF NATURAL ECOSYSTEMS FOR CLIMATE CHANGE!

Terrestrial and marine natural ecosystems are the basis for preservation of biological life on Earth. They have existed almost unchanged for millions of years and all this time have supported climate stability, biochemical flows, global water circulation and many other processes, irreplaceable and essential for preservation of life on our planet. Undisturbed natural ecosystems maintain the Earth's temperature, suitable for human life.

The laws of nature are the basis of life on Earth, and all the laws of human society that regulate economic, political, social and cultural relations are secondary to them and must take into account the biosphere’s operating principles and man’s place in it.

However, over the past decades, human activities aimed at meeting the needs for food, energy and 
water have caused unprecedented changes in ecosystems, including land degradation and deforestation. These changes have helped improve the lives of billions of people, but at the same time, they have destroyed nature's ability to regulate the environment and maintain the climate.

According to current estimates, more than 75% of natural ecosystems are subject to degradation and loss of their functions, which undermines all efforts to preserve the climate and threatens the achievement of SDGs, including hunger, disease and poverty eradication. 

Humanity is standing on the edge of a precipice. Over-threshold disturbance of ecosystems leads to
irreversible loss of the gene pool, up to complete disappearance of ecosystems. In the face of growing efforts and understanding of the threat of climate change, it is now necessary to recognize and support the unique role of natural ecosystems in preserving the climate and a vital environment. International climate policy adjustments and fundamental changes in national development strategies are required.

We call to wake up and recognize the fundamental and irreplaceable value of natural ecosystems and for strong and urgent action, including:

1.  To recognize the goal of preserving natural ecosystems as humanity’s highest priority and stop their further destruction through adopting a global moratorium on any further development of territories still untouched by human activities, with international support mechanisms, including funding.
2.  Promotion of large-scale natural reforestation is an urgent task. Climate-regulating functions of forests, associated with the ability to retain soil moisture and maintain continental water transfer, are their main value, which are orders of magnitude higher than the cost of wood. Undisturbed forests should be completely removed from economic activity by law and allocated to a separate category with the maximum degree of protection. 
3. At all levels, from international to regional, national and local, it is necessary to review ongoing development strategies and take urgent measures to protect natural ecosystems and wildlife. It is necessary to adjust all sectoral policies, including agricultural practices, in order not only to meet the demand for food, but also to minimize the burden on natural ecosystems
4. A transition from conventional sectoral management to basin and ecosystem management is required, including raising the status of nature conservation goals. Water resources management should ensure that natural ecosystems are guaranteed priority in water supply that is necessary for their conservation, as well as protection and restoration of aquatic and other ecosystems - from mountains and glaciers to deltas and reservoirs.
5. Measures aimed at preserving natural ecosystems also require a review of existing incentives and tools and creation of new ones, so that ecosystem services are no longer perceived as free and unlimited, and their management takes into account the interests and roles of the populations and local communities which directly depend on them and are their custodians.

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International Socio-Ecological Union, Eco-Forum (of 54 public organizations) of Kazakhstan, 
Association (non-governmental organizations) «For Sustainable Human Development of Armenia»,
Eco-Forum (independent non-governmental organizations) of Uzbekistan, as well as professional and non-governmental organizations of Armenia, Moldova, Russia, USA and others

Can our bacteria and virus friends save us? The rise of psychobiotic therapies for the human holobiont

 

The effect of the gut microbiota on the brain in the human holobiont is an incredibly complex story. It is described in a review appeared in 2019 on the "Physiological Reviews" journal. Not so much a review as a whole treatise, it is huge, with 1694 references in the bibliography!!! You can find a more readable summari on the blog "gut microbiota for health"

Before going on, a note of caution: as it is the rule nowadays, most of what appears in the medical literature is biased or simply false as the result of the widespread corruption of the medical research system. This review has been partly sponsored by the pharmaceutical industry, nevertheless it seems reasonably free of hype for specific treatments or wonder drugs. Its approach is correctly evolutionary, for instance when they say:

"Given that there has never been a time when mammals existed without microbes (apart from under highly restrictive laboratory conditions), there has also never been a time when the brain has been without signals from the gut, and it is important to consider the relationship between the host and its microbiota from an evolutionary perspective"

And they have it perfectly right when they use the magic word "holobiont"!!!

"The hologenome theory may even account for complex biological phenomena such as certain behaviors. For instance, behavior that facilitates social interaction among holobionts might be considered evolutionarily adaptive/advantageous as it gives rise to greater transmission of microbiota, thereby enhancing genetic variation (1285, 1286, 1689). In light of these inextricable links between the microbiota and the brain throughout evolutionary history, it is imperative for the study of our own biology (and that of the entire animal kingdom) to understand how microbial symbionts influence brain physiology and behavior."

The main point of the paper is that the human brain is strongly influenced by the gut, that our behavior, to the point that modifying the gut composition can be seen as a form of "psychobiotic therapy" that improves plenty of pathological situations and improves also the quality of life, for instance helping against depression. 

Seeing how the world is behaving nowadays, it would seem that the human microbiome is in bad conditions, indeed. Especially the gut of our leaders seems to have been invaded by micro-monsters instead of the beneficial gut flora that's normal for humans. And, surely, the various habits of hyper-cleaning, distancing, and self-suffocation that became fashionable during the past two years didn't help in improving the situation. Maybe, at this point, only our bacterial and viral friends can help us (with a little support also from our friendly archaea and fungi). Go, friends, go! We need you badly! 

The epidemic of obesity keeps getting worse. What's a good holobiont supposed to do?

 

The obesity epidemic keeps expanding. The above are, I believe, the most recent data available for the US. The COVID-19 lockdowns and isolation measures are reported having made things even worse. This trend is simply horrendous: what the heck is happening to humankind? (Disclaimer: I am not a nutritionist, I am just someone who is fascinated by data and trends. And, of course, we are all interested in our health! Here, I report some data I found, hoping you may find them useful. Don't take them as the last word on the subject. As always, before acting on things that affect your health, do your own search and use your judgement about what works for you.)

The obesity epidemics had a considerable boost by the lockdowns during the Covid-19 pandemic. Coupled with the opposite effect, that obesity is a risk factor for people who contract Covid, you have a remarkable disaster in the making. With several Western countries having percentages of obese people close to or higher than 50%, one wonders what's going to happen in the future. Why are the human holobionts in such a poor shape?

The story is complicated, and I don't pretend to say anything new. I just want to attract your attention to some recent studies that I think shed some light on the mechanism of human obesity (but even our fellow dog holobionts are suffering from obesity). 

First, the work by Raubenheimer and Simpson on the food preference of various animals, summarized in a recent book titled, "Eat like the Animals." (Mariner Books, 2021). Their discovery is easy to summarize: it seems that most living beings have a specific set point in their needs for the main nutrients. They seek for a specific balance among proteins, fats, and carbohydrates. In particular, they aim at a minimum intake of protein. If animals are fed an unbalanced diet, for instance, poor in protein, they will tend to eat more food until they reach the right level. Raubenheimer and Simpson call this the "protein leverage hypothesis:" 

"In a protein-poor but energy-rich food environment, humans will overeat carbs and fats to try to reach their protein target. However, when the only available diet is rich in protein, human will underconsume carbs and fats"

Since excess carbohydrates are stored in the body as fat, we can say that one of the causes of the obesity pandemic is that the human diet in Western countries is overstocked in carbohydrates. 

Here comes Robert Lustig and his book "Metabolical" (Yellow Kite 2021) where he minces no words on how this is not only true but also a profitable strategy for the food industry. They discovered long ago that if they put more and more carbohydrates (sugars) in the food they sell, then people will get fat, they will eat more, and that will increase their profits. Just like sick people are a boon for medical doctors, obese people are a boon for food producers. 

You don't believe that? Let me show you a picture I took a few days ago in an Italian supermarket:

There are four kinds of regular mayonnaise on sale, plus a fancy one with no eggs. Can you guess which is the only one that does not contain added carbohydrates? Let me tell you, it is the most expensive one among the regular ones. All the others contain sugar. Maybe it is not the same for all mayonnaise brands on the market, but I think it is significant. Food companies do add sugar everywhere, even when it is not called for by the traditional recipes. They deny that, but it is written on the list of the ingredients (I have pictures, if you don't believe me!)

Now, it may well be that there is much more to obesity than just carbohydrates, but I think that these results point at an important cause of the problem. There are data showing that what we are seeing may be a delayed effect of "peak sugar" that occurred around the year 2000. From then on, the amount of sugar consumed in the U.S. has been going slightly down. But it remains high. 

The beauty of this is that, if it is true, with obesity we don't have such a wicked problem as others, say, global warming. We know that to avoid global warming, we should stop burning hydrocarbons, but it is also true that we can't just stop: billions of people would die. But we could stop, or at least strongly reduce, the extra carbohydrates added to processed food, and we could do it today. Nobody would die, but the problem would be eased and many people would be healthier! But this is the way things are in the world: no problem can ever be solved when there is somebody making money if it remains unsolved. 

To make you happier, let me show you some data that tell us that a little excess weight (a little!) is not necessarily bad for your organism. Here are some data from Malcolm Kendrick's wonderful book "The Clot Thickens" (Columbus 2021). 

BMI stands for "Body Mass Index" and the overall lowest risk of death is for a BMI of 25-30 that's normally classified as "overweight" (if you want to know, I am at BMI=27). Being underweight is a larger risk than being obese! But obesity has many other problems, not least in terms of self-esteem. 

In the end, remember that you are a holobiont and that for hundreds of millions of years your holobiont ancestors never ate anything that was processed in an industrial plant. You are a fine-tuned machine that includes trillions of friendly viruses and bacteria living in your guts. They want a balanced diet of fats, protein, fiber, and not too much in terms of carbohydrates (but you need them, too!). Try to make them happy, and you'll be happier, too!

Gaia's One Billion Years Task: Colonizing the Land

 

Gaia as the sea Goddess Grammamare according to Hayao Miyazaki's interpretation in the film "Ponyo"

Imagine a time machine that brings you back to the Earth of one billion years ago, right in the middle of the eon called the "Proterozoic." First of all, you need an oxygen respirator, otherwise you'll die of suffocation in a few minutes. You also need a wide-brimmed hat and an outfit that covers your limbs in such a way as to protect your skin from the ultraviolet radiation. It is your planet, but in this period it is not especially friendly to a metazoan as you are.

You walk a few cautious steps onward. In front of you, the blue sea. You turn around: an expanse of dry rocks that continues all the way to the horizon. No traces of anything green that you can see: no plants, no insects, no birds, nothing like that. Above you, the sun is bright in the blue sky. You notice that it is a little less bright than you are used to seeing it, in your time. No traces of clouds: it is what you expected: no trees means no evapotranspiration of water vapor, no volatile organic compounds to function as nucleation sites for the water droplets that form clouds. 

You walk toward the sea. There are mainly rocks, but also some sandy places: small patches of beach. If there is a beach, there has to be a river, somewhere, that created it. You see it, not far away. It is completely dry, its bed going straight through the rocky landscape from the hills in the distance. Rains, when they arrive, must be torrential downpours that come and go quickly. 

You kneel on the beach, in front of the sea, lifting some water with your cupped hands. You know that it should be less salty than the seawater you are used to in your time, and you are tempted to taste it to confirm. But that is not a good idea. That water is brimming with microorganisms, most of them unlike anything your immune system is used to. You drop the water on the surface of a rock, where it forms a dark spot that rapidly evaporates and disappears. 

Standing up in front of that alien sea, you look at the gentle waves coming and going. You know that there are no fish in there. No crabs, no seashells, no seaweed, nothing like that. But there are enormous numbers of microorganisms. They are photosynthesizing, eating each other, reproducing by splitting themselves in two. They can live only in water. Is there life on the dry rocks on the shore? Maybe some of those microscopic creatures survive there, maybe even thrive, perhaps algae or even ancestors of modern lichens. But they are just eking out a precarious existence. They are invisible to the naked eye, and their time has not come yet.

On the horizon, an enormous orange moon rises as the sun slowly fades on the opposite side. You keep looking at the dark waters in front of you. Just under the surface, you glimpse something that looks like a pair of large eyes. You think you see her just for a moment, Gaia in her form of sea goddess, languidly swimming in the calm sea. 

____________________________________________________

Back to your time machine. You dial 350 million years before your time, the start of the Carboniferous Period. You press the button. 

You emerge out of the machine, breathing the fresh air, smelling something you had never smelled before. Whatever it is, the air is humid, rich in oxygen. You are in a small clearing, in front of you, there is a pond surrounded by a lush forest. Trees, tall trees, forming a full canopy under the low clouds, swept by a gentle wind. The place is eerily silent: no birds, no insects, nothing like that. Yet, you recognize the place: this is your planet, Earth, not yet the way it will be in the future that is your time, but a familiar world. 

As you stand, a noise comes to you: a buzz. You see something flying away, an insect of some kind. It starts raining. It is a warm, gentle downpour that wets you rapidly, but ends quickly. It has been enough to disturb the creatures living under the low bushes. You have a glimpse of them scuttling away: tetrapods, early amphibians. They jump into the water of the pond and then disappear. They are your ancestors, the ancestors of all the metazoans that will move on land in the future that's your time. 

As you walk, splashing your boots on the mud, you wonder how Gaia pulled this incredible trick: transforming the bare rock of entire continents into lush forests. While you think that, you have a glimpse of a pair of bright eyes staring at you from the canopy. You look up, and they disappear, leaving only a Cheshire-cat smile of the Goddess of the Forests, then she vanishes among the branches.

Images of the Goddess courtesy of "Mon Seul Desir"

Good Advice for Holobionts

 

h/t Ilargi Meyer

Gaia and the Savanna Monkeys. 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 herbivores may have subsisted on decaying plant matter, or perhaps on ferns. 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 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 eat leaves on the high branches of trees. A bipedal stance makes the creature able to stand up, balancing on its tail. 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). That's a bit silly, 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: 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 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 during the Cretaceous.  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 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 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 herbivore, 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!

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, the 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 "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. So, 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 could not 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 freezing 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. 

The rise of the Savanna Monkeys  

Primates are arboreal creatures that had 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 was a disaster. They were not equipped to live in savannas: they were slow on the ground, just an easy lunch for the powerful predators of the time. Primates also never colonized the northern taiga. It was probably 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. So, "boreal monkeys" do not exist (actually, there is one, but it is not exactly a monkey!).  

Yet, during the Pleistocene, the shrinking of the tropical forests forced some monkeys to move into the savanna, leaving their comfortable living on tree branches. The Australopythecines, (image source) appeared about 4 million years ago. We may call them the first "savanna monkeys." 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. 

The 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? 

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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.