Book Summary: “Revolutions That Made the Earth” by Tim Lenton & Andrew Watson


Revolutions that Made the Earth

Title: Revolutions That Made the Earth
Author: Tim Lenton & Andrew Watson
Scope: 5 stars
Readability: 3 stars
My personal rating: 5 stars
See more on my book rating.

Topic of Book

The authors present a theory for how and why the Earth and Life have changed over time.

Key Take-aways

  • The history of Earth has been dominated by a few revolutions:
    • Evolution of simple life
    • Dramatic increase of oxygen in the atmosphere
    • Evolution of multi-celled organisms
    • Evolution of humans with culture and language.
  • Each revolution was triggered by a global catastrophe that forced an evolutionary adaptation involving:
    • Higher level of organization
    • Greater energy utilization
    • Faster information processing
    • New means of recycling organic elements (Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus and Sulfur)
  • Life on earth has altered earth so that life can flourish.

Important Quotes from Book

This book tells a strange and fascinating story—the entangled histories of life and this planet. Our central idea is that a very few profound revolutions have made the Earth as we know it. Each revolution can be traced back to unlikely innovations in the evolution of life, and each involved radical changes in the non-living environment.

The revolutions we trace are best viewed as fundamental transformations of the whole system of life coupled to its planetary environment, which we call the ‘Earth system’. What exactly is that? Well, we mean the many processes that interact together to set the living conditions at the surface of the planet—atmospheric and ocean composition, temperature and climate, and the life on the land and in the ocean. All-encompassing as this definition is, it nevertheless describes a very coherent system whose properties can only be fully understood by considering it as a whole.

Why ‘revolutions’ that made the Earth? Well, the changes we describe were fundamental ones for life and the planet. They were triggered by new evolutionary innovations, resulting in great and often violent shifts in the environment, which in turn killed off many existing species. Out of this chaos emerged a new system and new life forms.

Though each revolution is different in the detail, they share an overall pattern. Each has involved major reorganization in the system, and at each it has moved stepwise towards greater energy utilization, greater recycling efficiency, faster processing of information, and higher degrees of organization. Each revolution has been characterized by near catastrophes and with no certainty of a successful outcome—the revolutions only appear ‘successful’ because we are looking at them with hindsight, and because had they failed, we would not be here to remark on them.

The Earth has gone from a system of physics and chemistry alone to one including biology, and therefore evolution.

Looking inside a human, or any other complex organism, reveals nested structures, one layer inside another, like Russian dolls. As we go deeper into this structure we also go further back in time.

The first layer of organization can be seen with a light microscope at quite low magnification. All animals, and also plants and many fungi, are multi-cellular, made up of trillions of cells (about 10 trillion in us humans, for example) which act in supremely close cooperation, mostly using chemical signalling mechanisms to coordinate their actions. The adoption of the multi-cellular habit has enabled a huge increase in the complexity of size and form of living things.

If you turn up the magnification to look inside a single one of your cells, you’ll see a second layer of organization. The cell itself is also a community. A typical animal, plant or fungal cell contains many ‘organelles’, different kinds of internal structures bounded by membranes that separate them from the cytoplasm in which they reside.

One of the great recent advances in our understanding of evolution has been the realization, championed by Lynn Margulis, that ancestrally, some of these organelles were derived from free-living bacteria.

A third level of organization is apparent at still higher magnification, in the genetic material in your cells (or their symbionts). This is the population of genes which resides in the molecules of DNA. The genetic information is written in a universal code, shared by all living things on the planettoday, so you would find the same code inside the bacteria and archaea in your gut, for instance. The genetic code evolved before the archaea, bacteria and eukaryotes split from one another, back at the very origin of life.

The revolutions of Earth are marked therefore not only by physical and chemical changes to the Earth environment, but also by increases in the complexity of the organisms living on it. They are also marked by increasing use of energy, increased efficiency in recycling materials, and increased information processing.

To achieve these massive increases in productivity, the biosphere had to do more than just evolve new means of capturing energy—it also had to find new ways of supplying the materials needed by life. All organisms build their bodies by transforming materials taken in from their environment (‘food’ in the broadest sense) and excreting waste products.

Many chemical elements are essential for all life forms, the ‘big six’ being carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulphur (S), but there are a host of other ‘micro-nutrients’ that life requires in at least trace quantities.

The Earth as a whole contains large amounts of all the elements required by life but they are mostly locked up in the core, magma and crust, and their proportions are rather different to life’s requirements. Of the big six essential elements for life, all but oxygen are relatively scarce in the Earth’s crust and interior. A significant mass of the lighter elements, including those essential to life, had to make a late arrival in the meteorite bombardment that also brought water to the Earth

The process of nuclear synthesis had left an abundance of even atomic numbers (such as carbon, atomic number 6, and oxygen, atomic number 8) over odd atomic numbers (such as nitrogen, 7, and phosphorus, 15). This is the fundamental reason why nitrogen and phosphorus are potentially limiting nutrients for growth.

To achieve these massive increases in productivity, the biosphere had to do more than just evolve new means of capturing energy—it also had to find new ways of supplying the materials needed by life. All organisms build their bodies by transforming materials taken in from their environment (‘food’ in the broadest sense) and excreting waste products.

The mark of each successful revolution of the Earth system has been a coupled increase in both energy processing and material recycling by the biosphere. The two must go hand in hand.

Initially in a revolution things can go awry, often with biological innovations leading to an increase in carbon burial and the removal of essential materials from the surface system, followed by wild oscillations (see Fig. 2.1 ). But such a state of affairs cannot persist and it appears to force new innovations that lead to a recovery of recycling and a reduction in organic carbon burial back to the original flux.

Life depends on energy and matter transformations, but something else is essential to it as well. This is heredity—the passing on of information to new generations. Over time the increasing complexity of the biosphere has gone hand in hand with the ability to pass on progressively more information.

We human, conscious beings are only here because of a remarkable series of revolutions in Earth history. We have awoken to find ourselves in a system built on a series of previous ones. The transitions between these systems each involved qualitative changes in energy processing, material recycling and information transfer by the biosphere. These were reflected in a changed overall state of the non-living environment and a diversification or differentiation of the Earth system into a wider range of environments (or niches). Earlier biospheres are still with us just as in the major transitions of evolution earlier units of evolution are still with us. In ecological terms, the addition of a new layer at each revolution of the biosphere represents a form of niche construction on a grand scale.

For the Earth system then, we can list the following revolutions:

1) Inception : This is the revolution about which we know the least, both in terms of the mechanisms involved and their timing. It represents the establishment of a global biosphere, beginning with the

origin of life. We are not even sure that life originated on Earth, but it was probably present by 3.8 billion years ago, and certainly by 3.5 billion years ago. As life diversified, recycling loops must have soon established. At first the energy for the biosphere came from chemical gradients, but at some point an early form of photosynthesis got going, greatly increasing the energy captured by the young biosphere.

2) Oxygen : This revolution began in the Archean with the origin of water-splitting oxygenic photosynthesis, probably sometime before 2.7 billion years ago. This generated an order of magnitude increase in energy supply to the biosphere.

3) Complexity: The presence of oxygen in the atmosphere and surface ocean provided an energy-rich environment in which eukaryotes began to flourish and diversify, developing much larger and more complex cells and genomes, and the ability to transmit more information to their offspring… In the aftermath of the great glaciations, with a high oxygen concentration now in the atmosphere, the first multi-cellular animals flourished. Once again energy use by the biosphere increased, and the advent of animals allowed complex food webs with multiple trophic levels to develop. The arms race that ensued between the eaters and the eaten left its most striking mark in the fossil record with the advent of hard shells as protection, recorded in the Cambrian explosion around 540 million years ago. The familiar ecology that characterizes the modern world, the Phanerozoic Eon, had been born.

4) Us?:  From 400 to 350 million years ago, the rise of vascular plants on the land surface doubled energy capture by the biosphere. This led to a reduction in atmospheric carbon dioxide, planetary cooling, and further (but compared to the snowballs, minor) glacial episodes. Fungi evolved to consume and break down woody plant material and recycling was boosted again. Animals followed plants onto the land surface and under the high oxygen atmosphere they became larger and more mobile, promoting the evolution of nervous systems and the brain. Through a period of climate cooling, culminating in periodic glaciations, a range of bipedal ape species evolved and one of them developed a surprising language faculty, enabling them to transmit much more information to their offspring. Because of this they were able to develop technical skills to a high degree, including the use of fire, tools and weapons. On emerging from the last glaciation these Homo sapiens spread worldwide, causing widespread extinction of other species, inventing farming and writing, and congregating in larger numbers in towns and cities. The rest, as they say, is history.

Oxygen Revolution

Whether your food came in the form of animal, plant or fungal matter, the energy it contains was first captured from sunlight by a photosynthesizer—most likely a green plant—which split water, liberatingoxygen gas, and fixed carbon from carbon dioxide into organic matter. Although green plants dominate the land surface today, this process of oxygenic photosynthesis first evolved in a water-borne cyanobacterium, and they are ultimately responsible for the oxygen-rich air that we breathe.

Aerobic metabolism yields an order of magnitude more energy per molecule of organic matter broken down compared to anaerobic pathways such as fermenting lactic acid. This high energy yield is absolutely critical to our existence.

Oxygen has risen from essentially zero to its present levels in a series of steps, over the past few billion years. The most striking of these steps was the Great Oxidation event around 2.3 billion years ago, which neatly divides the history of the planet into two halves.

Thus, producing an oxygen rich atmosphere won’t happen on planets that are too small as they lose their atmosphere altogether, and it won’t happen on planets that are too large as they will not oxidize within their lifetime.

This adds an extra element of difficulty to the revolution we have been describing in this and the last two chapters. The upshot is that out there in the cosmos, there may be very few planets with oxygen-rich atmospheres, and thus very few that can support intelligent, observer life forms.

Single-celled organism to Multi-celled organisms:

Most importantly, multi-cellularity could flower because the eukaryotes evolved a new and much larger genome. It could contain and use orders of magnitude more information than could the prokaryote genome. This was a necessary pre-condition, because multi-cellular life needs to store much more information in its genome than does single-celled life.

Multi-cellular organisms are more complex than single cells, and they need a correspondingly bigger library in which to store their descriptions. The Neoproterozoic revolution was therefore based on an information revolution.

The difference between the eukaryote and prokaryote genomes is something like the difference between a computer in the 2000s and one in the early 1980s. The basic code in which information is stored remained the same in prokaryotes and eukaryotes, and many of the individual genes—fragments of program—were also conserved, but the organization of the chromosome, the operating system, so to speak, was radically upgraded. The upgrade allowed orders of magnitude more storage and faster copying.

Their ability to ingest other cells enabled them to build a much more complex cell, including vital components that were once freeliving prokaryotes.

We know eukaryotes only evolved once, and from this and the complexities of their origin, we can be fairly sure it was a difficult evolutionary transition.

We hypothesize that photosynthesizing life colonized the late Proterozoic land surface, liberating phosphorus from rocks, and driving up atmospheric oxygen, but what could have been the responsible organisms? Plants do not appear on the land surface until much later, around 470 million years ago. The first organisms to get a foothold are …  lichens. Lichens are remarkable not only for their ability to colonize bare rock but because they are a partnership between quite different types of life.

The Swiss botanist Simon Schwendener was the first to realize that lichens are not single organisms, but a symbiosis between fungi and algae, across two of the fundamental divisions of life.

The fundamental reason that fungal-photosynthesizer associations are so successful… is that the needs of the two partners are so well matched. Photosynthesizers can fix carbon and derive energy out of water and carbon dioxide, providing they also have access to nutrients. Fungi can access nutrients out of rock, but the trick of photosynthesis has eluded them. Together they formed (and still form) a partnership which could prosper in environments where neither by itself could survive.

In the modern world then, colonization follows a sequence starting with lichens, after which bryophytes [mosses and liverworts] appear, and only after that do larger, more recently evolved plants begin to establish themselves. This is suggestive that a similar sequence of events occurred during the first colonization of the land surface.

Let us assume then that land colonization by eukaryotes began at some point around 800 million years ago. We can’t as yet call the organisms responsible ‘plants’, but we assume they were associations of fungi with algae and/or cyanobacteria—proto-lichens. They began to accelerate the chemical weathering of continental rocks, and this would have had two fundamental effects on the atmosphere: increasing oxygen, and reducing carbon dioxide concentrations.

Animals:

There is something special about animals. They are not the only multi-cellular life forms on Earth—plants, fungi, and various marine algae have independently evolved the multi-cellular habit.

However, animals are committed to multi-cellularity in a way that that these other eukaryotes are not. Animal cells usually become terminally differentiated. Most animal cells are mortal, and doomed eventually to come to the end of a cell line, when the genes in them cannot be passed on. To survive, the genes rely on the fact that they are identical to those in the reproductive cells of the animal. This isn’t in general true of plants, fungi, or multi-cellular algae, in which cells remain ‘totipotent’, meaning that each cell can potentially give rise to the whole organism, and may do so if it is separated.

The individual cells of multi-cellular organisms display a form of altruism. They do not reproduce indefinitely without regard to their neighbour cells, consuming as much resource as they can, but are constrained by their shared genes to cooperate.

The evolution of animals with the capacity (i.e. a mouth, gut and anus) to eat other creatures has been described as the defining moment for the ecology of the planet. Beforehand most evolution proceeded at a sedate pace steered by the environment, and ecosystems lacked complex webs of feeding relationships. Afterwards there were co-evolutionary ‘arms races’ between the eater and the eaten: Prey evolved new ways to avoid predators and predators evolved new ways to eat prey. The result was a much more rapid turnover of types of life in the fossil record. Traditionally this transition was viewed as happening in the Cambrian, but recent work now places the major increase in the rate of turnover of fossil lineages during the Ediacaran interval, consistent with the appearance of bilaterians.

The major new arrivals of the first part of the Phanerozoic, called the Paleozoic, were the land plants. In greening the land surface they doubled the amount of free energy captured by the biosphere, and transformed the face of the Earth. The productivity per unit area on land today is, on average, more than double what it is in the ocean. To achieve this required the emergence of the most remarkable systems for acquiring and recycling water and nutrients to have graced the planet thus far. Probably the best example is the nutrient phosphorus. Plants and fungi selectively extract phosphorus from rocks and it is then recycled around fifty times in modern ecosystems. The supply of other elements such as potassium, iron, calcium and magnesium is also enhanced by biological rock weathering and recycling within ecosystems. Nitrogen and carbon are fixed from the atmosphere, and they too are internally recycled. Recycling is essential to maintain the high productivity that has enabled the land to support the greatest flowering of biological diversity the Earth has yet seen.

‘megaphylls’.

As plants evolved and more complex ecosystems developed, they made the land surface a much damper place. They did this by creating organic rich soils that hold more water, and by tapping into progressively deeper water reserves in the ground. By actively pumping up water and returning it to the atmosphere, and by reducing runoff from the land surface, plants increased the recycling of water from the land to the atmosphere. This has increased precipitation over land, which in turn has increased the amount of vegetation biomass the Earth can support today by an estimated factor of three.

As well as a shortage of water, the first plants were faced with a profound shortage of nutrients, because many essential elements were locked up in rocks.

Today therefore, a mature land ecosystem is a grand coalition that involves bacteria, archaea, plants, animals and fungi all working in concert to find, fix and recycle nutrients and carbon. The Rhynie Chert shows that the basics of this system were in place 400 million years ago.

Mammals:

[55 million years ago], the three key orders that emerged then were the Primates (to which we belong), the even-toed ungulates (Artiodactyla), and the odd-toed ungulates (Perissodactyla), which taxonomists refer to as the ‘APP’ orders.

But there are intriguing signs that all three modern mammal orders appeared in a remarkable time of climatic upheaval called the Paleocene- Eocene Thermal Maximum (or ‘PETM’ for short).

During the PETM, the modern (APP) orders of mammals appeared within 10 000 years or so of each other in North America, Europe and Asia. Primates were the last of the three orders to arrive roughly 10 000 years into the event. The earliest APP species were significantly smaller than their immediate descendants… The rapidity of appearance of major orders of mammals during the PETM is truly remarkable, and if borne out by the ongoing accumulation of evidence, indicates that evolution can operate surprisingly rapidly.

The subsequent evolution of modern mammal orders, including our own, is linked with a profound shift in terrestrial ecosystems; the rise of grasslands. Grasslands provided a habitat in which our ancestors ultimately evolved, as did many of the animals they domesticated, whilst grasses provide our major food crops.

An overall driver for the spread of grasslands was a cooling and drying of the climate over the last 50 million years.

Humans

Perhaps the best candidate for an evolutionary innovation behind all of these is the development

of ‘natural’ language with a universal grammar (common word order) and syntax. The evolution of natural language represented a revolution in the transmission of information, which decoupled human social evolution from gene-based evolution giving it a ‘genetic material’ analogous in many ways to DNA, that could be transmitted both to contemporaries and future generations. This allowed cultural evolution to accelerate, and new levels of social organization to emerge. The result was the rapid development of human imagination and culture.

The key thing about sharing a universal grammar is that it vastly increases the capacity of those sharing it to exchange complex information. With the emergence of natural language, the accumulation and spread of information was no longer tied to biological reproduction ( 9 ). Thus, the first group of humans to acquire universal grammar would have had a formidable advantage over other groups, especially in a changing environment, because they could adapt much faster.

Conclusion

There are several common features to the great revolutions that have made the present Earth. These can be broadly categorized as the main characteristics of change, and the features that must emerge for it to be ‘successful’—in the sense that permanent change persists and includes a thriving biosphere.

Two out of the three past revolutions have been underlain by a step change in information transmission between living organisms… The original example was the origin of the genetic codesupporting prokaryote cells—and of cells supporting the mechanics of genes. The evolution of the eukaryote genome, simultaneously with the new structure of the eukaryote cell, was another. These revolutions in information and organization do not of themselves lead to changes in their planetary environment. Rather, they light a fuse that can be very slow burning.

Changes of the planetary environment also require revolutions in energy and matter flow through the biosphere. Virtually all the important metabolisms of life evolved among prokaryotes, on the early Earth. The origin of DNA and prokaryote cells was the fuse for them. The most important and difficult-to-evolve example was the origin of oxygenic photosynthesis, an essential precondition for the Great Oxidation. The subsequent revolution eventually involved eukaryotes taking this already existing metabolism onto land and evolving ways to mine rocks for nutrients, with the consequence of boosting productivity both on land and in the ocean. Still these innovations in metabolism did not necessarily nor immediately lead to planetary scale disruption.

The revolutions did however cause such disruption at the global scale once they began to interact with fundamental feedbacks in the Earth system.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s