The Invisible Groundwork

If I visited Earth three billion years ago, I’m pretty sure I would have been disappointed.

If I visited Earth three billion years ago, I’m pretty sure I would have been disappointed.

I would have found water in the form of a warm ocean under a hazy sky. The atmosphere would have been a mix of nitrogen, carbon dioxide and methane, with almost no free oxygen. I would have found, though I would have to look carefully, a thin layer of microbial life on surfaces in shallow water. Mats, or biofilms, of single-celled organisms, prokaryotes, that lacked a nucleus, some of whom were photosynthetic, happily doing their chemistry in the sun.

But that’s about it. I wouldn’t have found animals. Nor plants. Nor fungi. No eyes, no legs, no shells, no brains. No real movement, no trees swaying in the breeze and no herds of animals roaming plains. Just a sodden planet with some microbial life on it, doing more or less the same thing it had been doing for the previous five hundred million years, and would continue doing for the next two billion.

Life emerged on Earth somewhere around 3.5 to 4.5 billion years ago (probably closer to the latter). The details are widely debated for all sorts of reasons, with the earliest claimed microfossils being contested, and the chemical signatures attributed to that early period being ambiguous. But it is safe to say that by about 3.5 billion years ago, “something” was alive. There were single celled prokaryotes. Organisms without any of the internal architecture found in plant or animal cells. Small, simple, and very successful.

For the next two billion years, that was the state of play. Bacteria, archaea, their viruses, and nothing else. The fossil record for this period is almost monotonous. Microbial mats. Stromatolites, which are layered mounds of sediment trapped by photosynthetic microbes, accreting slowly in shallow seas. The planet was alive, and it was certainly busy, what with all that cell growth and metabolism, but it was all at a scale you can’t see. It was uneventful at the scale you can.

Then, roughly 540 million years ago, the Cambrian explosion happened. In about twenty million years, which is an eyeblink when your baseline is billions, nearly every major animal body plan appeared in the fossil record. Arthropods. Molluscs. Chordates. Brains, blood and barnacles, as I like to say. Eyes, guts, exoskeletons, segmented bodies, bilateral symmetry, predation, burrowing, swimming. Complex animal life, vaulted into existence after a very long pause.

The temptation for us biologists is to treat the preceding billions of years as a preamble, a holding pattern before the main event. But if we believed this, we wouldn’t be more wrong. To understand why, you need to know what that thin, glistening film of life was doing during those three billion years.

You tend to not hear about it as much, but the oxygenation of the atmosphere was the single most consequential event in the history of life on Earth. More consequential than the Cambrian explosion, more consequential than the colonisation of land, more consequential than the evolution of intelligence. And it was carried out by these microbes I have been talking about, through nothing more than photosynthesis.

Cyanobacteria, the group of bugs responsible for all photosynthesis at that time, evolved the ability to use sunlight to split water molecules and fix carbon, releasing oxygen as a byproduct. They didn’t do this in order to oxygenate the atmosphere – evolution doesn’t plan anything. They did it because it was a useful way to get energy. Oxygen was toxic waste.

This waste accumulated, albeit very slowly. For hundreds of millions of years, the oxygen produced by cyanobacteria was hoovered up by chemical reactions. The planet was a “highly reduced” place, so it easily absorbed whatever small amounts of oxygen the cyanobacteria produced. The oxygen oxidised dissolved iron in the oceans, or it reacted with volcanic gases, meaning that it was consumed before it could build up. The banded iron formations in ancient rocks (those red-and-grey striped sedimentary layers) are the record of this process. Oxygen being produced and immediately consumed. You can think about it as the planet rusting rather than breathing.

It took somewhere between several hundred million and over a billion years for oxygen production to outpace oxygen consumption. The Great Oxidation Event, around 2.4 billion years ago, marks the point at which free oxygen finally began to accumulate in the atmosphere. Even then, levels were nowhere like what they are today. It took another billion-plus years to approach modern levels, and that was mostly thanks to a second major rise around 800 to 540 million years ago.

So far, our analysis says that two billion years passed while cyanobacteria produced oxygen, with no discernible change in the planet for most of that time. If I could have observed the planet over those billions of years, I would have seen painfully little progress from one millennium to the next. Nonetheless, without the invisible changes, nothing that followed could have followed.

Aerobic respiration extracts energy from food using oxygen, and it yields roughly eighteen times more energy per glucose molecule than anaerobic alternatives like fermentation. An organism chugging away on fermentation doesn’t grow large, and won’t be able to move fast. Most importantly for my point here, it couldn’t sustain the energy-intensive processes that complex multicellular life requires. I’m talking about cell differentiation, manufacturing nervous systems, growing muscles, and deploying immune responses, to name a few. Large, active, complex organisms require atmospheric oxygen. The energy budget for building an animal on fermentation does not work.

This means that for the first two to three billion years of life on Earth, the change to complex multicellular life was being held back by atmospheric chemistry itself, not by a lack of evolutionary innovation, or evolvability.

There was another precondition being assembled during those billions of years, and that was the structure of life itself. Eukaryotic cells, those complex cells with nuclei and organelles that make up every animal, plant, and fungus, did not exist at all for the first two billion years of life. They arose somewhere between 1.5 and 2 billion years ago, and this necessitated a specific sequence of prior events. The leading theory, supported by strong evidence, is that eukaryotic cells arose through a merger of cells: specifically, an archaeal cell engulfed a bacterium, and instead of digesting it, the two organisms entered a partnership that became permanent. The engulfed bacterium became the mitochondrion, the organelle that performs aerobic respiration in every eukaryotic cell alive today.

So, what were the prior conditions that were needed? Bacteria had to be capable of aerobic respiration (which required oxygen, which required cyanobacteria to already have done the heavy lifting on that front), archaeal cells with the right membrane chemistry and metabolic profile, and an ecological context in which the partnership could stabilise. You could even say that the preconditions had preconditions.

Once eukaryotic cells existed, it took another billion years or so before complex multicellular organisms appeared. The period from roughly 1.8 to 0.8 billion years ago is sometimes called the “Boring Billion”, a stretch of Earth history in which, as far as the fossil record shows, almost nothing of macroscopic interest happened.

We now know that during that time the genetic toolkit for cell differentiation was being assembled, the regulatory architecture for multicellularity was developing, and the ecological conditions under which multicellular life would be advantageous were forming. All these developments were invisible to the naked eye and all of them were essential.

Then the Cambrian explosion produces complex animal life, and now we have the familiar story of life on earth: fish give rise to amphibians, then we get reptiles and then along comes the mammals, and rather recently, humans. But that familiar story, with its teeth and wings and predators, covers only the last fifteen percent of life’s history. The first eighty-five percent was groundwork. Without it, none of the visible drama could have occurred.

The same pattern appears at smaller scales. Before a developing embryo produces anything recognisable such as a limb, or an eye, it spends the vast majority of its early existence on processes that look like nothing is happening. It spends its time doing cell division, and establishing body axes and making the three germ layers from which all tissues will eventually derive. Externally, none of this looks like progress. But the visible, recognisable structures that appear later appear only because weeks of molecular groundwork made them possible.

This is where a certain kind of writer would tell you that your years of struggle are secretly the groundwork for your future triumph. I don’t think I want to do that.

The cyanobacteria were not working towards the Cambrian explosion. They were blind to the future, had no goal and no expectation of a payoff. They were simply metabolising, winning at life, and the atmospheric transformation was a side effect. For every metabolic innovation that turned out to be a precondition for later complexity, there were many others that lived out their existence for millions of years and then vanished, enabling nothing, leaving no descendants. We tend to say that the groundwork that led to the Cambrian explosion was essential because from our vantage point, we can see what happened subsequently. The microbes doing the work had no way of knowing (and they obviously didn’t care) which of their activities would lead to something new.

The honest version is that the preconditions for significant change take far longer to accumulate than the change itself takes to happen. During the accumulation phase, you usually cannot tell which activities are building preconditions and which are not. The accumulation phase looks like nothing is happening because, at the scale you can perceive, nothing is. The lineages that stopped during the groundwork phase are the ones that are not here.