A new and exhaustive genetic study just published in ‘Nature’ has just called into question some of the most entrenched dogmas of evolutionary biology: the first complex cells emerged a billion years earlier than previously thought. And they did, furthermore, … in a world where there was not yet enough oxygen.
Until recently, science was convinced that the emergence of complex organisms, which eventually gave rise to animals and, eventually, us, happened relatively recently, about 630 million years ago, in a sudden ‘burst’ of biological creativity. But the new study, led by the University of Bristol, forces us to reconsider a good part of that history.
And it seems that the path to complexity was not a kind of ‘final sprint’ of nature, but rather a slow, tortuous and, above all, extremely ancient marathon. The ‘machinery’ of complex life, in fact, began to develop almost a billion years earlier than we believed, and it also did so in conditions that until now we considered impossible: in a suffocating world lacking oxygen.
Prokaryotes and eukaryotes
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Broadly speaking, there are two different kinds of life on our planet. On the one hand, there are prokaryotes: simple single-celled organisms, such as bacteria and archaea, which are distinguished by a crucial characteristic: they do not have internal divisions. Its genetic material, in fact, is dispersed throughout its interior, without differentiated structures. It would be like a one-room studio: small, efficient, but very simple. Prokaryotes were the first to arrive, they have been here since the beginning, and it is estimated that they appeared more than 4 billion years ago.
And on the other hand there are, we are, the eukaryotes. This group, which appeared on Earth between 1,500 and 2,000 million years ago, includes algae, fungi, plants and, of course, all animals, including us. Eukaryotic cells are immensely more sophisticated. And if prokaryotes are a ‘study’, eukaryotic cells are mansions: they have specialized rooms (organelles), an armored command center where the DNA is stored (the nucleus) and their own power plants (the mitochondria). Without them, complex life (animals and plants) would never have been able to develop.
The path to complexity was not a kind of ‘final sprint’ of nature, but a slow, tortuous and, above all, extremely ancient marathon.
Now, how and when did a simple bacteria make the ‘leap’ to become a much more complicated eukaryotic cell? Classical theory told us that this occurred ‘a short’ time ago (in geological terms) and that it was previously necessary for the atmosphere to be filled with oxygen to supply the energy necessary for this change. But the Bristol researchers, along with collaborators from the University of Bath and the Okinawa Institute (OIST), have discovered that this idea was wrong.
To do this, they turned to a technique known as ‘molecular clocks’, analyzing hundreds of gene families to trace their history back in time, like someone following breadcrumbs in a forest to find the origin of the path. Thus, by combining this genetic data with the fossil record, the team created a tree of life with unprecedented temporal resolution.
The great ‘leap’ of life
The main conclusion of the study is a true revolution: the transition towards complex life began 2.9 billion years ago. That is, more than a billion years earlier than previously assumed. But the most surprising thing is not only that result, but the order of the factors. Because until now it was taken for granted that the primitive cell first acquired the mitochondria (the energy plant that uses oxygen) and that, thanks to that ‘shot’ of energy, it was later able to develop its complexity (nucleus, cellular skeleton, etc.). It is the so-called ‘mitochondria first’ hypothesis.
But the data from the Bristol team paint a totally new scenario that the researchers have named CALM (Complex Archaeon, Late Mitochondrion, or Complex Archaeon, Late Mitochondria). According to this model, our microscopic ancestors had already begun building complex structures, internal skeletons, and membrane transport systems long before mitochondria arrived. That is to say, life did not wait to have the ‘power plant’ installed to begin expanding the house. Structural complexity preceded energy complexity.
And all without oxygen
These findings have implications that shake the very foundations of geochemistry. Because if these first steps towards complexity occurred almost 3 billion years ago, it means that they took place in anoxic oceans, totally devoid of oxygen.
“The ancestor of eukaryotes – explains Philip Donoghue, paleobiologist at the University of Bristol and co-author of the study – began to develop complex characteristics approximately one billion years before oxygen was abundant.” In fact, mitochondria, which allow us to breathe oxygen today, arrived much later, and curiously coincided with the moment when the levels of this gas began to rise in the atmosphere.
Difference between eukaryotic and prokaryotic cell
Of course, this will also change the way we look for life on other planets. Because if it turns out that complexity can arise in worlds without oxygen, the range of places where we could find ‘advanced life’ expands considerably.
Until the publication of this study, the accepted chronology was quite conservative. It was assumed that bacteria dominated alone for 3 billion years and that only about 635 million years ago, after a global increase in oxygen, complex life took off for good.
Some previous clues
However, despite the novelty of the Nature study, we already had some prior clues that something did not add up. And, as ABC already published, there are intriguing antecedents, such as those discovered in July 2024 in the Franceville basin, in Gabon. There, a team led by Ernest Chi Fru, from Cardiff University, found fossils of supposedly complex organisms from 2.1 billion years ago.
That discovery, considered a ‘failed experiment’ of nature, suggested that life tried to make the ‘leap’ to complexity much earlier, taking advantage of a temporary spike in oxygen caused by underwater volcanoes and cyanobacteria, but that it ‘turned back’ and became extinct when conditions worsened.
The new genetic study does not necessarily contradict the existence of failed experiments like the one in Gabon, but rather gives them a much deeper theoretical framework. It tells us, in effect, that the genetic machinery for complexity did not emerge from nowhere, neither in Gabon 2.1 billion years ago nor 600 years ago, because the genetic ‘bricks’ to build complex cells were being slowly cooked long before, 2.9 billion years ago.
Therefore, what Chi Fru once called a ‘first attempt that failed’ could actually be one of the first visible physical manifestations of that long invisible genetic process that the University of Bristol has just revealed. Life was simply rehearsing.
Why are we taking so long?
But if machinery started working 2.9 billion years ago, why did it take us so long to see large animals and plants? Gergely Szöllősi, another of the study’s authors, summarizes it with the concept of ‘cumulative complexification’. In other words, you can’t build a skyscraper in one day.
The evolution of complex life, in fact, was not a one-time event, but an agonizingly slow process. First we had to ‘invent’ the internal tools (the nucleus, the cellular skeleton, etc.) in a world without air. Then, we had to wait for the fusion with a bacteria that would give rise to the mitochondria. And finally, we had to wait even longer for the planet itself to change, filling with oxygen, so that this machinery could function at full capacity and create the biodiversity that we enjoy today.
The old idea that Earth was a boring place, inhabited only by ‘dumb’ bacteria for most of its history, seems doomed. In the depths of those dark, oxygen-depleted oceans, almost 3 billion years ago, nature was already silently working on the design of the cell that, eons later, would give rise to the creatures trying to understand it.
