The Tree of Life is dizzying, and time—3.8 billion years—is the engine that explains everything. The most cited study, published in PLoS Biology, estimates that there are 8.7 million species on Earth, including 6.5 million terrestrial and 2.2 million aquatic species.
Imagine a population splitting into two geographically isolated groups. After a million years, accumulated mutations make them different enough to no longer reproduce together: two species are born where there was only one. Each of these two species can, in turn, split, and so on.
By analyzing the fossil record, researchers estimate that the average lifespan of an animal species is on the order of 1 to 10 million years before it goes extinct or transforms. For complex eukaryotes, the most commonly cited figure is 2 million years for a lineage to split into two distinct species.
If a lineage splits on average once every 2 million years, over 3.8 billion years, this represents 1,900 successive divisions per lineage. In arborescent growth, this theoretically gives about \(10^{572}\), an unimaginable number that remains infinitely greater than the number of atoms in the observable universe (estimated at about \(10^{80}\)). If a lineage splits on average once every 10 million years, this gives about \(10^{114}\), a number that remains truly unimaginable.
This unimaginable number is a theoretical limit in a world without extinction, without ecological saturation, and where all lineages would bifurcate regularly and simultaneously every 1 to 10 million years. In reality, massive constraints crush this theoretical potential of pure doubling before it can even express itself.
Constraints actually form a cascade filter: a lineage must overcome all of them simultaneously to result in a stable and lasting species, which explains why the number of real species is far from the theoretical potential.
Paleontologists estimate that the total number of fossilizable species that have existed since the appearance of complex animals, about 540 million years ago (beginning of the Cambrian), is between 5 and 50 billion. But this figure only concerns organisms that have left traces in the rocks, less than 1% of actual life. By correcting this bias and including all life forms since microbial origins, the total number of species that have ever existed on Earth could be between \(10^{12}\) and \(10^{15}\), or between one trillion and one quadrillion extinct species.
The numbers speak for themselves: over 3.8 billion years of evolution, extinction is the rule, not the exception: more than 99.9% of all species that have ever existed are gone. In the history of life, survival is the exception.
All living species today combined represent only a billionth, perhaps less, of the total biological diversity that Earth has produced since the origin of life. Natural selection is relentless: it mercilessly eliminates unfit lineages, continuously filtering the variations of life, leaving only those that meet the immediate demands of their environment.
Thanks to the work of Carl Woese (1928-2012) and his collaborators in the late 1970s, we know that life is divided into three major domains. This revolutionary classification is based on the analysis of ribosomal RNA, a molecule present in all living beings.
The three domains that form the main branches of the tree:
N.B.: Archaea are genetically closer to eukaryotes than to bacteria. Eukaryotes are actually a branch derived from the domain of archaea. Humans share a more recent common ancestor with a thermophilic archaeon from hot springs than with a gut bacterium.
| Domain | Lineage | Example Organism | Estimated Appearance | Key Characteristic |
|---|---|---|---|---|
| Bacteria | Proteobacteria | Escherichia coli | ~ 3.5 billion years ago | Very diverse group, includes many pathogenic and symbiotic bacteria. |
| Bacteria | Cyanobacteria | Spirulina | ~ 2.4 billion years ago | Only bacteria capable of oxygenic photosynthesis (oxygen production). |
| Archaea | Euryarchaeota | Methanobrevibacter | ~ 3.8 billion years ago | Includes methanogens (producing methane) and extremophiles. |
| Archaea | Asgardarchaeota | Lokiarchaeum | ~ 2 billion years ago | Recently discovered archaea, genetically closest to Eukaryotes. |
| Eukarya | Animals (Metazoa) | Homo sapiens | ~ 800 million years ago | Multicellular heterotrophic organisms (feed on other beings). |
| Eukarya | Plants (Viridiplantae) | Arabidopsis thaliana | ~ 1.5 billion years ago | Photosynthetic organisms with cellulose cell walls. |
| Eukarya | Fungi | Saccharomyces cerevisiae | ~ 1 billion years ago | Osmotrophic organisms (absorption) with chitin cell walls, close to animals. |
| Eukarya | Protists | Amoeba proteus | ~ 1.8 billion years ago | Paraphyletic group (catch-all) including all non-animal, non-plant, and non-fungal eukaryotes. |
N.B.: The dates indicated are estimates based on molecular clocks and fossils. The first signs of life (prokaryotes) appeared about 3.8 billion years ago. The age of the Earth is estimated at about 4.54 billion years.
You, me, the cheetah, the button mushroom, the giant sequoia, and the bacterium share a common ancestor: we all descend from LUCA (Last Universal Common Ancestor), a unicellular organism that lived about 3.5 to 4 billion years ago. LUCA is not necessarily a single individual. LUCA rather corresponds to a population of primitive organisms probably living around hydrothermal vents and massively exchanging genes directly between individuals, without going through reproduction.
In popular science, LUCA is often presented as A single organism, a bit like the "Adam and Eve" of life. This is convenient for explanation, but inaccurate. LUCA probably represents several related organisms because a population allows for more genetic diversity than a single individual.
But LUCA itself is the product of a very long selection. Before LUCA "won," there were likely billions of billions of attempts at the emergence of life: proto-cells, self-replicating systems, primitive metabolisms that appeared and disappeared without leaving descendants. This is called the period of Darwinian prebiotic chemistry, which would have taken place in a window of about 100 to 400 million years, between the end of the Late Heavy Bombardment and the appearance of LUCA.
The Tree of Life reveals a story that is both grand and fragile: that of life that appeared billions of years ago, shaped by an infinite succession of random events, catastrophes, and selections.
Our existence is the fruit of a contingency so singular that it could not be reproduced. It reminds us that life, in the form we know it, is precious, rare, and unique. Rather than desperately searching for twins of humanity in the stars, we should marvel at our own presence and ensure that this exception does not become, through our fault, a new extinction.
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