The Tree of Life: Billions of extinct species and a single ancestral community

A single family for an unimaginable number of species

Current diversity of life

The Tree of Life is dizzying, time: 3.8 billion years is the engine that explains everything. The most cited study, published in PLoS Biology, suggests the figure of 8.7 million species present on Earth, of which 6.5 million terrestrial and 2.2 million aquatic.

The mechanism of speciation

Imagine a population splitting into two geographically isolated groups. After a million years, the accumulated mutations make them sufficiently different to no longer be able to reproduce together: two species are born where there was only one. Each of these two species can in turn split, and so on.

The dizzying theoretical calculation

By analyzing fossil records, researchers estimate that the average lifespan of an animal species is around 1 to 10 million years before it becomes extinct or transforms. For complex eukaryotes, the most often 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 that represents 1,900 successive divisions per lineage. In tree-like 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.

A world that never existed !

This unimaginable number is a theoretical bound in a world without extinction, without ecological saturation and where all lineages would branch regularly and simultaneously every 1 to 10 million years. In reality, massive constraints crush this theoretical potential of pure doubling before it can even be expressed.

• Extinction: five mass extinctions alone have swept away between 75 % and 96 % of species each time, brutally reducing the tree to a few surviving branches.
• Ecological saturation: each ecosystem has only a limited number of niches, i.e. combinations of conditions in which a species can survive and reproduce. As long as these niches are all occupied, an isolated population finds neither the resources nor the space needed to diverge long enough to become a full-fledged species.
• Insufficient geographic isolation: speciation requires that two populations remain separated long enough to accumulate irreversible differences. Yet geographic barriers, mountains, seas or deserts, are rare, unstable and temporary. Without this lasting isolation, populations mix again and divergence is canceled before it succeeds.
• Genetic constraints: the vast majority of mutations are neutral or lethal. Favorable mutations allowing true adaptive divergence are extremely rare, and genetic drift can eliminate by pure chance a mutation that is nevertheless advantageous before it spreads in the population.
• Interspecific competition: a nascent species must assert itself against already established and better adapted species, occupying similar niches. The failure rate of these new lineages is considerable before they even reach a population size sufficient to be viable in the long term.

From theory to reality: the extinction filter

The constraints actually form a cascade filter: a lineage must overcome them all simultaneously to result in a stable and durable species, which explains why the number of real species is very 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 concerns only organisms that have left traces in rocks, i.e. less than 1 % of real life. By correcting this bias, and including all life since microbial origins, the total number of species that have ever existed on Earth could be between \(10^{12}\) and \(10^{15}\), i.e. between one thousand billion and one million billion extinct species.

Current life: one billionth of life's history

The figures are unequivocal, 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 have disappeared. In the history of life, survival is the exception.

All living species today combined represent only one billionth, perhaps less, of the total biological diversity that Earth has produced since the origin of life. Natural selection is ruthless: it mercilessly eliminates maladapted lineages, relentlessly filtering variations of life, allowing only those that meet the immediate requirements of their environment to survive.

The three master branches of the tree of life

Thanks to the work of Carl Woese (1928-2012) and his collaborators in the late 1970s, we know that life is divided into three great domains. This revolutionary classification is based on the analysis of ribosomal RNA (16S ribosomal RNA molecule in prokaryotes and 18S in eukaryotes, used as a molecular clock), a molecule present in all living beings.
The three domains that form the main branches of the tree:

• Bacteria: Prokaryotic organisms (without nucleus), with extraordinary metabolic diversity. They colonize all environments, from hot springs to the human intestine.
• Archaea: Long confused with bacteria, they possess unique genetic and biochemical characteristics. Many are extremophiles, living in extreme environments (high temperatures, acidity, salinity).
• Eukaryotes: Organisms with complex cells containing a nucleus. This domain includes animals, plants, fungi and protists. It is the branch to which we belong, an infinitesimal part of the diversity of life.

N.B.:
Archaea are genetically closer to eukaryotes than to bacteria. Eukaryotes are actually a branch derived from the archaeal domain. Humans share a more recent common ancestor with a thermophilic archaeon from hot springs than with an intestinal bacterium.

Summary table of the main groups of the Tree of Life

N.B.:
The dates indicated are estimates based on molecular clocks and fossils. The first signs of life (prokaryotes) appear about 3.8 billion years ago. The age of the Earth is estimated at about 4.54 billion years.

What happened before LUCA ?

LUCA is not a single individual

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.

Popularization vs scientific reality

In popularization, LUCA is often presented as a single organism, a bit like the "Adam and Eve" of life. It is convenient for explanation, but inaccurate. LUCA probably represents several related organisms because a population allows more genetic diversity than a single individual.

The period of Darwinian prebiotic chemistry

But LUCA is itself the product of a very long selection. Before LUCA "won", there were very probably 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.

What to remember

The Tree of Life reveals to us a history both grandiose and fragile: that of a life that appeared billions of years ago, shaped by an infinite succession of random events, catastrophes, and selections.

Our existence is the fruit of such a singular contingency that it could not reproduce itself. It reminds us that life, as we know it, is precious, rare, and unique. Rather than desperately seeking twins of humanity in the stars, we should marvel at our own presence and ensure that this exception does not become, by our fault, a new extinction.