Convergent Evolution: chance, necessity… or mirage?

What is convergent evolution?

A simple definition

Convergent evolution is one of the most fascinating phenomena in evolution. It shows that species from very distant lineages, separated by hundreds of millions of years, can develop astonishingly similar traits. These resemblances, presented as the result of separate evolutions, raise the question of whether the mechanisms invoked are really sufficient to explain such close solutions.

Millions of years reinventing — or repeating — the same wonders

Swimming fast to escape a predator, perceiving light in the depths, conquering the air, withstanding temperature variations, optimising gas exchange in oxygen-poor environments, enduring extreme pressures, saving energy during shortages, detecting chemical signals, or camouflaging to escape predation: so many constraints imposed by the environment.

According to the theory, the evolutionary mechanism, devoid of plan, purpose or design, explores and selects, generation after generation, the most effective solutions. But when very distant lineages seem to arrive at the same answers, again and again, the question arises: does nature really reinvent, or does it follow more signposted paths than we think? Everything happens as if evolution were pushed, almost in spite of itself, to repeat some of its inventions.

An ineluctable tendency… or something that resembles one

This phenomenon seems to challenge the idea of an entirely contingent and haphazard evolution. It would no longer be merely a matter of chance, but of a form of recurrent tendency: natural selection, like an inflexible engineer, would push species towards a limited set of effective solutions. As if the laws of matter and energy imposed, in the background, certain possible forms of life. Wings for flying, eyes for seeing, venoms for neutralising prey… so many innovations that appeared in lineages with no direct link, to the point of suggesting that nature might be following an instruction manual we are only beginning to glimpse.

Stephen Jay Gould's question: chance or the inevitable?

The palaeontologist Stephen Jay Gould (1941–2002), who saw evolution as a largely unpredictable and contingent process, nevertheless recognised that convergence raises a troubling question: what if certain forms of life were not only the product of chance, but also the expression of deeper constraints and perhaps inevitable ones? In his essay Wonderful Life, he imagined "replaying the tape of life" and asked: if evolutionary history began again, would we see the same creatures reappear, or at least the same solutions to the challenges of existence? Convergence seems to suggest a repetition… but also leaves doubt about what, in evolution, really belongs to chance.

A truly predictable diversity?

This idea opens a vertiginous perspective: the diversity of life, despite its apparent profusion, might fit into a framework more predictable than we imagine. The constraints of the physical world would then act as a filter, allowing only certain innovations to emerge, those that, curiously, end up appearing in species with very distant origins. A lesson in humility, perhaps, but also an invitation to ask whether there might exist deeper rules, still poorly understood, that orient the inventiveness of nature.

Table of convergent evolution in the living world — or what resembles it

The table below lists several frequently cited cases of convergent evolution, specifying the groups concerned, their adaptive function, the estimated age of their divergence and a representative example.

The troubled history of vision

An unexpected common ancestor

Among all the examples put forward for convergent evolution, the eye is undoubtedly the most spectacular, and perhaps also the most disconcerting. For the more biologists explore its origins, the more the idea of a simple convergence and obvious seems to become complicated.

The common ancestor of octopuses and humans was a primitive bilateral worm, living about 550 to 600 million years ago (late Precambrian). This animal did not possess a structured eye with lens, retina and pupil, but it was not devoid of all visual equipment either: it already carried rudimentary photoreceptors, the Pax6 gene (master gene of eye development, conserved from the fly to the human via the octopus), a true conductor of ocular development, as well as an ancestral opsin (primitive photosensitive protein common to all animals, molecular precursor of all current visual pigments) shared by the entire animal lineage.

The role of Pax6

This is precisely where the notion of convergence becomes less obvious. When Walter Jakob Gehring (1939–2014) showed in 1983 that injecting the mouse Pax6 gene into a Drosophila could trigger the formation of an eye on an antenna, he revealed that the founding genetic programme of the eye is common to animals separated for hundreds of millions of years. The octopus and the human therefore did not perhaps "reinvent" the eye from nothing: they seem rather to have reactivated, each in their own way, the same ancestral molecular kit, leading to astonishingly similar architectures.

Atavism: a challenge for convergence

What is atavism?

Atavism refers to the reappearance, in a modern organism, of an ancient character that disappeared millions of years ago. This phenomenon shows that certain traits are not really "reinvented" by evolution: they can simply be reactivated from ancestral genetic programmes still present in the genome, but silent. Chickens that develop teeth, whales that reappear with legs, or snakes that manifest limb buds illustrate this capacity of life to reactivate ancient structures rather than recreate them.

Consequences for convergence

Atavism therefore complicates the interpretation of convergent evolution: what is sometimes presented as an independent invention might be only a reactivation of a pre-existing genetic module. In other words, certain "convergences" might not be new solutions imposed by environmental constraints, but possible returns to ancient states inscribed in the deep history of life.

The foundations laid by Darwin remain solid

The three Darwinian pillars

The framework proposed by Charles Darwin (1809–1882) in 1859 rests on three major principles: descent with modification from common ancestors, hereditary variation between individuals, and natural selection as a sorting mechanism. These three pillars are today confirmed by genetics, palaeontology, molecular phylogeny and direct observation. No serious member of the scientific community questions them.

A confirmed and enriched framework

Darwin had grasped the essentials, even if the mechanisms he proposed are no longer sufficient to explain everything. Biology in the 21st^e century does not contradict his work: it extends, completes and enriches it, sometimes beyond what we imagined. The contributions of modern genetics and recent palaeontological data confirm the power of the Darwinian framework while nuancing it.

What to remember: convergence vs contingency

A possible predictability

Convergent evolution would show to what extent the environment and physical laws can orient evolution towards solutions that resemble one another, without being entirely left to chance. It thus opens up the possibility — and not the certainty — that forms of extraterrestrial intelligence might emerge elsewhere, if similar constraints are exerted there. Eyes, wings or analogous social structures might then appear… or suggest that life sometimes follows more signposted paths than we imagined.

Irreducible contingency

But this theoretical possibility must not mask a deeper reality: evolutionary trajectories remain largely shaped by unique sequences, impossible to anticipate.

And yet, despite these troubling resemblances, terrestrial life remains the product of an extraordinarily contingent history (in evolution, contingency designates the unique and non-reproducible character of a sequence of events: a history that happens only once and cannot be replayed identically.), shaped by a succession of improbable and non-reproducible events. Nothing suggests that this journey, with its ruptures, accidents and detours, could be reproduced elsewhere; everything suggests on the contrary that it is specific to Earth and to its unique history.