Would Life Evolve the Same Way Again?

Erin McKenney, Aram Mikaelyan, Autumn Sylvestri, Aurora Toennisson, Michael Welsh

This post emerged from a question raised by grad student Michael Welsh: If Earth's evolutionary history were restarted under identical abiotic conditions, would life unfold in the same way, or diverge entirely?

If earth’s evolutionary history were reset with identical abiotic conditions, would life unfold the same way? Or would new forms – perhaps completely novel ones – emerge? If they were novel, how novel might they be?

The question foregrounds a long-standing debate between evolutionary contingency and evolutionary determinism – is it fate or fluke that determines evolutionary trajectories? Contingency holds that life’s trajectory is historically unique, shaped by random mutations, extinctions, and environmental fluctuations. Under this view, the emergence of eukaryotes and multicellularity may have been improbable, with microbial life remaining the dominant form in other scenarios.

Determinism, on the other hand, suggests that certain forms and functions are expected to repeatedly emerge because they represent optimal solutions to common challenges. Structures like limbs, eyes, vascular systems may appear not by coincidence but through physical and ecological constraints that steer evolution toward predictable outcomes.

This raises the question of how we define "similarity." Morphological traits may diverge, yet organisms can still fulfill analogous ecological roles – herbivores, decomposers, predators. In this sense, functional convergence may occur even in the absence of structural resemblance. A repeat of evolution might not produce humans or birds, but it might yield creatures that fill similar niches.

Underlying this is the idea that natural selection filters random variation, but that the filter itself is shaped by physics. The Constructal Law, a principle from thermodynamics, proposes that all flow systems - whether rivers, lungs, or blood vessels - evolve to increase access to flow. If evolution operates within such constraints, then certain phenotypes may be not just likely, but inevitable under given conditions. Still, physical constraints do not “supersede” random mutation; rather, they influence which mutations persist. Selection does not design outcomes, but it favors traits that incidentally enhance survival and reproduction – especially those that improve flow, function, or energetic efficiency.

Symbiosis offers a case in point. Organelles such as mitochondria and chloroplasts originated from ancient symbiotic events, but why haven’t similar integrations occurred more frequently? Insects, for instance, host endosymbiotic bacteria that are membrane-bound, metabolically integrated, and vertically transmitted – yet still fall just short of full organelle status. Whether such symbionts cross the threshold into organelles depends not just on structure, but on how they are inherited, controlled, and encoded within the host.

The framing of evolutionary outcomes as either contingent or constrained misses an important middle: that constraints can emerge over time. Evolution is not only shaped by randomness or laws of physics – it also shapes its own rules through developmental and ecological feedbacks. Certain outcomes become more likely not because they are predestined, but because earlier steps set the stage for later canalization.

In the end, the question is not whether life would be the same, but which aspects of it are repeatable. Morphologies may differ. Ecologies may be filled by new players. But the functions – energy capture, nutrient cycling, reproduction – may recur in familiar forms. Perhaps evolution does not return to the same points, but converges on similar solutions to recurring problems.

Physics and chemistry constrain the starting conditions. 

Mutation and selection guide the emergent complexity. 

And contingency decides which possibilities become "real."

Note: This post was developed as a pedagogical exercise in collaborative synthesis and argument literacy, emerging from a discussion among three students and two faculty members in the M-lab Fun chat group, exploring how microbiomes challenge conventional views of inheritance and evolution.