Does evolution repeat itself. Does it rhyme? Chance vs Necessity.

The ancient covenant is in pieces; man knows at last that he is alone in the universe’s unfeeling immensity, out of which he emerged only by chance. His destiny is nowhere spelled out, nor is his duty. The kingdom above or the darkness below: it is for him to choose.”― Jacques Monod, Chance and Necessity: An Essay on the Natural Philosophy of Modern Biology

50 years ago, Jacks Monod argued in “Chance and Necessity” that the origin of life on earth was a freak accident and that we are alone in the universe (via Nick Lane, “The Vital Question”). In his 1989 book “Wonderful Life”, Stephen Jay Gould argued that historical contingency is central to evolution and illustrated his view with the now-famous thought experiment of replaying life’s tape. Gould’s conclusion was:

Replay the tape a million times… and I doubt that anything like Homo sapiens would ever evolve again.”

This is also the idea that Nicholas R. Longrich exposed a few months ago in article for The Conversation:

Our evolution wasn’t like winning the lottery. It was like winning the lottery again, and again, and again.”

The Conversation

However, it is not a universally held view. The power of natural selection to find the limited set of high-fitness solutions to the challenges imposed by environments could, in principle, make those outcomes deterministic. One school of thought, called convergent evolution, says that random effects eventually average out so that evolution converges, tending to produce similar organisms in any given environment. And striking examples of convergent evolution do exist:

  • Australia’s entire evolutionary history, with mammals diversifying after the dinosaur extinction, parallels other continents.
  • Dolphins and extinct ichthyosaurs evolved similar shapes to glide through the water,
  • Insects, birds, bats and pterosaurs, convergently evolved flight.
  • Eyes evolved not just in vertebrates, but in arthropods, octopi, worms and jellyfish.
  • Vertebrates, arthropods, octopi and worms independently invented jaws.
  • Legs evolved convergently in the arthropods, octopi and four kinds of fish (tetrapods, frogfish, skates, mudskippers).

The palaeontologist and astrobiologist Simon Conway Morris thinks that convergence is one of the best arguments for Darwinian adaptation:

One can say with reasonable confidence that the likelihood of something analogous to a human evolving is really pretty high. And given the number of potential planets that we now have good reason to think exist, even if the dice only come up the right way every 1 in 100 throws, that still leads to a very large number of intelligences scattered around, that are likely to be similar to us.

BBC Science Focus

Arik Kershenbaum, a zoologist at the rarified British institution, wrote a whole book about the concept of alien evolution, where he argues that because some evolutionary challenges are truly universal, life throughout the cosmos may share certain features.

Here’s the catch, according to Longrich.

All this convergence happened within one lineage, the Eumetazoa. Eumetazoans are complex animals with symmetry, mouths, guts, muscles, a nervous system. Different eumetazoans evolved similar solutions to similar problems, but the complex body plan that made it all possible is unique. Complex animals evolved once in life’s history, suggesting they’re improbable.

Many critical events in our evolutionary history are unique and, probably, improbable.

  • The complex, eukaryotic cells that all animals and plants are built from, containing nuclei and mitochondria, evolved only once.
  • Sex evolved just once.
  • Photosynthesis, which increased the energy available to life and produced oxygen, is a one-off.
  • So is human-level intelligence.

Multicellularity with all it evolutionary benefits has traditionally been viewed as a major transition with large genetic hurdles to it. But new research suggests that even this barrier may not have been so difficult after all.

For all the powerful the idea of natural evolution is, our possibilities to actually explore through experimentation with it are limited. Our lifespans are ridiculously short in comparison with evolutionary processes timespans.

  • Photosynthesis evolved 1.5 billion years after the Earth’s formation,
  • Complex cells after 2.7 billion years,
  • Complex animals after 4 billion years,
  • And human intelligence, 4.5 billion years after the Earth formed.

Our planet is but one experiment and Gould thought that the the bad news is that we can’t possibly repeat the experiment. Yet, an article published in Science in 2018 argues that Gould’s gedankenexperiment has been transformed into a real experimental program, one in which increasingly sophisticated and audacious studies are exploring the roles of contingency and determinism at ever deeper levels.

Where to now? Clearly, evolution can be both contingent and deterministic, and often in complicated and fascinating ways. Recognizing this mixed nature will allow future research to investigate how contingency and determinism interact.

Blount, Zachary D., Richard E. Lenski, and Jonathan B. Losos. ‘Contingency and Determinism in Evolution: Replaying Life’s Tape’. Science, 9 November 2018.

The idea of natural evolution and Darwin’s theory in particular is now more than one century and a half old. However, I would bet that we ain’t seen nothing yet.

We are barely awakening to the complexity that a single but incredible powerful idea, an algorithm, can create. And if I had to / could bet, I would bet that we are not alone in the universe. But no, we are neither going to have a conversation any time soon with another convergent organism in/from another planet.


Featured Image: Donald Iain Smith, Which one’s the alien and which one’s the human? (via cnet)

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