If evolution's tape were rewound, would it reinvent eyes, wings and intelligence?
Stephen Jay Gould argued in 1989 that replaying the history of life would produce a very different result. Simon Conway Morris disagrees, pointing to how often unrelated creatures arrive at the same designs. Octopus eyes, bat wings and dolphin sonar suggest that nature keeps rediscovering good solutions, sometimes down to identical molecules.
Convergent evolution is the independent appearance of similar traits in lineages whose common ancestor lacked them. Flight is the textbook case: insects, pterosaurs, birds and bats each found their own way into the air. Bird and bat forelimbs are homologous, inherited from a shared four-legged ancestor, but their wings are only analogous. A bat flies on skin stretched across four extremely long fingers and its legs, while a bird's airfoil is built of feathers anchored to fused wrist and hand bones. Oddly, both carry an unusually high concentration of cerebrosides in their wing skin, which keeps it supple.
The camera eye arose separately in vertebrates, cephalopods such as squid and octopus, and even some jellyfish, from an ancestor with at most a light-sensitive spot. The cephalopod version is wired the opposite way, with nerves and blood vessels entering behind the retina, so unlike vertebrates it has no blind spot. Echolocation evolved separately in bats and whales, yet through the same genetic mutations. Herring, dolphins and extinct ichthyosaurs all converged on a torpedo shape for moving fast through dense water, and Australia's thylacine grew a skull strikingly like a fox's.
Convergence reaches into chemistry. The catalytic triad, an arrangement of three amino acids that powers certain protein-cutting enzymes, has evolved identically more than 20 times in unrelated enzyme families, because the physics of catalysis leaves few good options. Insects that tolerate toxic cardiotonic steroids across six orders repeatedly swapped the same amino acids in one ion pump; 40 percent of the relevant substitutions sit at just two positions.
Such repetition is what Conway Morris leans on. If constraints keep steering life toward similar body plans, he suggests, intelligence, already present in primates, corvids and cetaceans, is likely to emerge again. Gould's side would reply that the details of history still matter enormously.
Source: Convergent evolution
