In closing his 1859 abstract, On the Origin of Species, Charles Darwin wrote one of the most poetic passages in the history of science. “There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one;” Darwin wrote, “and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” Beautiful. And in this cherished passage, there’s one phrase that always jumps out at me.
What did Darwin mean when he mentioned “endless forms”? Sometimes I wonder if Darwin meant to underscore that evolution was an essential part of life. As long as there is life, that life will be molded into unpredictable species and varieties. Then again, Darwin may have been talking about the profusion of forms themselves. One of Darwin’s critical contributions to evolutionary thought was that evolution does not proceed along a golden road of increasing perfection, and no one knows the shape of future life. Who could outline the shape of life in millennia to come? The possibilities are endless.
But we know that not everything is possible in evolution. The future shape of life is constrained by what exists today, and quirks of history tucked deep into the past. These factors can be as simple as the limitations of gravity or as far-sweeping as a mass extinction that swiftly changed the prehistoric landscape. Evolution constantly weaves through contingency and constraint, and, sometimes, this produces convergence.
Consider the beak of the pelican and the maw of the baleen whale. These animals are only distantly related – they last shared a common ancestor over 305 million years ago. Yet their bodies are still organized in the classic vertebrate body plain (head, neck, body, tail) and they feed in the same way. Due to the physics of feeding in water, and the biological properties of skin and bone, there are only so many ways to scoop mouthfuls of fish from the sea. No surprise, then, that the mouths of these distantly-related creatures have been adapted into analogous forms. From roughly similar anatomical starting points, both bird and aquatic mammal became pouch-mouthed bulk feeders.
Whales are also fish copycats. Earlier this year, University of Oxford paleobiologist Matt Friedman tracked the evolutionary similarities between baleen whales and enormous filter-feeding fish. (In fact, the biggest fish in the sea today is the filter-feeding whale shark Rhincodon typus.) Not only did whales and fish evolve to take up the similar filter-feeding niches in the seas, Friedman found, but both fish and mammal became adapted to living large along the same evolutionary pathway.
The key to Friedman’s study was a roughly 180 million year old fish called Ohmdenia. When he reexamined the puzzling fish, Friedman found that the relatively small, toothy fish was the closest cousin to a major radiation of huge filter-feeders called the pachycormiforms. This group included the gargantuan Leedsichthys – a 30-foot Jurassic fish that sifted microscopic feasts from the sea.
Ohmdenia was not ancestral to such giants, but rather indicated what such as-yet-undiscovered progenitors might have been like. There’s a considerable morphological difference between the six-foot, blunt-toothed Ohmdenia and the toothless Leedsichthys. In Friedman’s reconstruction of the transition between these forms, though, the fish evolved in a similar way to early baleen whales.
Contrary to their name, the earliest recognizable members of the baleen whale lineage had mouths full of fearsome teeth. Eventually, however, forms such as Aetiocetus combined teeth and baleen, which was the start of a switch in diet. The whales lost their teeth, developed greater swaths of baleen in their mouths, and became larger. While the method of feeding in the fish was different – relying on rakers set along the gills rather than baleen – it appears that the forerunners of filter-feeding fish underwent similar changes in jaw shape, tooth loss, and body size. As Friedman illustrates, understanding convergent evolution isn’t only a matter of comparing similar forms. We must also understand the patterns that produced these corresponding creatures.
Friedman, M. (2012). Parallel evolutionary trajectories underlie the origin of giant suspension-feeding whales and bony fishes Proceedings of the Royal Society B DOI: 10.1098/rspb.2011.1381