I’m in the middle of reading Fires of Life: Endothermy in Birds and Mammals by Barry Gordon Lovegrove. It’s a detailed look at how these two classes may have become warm-blooded (endothermic) while other living beings have largely remained cold-blooded (ectothermic). Dr. Lovegrove, having done his work in the Southern Hemisphere, strove to highlight research often overlooked here on the other half of the planet, and he conveyed this in an informative, accessible writing style. Sadly he passed away in 2022 so I am unable to write to him about how much I have been enjoying this book, or how more than occasionally he mentioned topics that had me interrupting my book reading to dive down a Wikipedia rabbit hole. One of these concepts is exaptation.
I’ve been aware of the fact that a trait evolved in a particular species can sometimes be repurposed further down the line in its descendants. For example, bird feathers probably evolved initially for insulation, communication, or to shed water, but later were given the new task of aiding in flight. This is one of the best known cases of exaptation, but we see it again and again with both physical and behavioral traits. A dog licking its owner’s face in greeting is an exaptation of the submissive behavior of a wolf toward another in its pack, but toward a different species. The ovipositors of some lineages of wasp were modified over time into venomous stingers, and leaves became cactus needles. A fish’s swim bladder originated as a primitive lung, which itself was derived from part of the piscine digestive system. And so on.
But my favorite exaptation that Lovegrove brought up is how the primitive little cluster of pectoral bones that gave support to an ancient fish’s pectoral fins not only got repurposed into the limbs of the first tetrapods, but ultimately became the same bones in the forelimbs of birds and mammals–everything from penguins to bats to humans. In our most ancient fish ancestors, the bones weren’t even attached to the rest of the skeleton; it took millions of years of evolution for them to expand and ultimately connect with the structure of the spine and ribcage.
There’s just something compelling about closing my eyes and imagining all these bones in my forelimbs shrinking back into a body covered in scales, nestled behind blood-filled gills, and giving strength to the fins that helped countless fish steer and power their way through the water–and eventually onto land. I marvel at how these clusters of tiny little calcium nubs, barely distinguishable from pebbles, refined over the generations into the long, lean limbs of antelope or the framework for a hummingbird’s frantic flight–or disappeared entirely in a menagerie of snakes (and a few lizards.) The hands that type this article are infinitely more complex than their roots, and yet the same basic pieces are all there, stretched and reworked to dizzying new purposes.
It’s not just those bones that have come a long way from our fish ancestors, either. The gill arches of our fish ancestors–one of which gave rise to the basis of the pectoral bones described above–also became the tetrapod’s embryonic pharyngeal arches developing into bones, muscles, arteries, and nerves that make up parts of our face, ears, and neck. The fifth pharyngeal arch is a ghost which surfaces with the rest around the fourth week of the human embryo’s development, but disappears back into the cellular muck within days, leaving no sign of its existence except, perhaps, a controversial artery. Yet it, too, is the descendant of the curved, mobile scaffolding of muscles, vessels and bones that held ancient gills open for life-giving oxygen in the water.
Exaptation–such an unremarkable word for such a profound phenomenon. Evolution is less about the spontaneous appearance of entirely new traits, and more about refining and repurposing existing ones. It’s not that we never get anything startling and unexpected any more, but life has had enough time to innovate that there’s a greater toolkit to play with than there was, say, at a point over two billion years ago, sandwiched between the buildup of oxygen in the atmosphere, and the dawning of the eukaryotes. And even these momentous events utilized prior materials, just as life coalesced from increasingly complex combinations of molecules that had been around since the earliest years of Earth’s existence.
In the end, exaptation is just another reminder that nature doesn’t waste; it only reuses, again, and again, and again. Carl Sagan famously said that “we are made of star-stuff”, and he was right. The stars are alive, and many of them are wandering around on nubs of bone stretched into fantastic forms.
Very interesting and amazing. Thanks for writing it up and sharing it.
Thank you; glad you enjoyed it!