3If you explore our genealogy back beyond about 370 million years ago, it gets fishy. Our ancestors back then were aquatic vertebrates that breathed through gills and swam with fins. Over the next twenty million years or so, our fishy ancestors were transformed into land-walking animals known as tetrapods (Latin for â€œfour feetâ€).
The hardest evidenceâ€“both literally and figurativelyâ€“that we have for this transition comes from the fossil record. Over the past century, paleontologists have slowly but steadily unearthed species belong to our lineage, splitting off early in the evolution of the tetrapod body. As a result, we can see the skeletons of fish with someâ€“but not allâ€“of the traits that let tetrapods move around on land.
You can get a feeling for how fish became tetrapods by looking at a fossil like Tiktaalik, shown here. These days, a lot of scientists are turning up clues about how a fish turned into this kind of creature, and how this kind of creature turned into creatures like us.
Along the way, a lot of genes changed. The genes in the egg of a fish encode the molecules that will produce the fins, gills, and all the rest of a fishâ€™s body. A different set of genes will produce a tetrapod. These days, scientists are finding some of the mutations that reprogrammed fins into feet.
But a new study in Nature puts a fascinating new wrinkle on our origins story. It suggests that our fishy ancestors already had the potential to develop the beginnings of a tetrapod body. They just needed some time on land to bring it out.
The authors of the new study, three scientists at McGill University in Montreal, studied fish called bichirs (Polypterus). Bichirs are the living remnants of a very old lineage of fishes, which split off from other fish lineages some 400 million years ago. While they mostly live in lakes and rivers, they will sometimes crawl across dry land with their fins. They can even sustain themselves on these journeys by breathing through primitive lungs. Hereâ€™s a video of how they walk.