Four hundred million years ago, some fish hoisted themselves out of the water and started a long evolutionary trek to personhood. But how did they learn to walk? A study published in this week’s Nature uses modern-day walking fish to mimic that transition.
In the experiment, researchers raised bichirs, which are fish with functional lungs and strong fins. In a pinch, these qualities allow them to walk on land. But this is something new: For eight months, a group of the primitive fish were raised entirely on land so that researchers could compare their development to specimens that grew up in normal, mostly aquatic conditions.
"I was most surprised early in the study when the Polypros [bichir] actually survived in the terrestrial habitat. That was amazing," said Emily Standen, the lead author and an evolutionary biomechanist at the University of Ottawa. "Then when we tested their behavioral and anatomical differences, we were really excited."
Standen and her colleagues thought that the bichir would develop differently if it grew up on land, giving them hints as to how a fish could go from water to earth as it evolved. Sure enough, the fish raised on land walked with a more effective gait. They placed their fins closer to their bodies and raised their heads up higher, which made them slip less than the aquatic walkers. Their skeletons also developed differently, with the bones that support the fins changing shape to support them in higher gravity.
The researchers also saw the fish acquiring more head and neck mobility, which would be important in a transition to life on land. "Fish generally don't have necks, as they can approach their food from any angle in a 3-D environment," Standen said. "Once on a 2-D terrestrial plane, head mobility becomes essential for feeding and other sensory perceptions."
While the changes are subtle, Standen said, they mirror what scientists have seen in the fossil record of fish-to-land-dweller evolution. So it was probably a similar kind of developmental flexibility that allowed the first fish to emerge from the water.