Millions of years before humans invented sonar, bats and toothed whales had mastered the biological version of the same trick - echolocation. By timing the echoes of their calls, one group effortlessly flies through the darkest of skies and the other swims through the murkiest of waters. It's amazing enough that two such different groups of mammals should have evolved the same trick but that similarity isn't just skin deep.
The echolocation abilities of bats and whales, though different in their details, rely on the same changes to the same gene - Prestin. These changes have produced such similar proteins that if you drew a family tree based on their amino acid sequences, bats and toothed whales would end up in the same tight-knit group, to the exclusion of other bats and whales that don't use sonar.
This is one of the most dramatic examples yet of 'convergent evolution', where different groups of living things have independently evolved similar behaviours or body parts in response to similar evolutionary pressures.
It is one of a growing number of studies have shown that convergence on the surface - like having venom, being intelligent or lacking enamel - is borne of deeper genetic resemblance. But this discovery is special in a deliciously ironic way. It was made by two groups of scientists, who independently arrived at the same result. The first authors even have virtually identical names. These are people who take convergence seriously!
Yang Liu from the East China Normal University had previously shown that echolocating bats share very similar versions of Prestin, even species that were only distantly related. This time, he sequenced the gene in even more bats as well as a wide range of whales. These included toothed species (dolphins, porpoises, orcas and sperm whales) that use sonar, and baleen species that don't.
Based on the DNA sequences of these Prestin versions, Liu drew a mammal family tree (a 'phylogeny'). It looked much like what you would expect, with the whales and bats clustering in separate family groups. But convert the sequences into amino acids and the picture changes dramatically. Suddenly, the family tree becomes utterly misleading. The echolocating mammals, be they bats or whales, are united as close relatives, to the exclusion of their rightful evolutionary kin.
Ying Li (see what I mean?) from the University of Michigan found a similar result. She sequenced the Prestin gene in the bottlenosed dolphin and compared it to sequences from other mammals. Again, she found that Prestin sequences place the dolphin as a close cousin of echolocating bats rather than species that it's actually more closely related to, such as cows.
At first, it might seem strange to see such strong convergence at the genetic level. After all, bats and toothed whales echolocate very differently. Bats create their sonar pulses using their voicebox while whales pass air through their nasal bones. Bats send their calls through air and whales send their through water. A single gene can't have accounted for these differences in production.
Instead, Prestin's role is in detecting the rebounding echoes. It is activated in the "outer hair cells" of the ear, which allow mammals to hear high frequencies. In echolocating species, these cells are shorter and stiffer than normal, making them exquisitely sensitive to the ultrasonic frequencies used in echolocation. Li thinks that the Prestin changes might have helped to tune the outer hair cells of echolocators to high-pitched noises.
Liu used his sequences to reconstruct what Prestin would have looked like the ancestor of all bats and the ancestor of all whales. Compared to these original versions, echolocating species have accrued the same set of 14 amino acid changes, whether they have wings or flippers. It seems that there are only very few ways, if not only one, for mammals to hear the ultrasonic sounds needed for biological sonar.
Exactly what these amino acid changes did to the Prestin protein, and how they led to the evolution of echolocation, is a mystery for another time. It will also be interesting to see if these changes have started cropping up in the Prestins of other animals with cruder forms of sonar, like oilbirds, swiftlets, shrews and tenrecs.
References:
Liu et al. Convergent sequence evolution between echolocating bats and dolphins. Current Biology in press.
Li et al. The Hearing Gene Prestin Unites Echolocating Bats and Whales. Current Biology in press.
More on convergent evolution:
Three desert lizards evolve white skins through different mutations to the same gene
Elephants and humans evolved similar solutions to problems of gas-guzzling brains
Venomous shrews and lizards evolved toxic proteins in the same way
Decay of enamel-forming gene linked to evolutionary loss of enamel
