The genetic basis behind one of the best-documented examples of evolutionary change in the fossil record has been revealed: how whales lost their hind limbs.
Ancient whales – four-footed land animals not unlike large modern dogs – evolved into graceful, streamlined swimmers through a series of small genetic changes during the whales’ embryonic development, report Hans Thewissen and his colleagues from the Northeastern Ohio Universities College of Medicine.
The research party began by exploring the embryonic development of whales’ cousins, the dolphins. These creatures are intriguing because, for a brief time during development, they sprout hind limbs. The limbs soon vanish, though, as the embryos reach the second month in a gestation period that lasts about a year.
“In most mammals a series of genes is at work at different times, delicately interacting to form a limb with muscles, bones, and skin. The genes are similar to the runners in a complex relay race, where a new runner cannot start without receiving a sign from a previous runner,” explains Thewissen.
In dolphins, however, at least one of the genes drops out early in the race, disrupting the genes that were about to follow it. That causes the entire “relay” to collapse, ultimately leading to the regression of the animals’ hind limbs. By analysing dolphin embryos, Thewissen has shown that the dropout is the SHH gene (dubbed “Sonic Hedgehog” by scientists), which is important at several stages of limb formation. “That’s why dolphins lose their ‘legs’.”
In whales, however, the story is more complex. Between 41 million and 50 million years ago, whales’ hind limbs did shrink greatly as these former land animals began a return to the sea. But their legs showed no change in the basic arrangement and number of bones, which proved that Sonic Hedgehog was still functioning. Its loss must have come later.
The dramatic loss of Sonic Hedgehog expression was not the genetic change that drove hind limb loss in whales. Instead, Thewissen and his colleagues conclude, whales’ hind limbs regressed over millions of years via “Darwinian microevolution”: a step-by-step process occurring through small changes in a number of genes relatively late in development.
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