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Evolution: The never-ending story
POSTED 5 FEBRUARY 2009
Bone fragments are laid out flat in a humanoid form, with large sections of the skull missing

Whale reveals "missing link"

Darwin, plagued by a shortage of fossils, yearned to see the "missing links" that, he knew, would show the transitional forms linking different species. Ever since, paleontologists have repeatedly been excited at the discovery of intermediate forms. One of the latest is a fossil from a prototype whale that demonstrates how this mammal moved from land to the water.

The whale in question, Georgiacetus vogtlensis, lived about 40 million years ago, "right at the time of transition," says Mark D. Uhen, curator of paleontology at the Alabama Museum of Natural History. "Georgiacetus was probably one of last ones with big hind legs. Right after that they became fully aquatic," he says, a process that took 10 to 12 million years.

"Lucy," an Australopithecus afarensis skeleton found in Ethiopia in 1974, is an extinct hominid that lived between 3.9 and 2.9 million years ago. Lucy is one of the "missing links" that illustrate the descent of modern humans.
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The earliest whales presumably swam with their rear legs, which disappeared once whales began swimming with a giant tail fluke.

Georgiacetus had a large pelvis, meaning it probably had rear legs, although none were found. Because the pelvis was not securely attached to the spinal column, Georgiacetus could not have walked on its rear legs, so it had to be a swimmer. But it lacked the flattened vertebrae found at the end of the spine in modern whales, so it probably had no fluke.

So how did Georgiacetus propel itself through the water?

By paddling with its rear legs, Uhen concludes. "It had to swim, but it did not have flukes, so I suggest it stuck its feet out behind it, and moved its back up-down, like today's whales, but instead of flukes, the surface of the feet moved the water, and that may have been how whales changed from terrestrial to aquatic."

Georgiacetus has four legs and duck-like webbed feet. Dorudon looks like a dolphin with shark teeth
Illustration by Mary Parrish, Smithsonian Institution.
According to paleontologist Mark D. Uhen, the tail-powered swimming of modern baleen and toothed whales evolved from the undulating hips of the primitive whale Georgiacetus. Notice that dorudon, which lived from 41 million to 33 million years ago, retained a stump of a rear leg; this has disappeared in modern whales.

Chatting with Charles: What would you say to Darwin?

Mark D. Uhen: "Darwin commented on whale evolution, and suggested that whales are pretty far out there for mammals, but even in this extreme case, he felt his theory could explain how it happened. The first thing they had to do in the water was to feed, and all the changes they underwent, at first, were related to becoming more effective at feeding. He was on the right track, but he had no fossils. I'd say, 'Thank you for getting us going on this path, we've taken your ideas and found overwhelming evidence for what you said.' 'The Origin' is just as relevant today as the day he wrote it."

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