PHOTOS: 7 Major "Missing Links" Since Darwin

PHOTOS: 7 Major
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Ambulocetus: The Walking Whale

Discovered in Pakistan in 1992, the fossil skeleton of 50-million-year-old Ambulocetus ("walking whale") suggests it was able to walk on four legs—on land and in the water.

Why it matters: Explaining the leap from land mammals to whales was another evolutionary headache for Darwin, who proposed bears as possible whale ancestors. Recently unearthed fossils trace whales to a doglike predecessor of hoofed plant-eaters, and genetic analysis has identified hippos as whales' closest living relatives.

Fossil expert Donald Prothero of Occidental College says Ambulocetus is the "most complete, best studied, and clearest case of something with a whale's head, the beginnings of an aquatic lifestyle with webbed hands and feet, but still fully quadrupedal."

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