By Corey Binns

updated 4:44 p.m. ET Aug. 14, 2007

Evolution is a tale of gradual change, but some animal alterations appear to have advanced by leaps and bounds.

Ancient four-limbed fish crawled out of the sea. Dinosaurs, insects and mammals took to the air. Our closest relatives straightened their backs and began walking upright on two legs.

But what made them do it? Charles Darwin taught us that evolution has no direction. Instead, living creatures exploit resources already available to them. So the answer eludes us there.

"For Darwin, the evolution of transitional forms was one of the most vexing questions with regard to his theory," said West Chester University biologist Frank Fish.

In "The Origin of Species," Darwin speculated on how natural selection could turn a land mammal into a whale. Since then, scientists have found clues to explain these animal transitions in the fossil and molecular records, as well as in energetic analyses, but the full story is yet to be discovered.

Into the deep
For instance, how terrestrial mammals moved back into the water and evolved into whales, seals and manatees continues to puzzle Fish and colleagues.

"Did the land-loving ancestors of these aquatic groups simply fall into the water, were they driven by hunger or did they seek safety while escaping from predators?" he asked. "The selection pressure must have been extreme as these new semi-aquatic mammals could not swim efficiently and there would have been additional large energy demands in thermoregulating in a highly thermally conductive medium."

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Studies conducted by Fish on the energy demands of swimming have shown that the locomotion transition from crawling land mammals to swimming whales, manatees and dolphins came from a sequence of morphological changes. This transition to living in the sea involved changes from paddling with limbs to undulating the body to oscillating a tail, such as the horizontal tail flukes in modern whale species.

When ancestors of whales, dolphins and manatees changed their swimming stroke from paddling with paws to graceful tail movements, Fish said, their swimming performance improved and used energy more efficiently.

With the new discovery of whale fossils, scientists have only recently been able to study the efficiency of limbless swimming. However, bones alone cannot tell us the whole story of how and why mammals lost their limbs in the water.

"Because only the bones have been preserved, we still do not know when these transitional forms started to insulate the body with blubber and how the fluke's design changed to generate large propulsive forces with high efficiency for high-speed swimming," Fish told LiveScience.

Early whales such as Ambulocetus most likely returned to the water, said American Museum of Natural History paleontologist Jack Conrad, because water free of big crocodilians at that time represented an unexploited resource.

"These early whales were basically playing the same game that crocodiles play: Wait for something to come get a drink and then pull it in the water for dinner," Conrad said. "This is also the same game that early land vertebrates, early amphibians and early relatives of crocs and dinosaurs were playing. These animals weren't necessarily 'on their way' to being anything; they were well suited to being exactly where they were."

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