The Blue Whale is the largest living creature on the planet. It is hard to believe it evolved from a land mammal roughly the size of a dog. While this is accepted as science fact, I have had many a discussion with those unwilling to believe this. But who cares~ no one believed in Max Planck in the first place, not even himself, and everyone was laughing at Charles Darwin when he suggested that we are closely related to macaques.
The cetaceans (whales, dolphins and porpoises) are marine mammal descendants of land mammals. Their terrestrial origins are indicated by:
- Their need to breathe air from the surface;
- The bones of their fins, which resemble the limbs of land mammals
- The vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish.
The question of how a group of land mammals became adapted to aquatic life was a mystery until discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.
This animal existed 50 million years ago by the rivers of southern Asia, walking on slender legs tipped with hooves, and takes to the water whenever it senses danger
It is called Indohyus (meaning India's pig)
and it is the earliest known relatives of today's whales and dolphins.
According to molecular evidence, the closest living relatives of whales are, quite surprisingly, the artiodactyls, a group of hoofed mammals that includes deer, cows, sheep, pigs, giraffes, camels and hippos, yet not a single one of them bears even a passing resemblance to whales and dolphins. Among the group, the hippos are evolutionarily closest and while they are at least at home in water, their family originated some 35 million years after the first whales and dolphins did.
Even though Indohyus had the elegant legs of a small deer and walked around on hooves, it also had features found only in modern and fossil whales.
It's jaws and teeth were similar to those of early whales, but the best evidence was the presence of a thickened knob of bone in its middle ear (involucrum). This structure helps modern whales to hear underwater, it’s only found in whales and their ancestors, and acts as a diagnostic feature for the group.
We should, however note that this animal IS NOT the direct ancestor of whales, it is only a sister group to the ancestors of whales. Got it?
So why did they go aquatic? Scientists speculate that whales developed from an Indohyus-like ancestor that fed on plants and possibly small invertebrates on land, but fled to water to escape predators. Over time, they slowly turned into meat-eaters and evolved to swim after nimble aquatic prey.
For a much more in-depth comparison see http://en.wikipedia.org/wiki/Evolution_of_cetaceans
Info gleaned from http://blogs.discovermagazine.com/notrocketscience
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