The Evolution of Marine Mammals
The Evolution of Marine Mammals
Whales and dolphins have had an on-again, off-again relationship with the sea. Life evolved in the sea some 3 billion years ago. When opportunities for food opened up on land, some animals, including the dolphins and whales, moved onshore to take advantage of the food supply. These ancestors were distantly related to cows and horses and have been described as wolf-like in appearance.
For some reason, probably again to find food, the descendants of these animals worked their way back into the sea, probably first foraging from shore before becoming full-fledged ocean dwellers. As new fossils are uncovered, scientists believe they may find further evidence that the ancestors of whales and dolphins moved on and off the land several times.
A fossilized jawbone and teeth recently unearthed in the Himalayan foothills suggest whales originated about 53.5 million years ago. This find is 3.5 million years older than any fossils previously found. The fossil is a new whale species named Himalayecetus subathuensis.
Scientists agree that dolphins and whales descended from land animals that were ungulates (like horses, cows, hippos, and camels). Now some researchers are finding some striking similarities between hippos, which spend much of their time partially submerged in muddy pools, and dolphins, which spend most of their lives submerged in the ocean.
Both may give off underwater sounds through fatty areas in their heads. The dolphin transmits sounds through a fatty area on its forehead known as the melon. Scientists thinks hippos, although they do have vocal cords, may also release sounds into the water through a roll of fat in their throats. Both may "hear" through their jaws. The University of California at Santa Cruz examined dolphin skulls to find evidence that sounds are conducted through the lower jaw to the dolphin's ears.
They al0so found similar structures in hippos that may make it possible for hippos to listen to sounds above the surface with their ears while simultaneously monitoring underwater sounds through its submerged jaw. They have a close blood match. Jerold Lowenstein of the University of California at San Francisco has compared blood serum of whales with many ungulates, including cattle, pigs, and gazelles. He found whales were more similar to hippos than to any other group (although DNA studies have not confirmed this).
