Marine Biology

How Whales Evolved



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A modern whale is warm-blooded, breathes air with lungs (through a blow hole on the top of the head), has a four-chambered heart, and feeds its young milk produced by mammary glands. Whales have ears that have been adapted to hearing sounds under-water, have either specialized teeth for catching prey or baleen for filtering food, have vestigial hind-limb bone structures, and modified fore-limbs that are used as flippers. When swimming, whales and dolphins move their spines in an up-and-down motion, like running mammals, instead of side-to-side like sharks and fish. Whales also have sensory bristles, which are the remnants of the hair that characterizes mammals.

Natural selection has produced the modern whale because whale ancestors had to compete and find resources to survive and reproduce. This resulted in anatomical changes that have taken place, for about 50 to 55 million years.

DURODON AND PRIMITIVE WHALES The oldest pieces of evidence for whale evolution are the remains of a primitive whale, the Durodon, that lived 45 to 25 million years ago in the Tethys Sea, which was located where present day Egypt is; the Mediterranean Sea is a remnant of the vast Tethys sea. The Durodon had a long snout with whale-like teeth and small, but complete hind limbs. The Dorodon had front flippers. Whale-like fossils with hind limbs, such as the Dorodon, indicate that there might be a land mammal ancestor for the whale.

PACKICETUS - A LAND MAMMAL ANCESTOR? The search for a land mammal ancestor has led scientists to another possible ancestor that lived 50 million years ago, in the Eocene. The Pakicetus had a whale-like skull with nostrils located near the end of the snout. There were numerous teeth in the elongated jaw that were similar to terrestrial mammal teeth. The jaw bone was also modified to pick up vibrations from the ground and transmit the vibrations to the ears; and the ears contain a distinct middle ear bone that is like a whale ear, an adaptation to hearing underwater.

AMBULOCETUS - THE WALKING WHALE The Ambulocetus was discovered in 48 million year old Tethys Sea deposits in Pakistan. This fossil was aptly named the "walking whale" because it had well-developed hind limbs and fore limbs that were used for walking. The fore limbs had four toes with small hoof on each toe. The teeth and ears were similar to those of modern whales.

ARTIOCETUS AND RODHOCETUS - MORE WALKING WHALES Recently, the remains of Artiocetus and Rodhocetus were also found in Pakistan and dated to be about 47 million years old. These mammals had nostrils located in a position almost midway between the end of the long snout and the top of the skull. They had well-developed limbs. The hind limbs have an ankle bone that is found only in the artiodactyls, which are hoofed, even-toed terrestrial ungulates. The ankle bone, or astragalus, allows the artiodactyls to use their limbs for flexing and stability in jumping movements; and allowed the Rodhocetus whale ancestors the ability to pull themselves out of water and walk on land.

BASILOSAURUS - WHALE WITH REDUCED HIND LIMBS Numerous fossil remains of the Basilosaurus have been found in Egypt and have been dated to around 37 million years ago. The Basilosaurus had hind limbs that are too small to hold the weight of the animal, however the hind limbs are well-developed enough to have a mobile knee.

DNA FINGERPRINTING - WHALES HAVE A COMMON ANCESTOR WITH HIPPOS! DNA fingerprinting of modern whales shows that the closest living relatives to the modern whales are the hippopotamids, even-toed artiodactyl ungulates. It can be inferred from the evidence available today that the modern whales might have evolved from a terrestrial mammal that was a common ancestor to both the whale and the hippopotamus!

In summary, the evidence shows that, over the past 55 million years:

1. Whales have changed by a gradual reduction in the hind limbs, from fully functional to vestigial.
2. Snouts have elongated and the nostrils have moved from the end of the snout to the top of the head.
3. Front limbs have been modified into flippers.
4. The mechanism of being able to hear underwater has been developed from the middle ear picking up vibrations in the jaw, to one of middle ear detection of vibrations by a specialized skull bone.
5. The tail has developed into horizontal flukes for moving in an up-and-down swimming motion.

Lastly, the genome of the modern whale, when compared to other mammals, maps out and points to the common ancestor of both whales and hippopotamids that might have lived as a terrestrial ungulate artiodactyl.

Whales possess the intelligence and tendency toward play that shows off their fascinating evolutionary past. The whale behavior of traveling in herds and migrating over long distances, while communicating to each other, also reveals their common evolutionary background with mammals.



Monastersky, Richard. 1999. The Whale's Tale (research on whale evolution). Science News (Nov. 6) and on the web at http://findarticles.com/cf_dis/m1200/19_156/57828404/print.jhtml

Thewisson, J.G.M., et al. 2001. Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature (Vol. 413) pp. 277-281.

Gingerich, P.D., et al. 2001. Origin of whales from early artiodactyls: Hands and feet of Eocene Protocetidae from Pakistan. Science (Vol. 293) pp. 2239-2241.




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