Cetacean Evolution

Dolphins are my favorite animals, and I’ve always been fascinated by their interesting evolutionary path. Dolphins and whales (that is, cetaceans) are mammals like we are: they breathe air, they give birth to live young, and their spines bend up and down like ours do (not side to side like those of sharks or other fish). And like us, they are descended from four-legged land mammals. It’s quite interesting to examine the sequence of changes that resulted in them more and more adapted to living in water.

Examination of a dolphin skeleton clearly reveals our closely shared ancestry. In addition to the similar spines (vertebral columns), in their flippers, dolphins have the same bones we do in our arms. Connected to the scapula (shoulder bone) is a short humerus (arm bone), then a short radius and ulna (forearm bones), followed by the tarsals (wrist bones), metatarsals (hand bones), and phalanges (finger bones)—dolphins (and whales) have five “fingers” inside their flippers. And often tiny, vestigial hind leg bones are found buried within the animal. In fact, there was a CNN article a couple weeks ago (“Could extra dolphin fins be legs?”) discussing a dolphin with a pair of caudal fins perhaps representing the ancient hind limbs:

TOKYO, Japan (AP) — Japanese researchers said Sunday that a bottlenose dolphin captured last month has an extra set of fins that could be the remains of hind legs, a discovery that may provide further evidence that ocean-dwelling mammals once lived on land.

Fossil remains show dolphins and whales were four-footed land animals about 50 million years ago and share the same common ancestor as hippos and deer. Scientists believe they later transitioned to an aquatic lifestyle and their hind limbs disappeared.

Whale and dolphin fetuses also show signs of hind protrusions but these generally disappear before birth.

If you’re interested in more about this fascinating progression, artist Carl Buell has started a series of posts on cetacean evolution at his blog Olduvai George (which I noticed courtesy of Pharyngula). He’s just posted the first in the series, with a nice commentary accompanied by his excellent illustrations. Take a look!

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