AP BIOLOGY

Detailed explanation-1: -Although whales are expert swimmers and perfectly adapted to life underwater, these marine mammals once walked on four legs. Their land-dwelling ancestors lived about 50 million years ago.

Detailed explanation-2: -The first whales appeared 50 million years ago, well after the extinction of the dinosaurs, but well before the appearance of the first humans. Their ancestor is most likely an ancient artiodactyl, i.e. a four-legged, even-toed hoofed (ungulate) land mammal, adapted for running.

Detailed explanation-3: -Cetaceans (whales, dolphins, and porpoises) are an order of mammals that originated about 50 million years ago in the Eocene epoch. Even though all modern cetaceans are obligate aquatic mammals, early cetaceans were amphibious, and their ancestors were terrestrial artiodactyls, similar to small deer.

Detailed explanation-4: -The theory is that some land-living ungulates favoured munching on plants at the water’s edge which had the added advantage of allowing them to easily hide from danger in shallow water. Over time their descendants spent more and more time in the water and their bodies became adapted for swimming.