The cetaceans (whales, dolphins and porpoises) are descendants of land-living mammals, and remnants of their terrestrial origins can be found in the fact that they must breathe air from the surface; in the bones of their fins, which look like huge, jointed hands; and in the vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish.
The question of how land animals evolved into ocean-going behemoths has been a mystery for a long time, owing to gaps in the fossil record.
However, recent discoveries in Pakistan have managed to solve many of these mysteries, and it is now possible to see several stages in the transition of the cetaceans from land to sea.
The most remarkable of the recent discoveries in Pakistan has been Ambulocetus, which looked like a three-metre long mammalian crocodile.
Ambulocetus was clearly amphibious, as its back legs are better adapted for swimming than for walking on land, and it probably swam by undulating its back vertically, as otters, seals and whales do.