The mammals in the sea once walked on land, and over 50 million years they became unrecognisable
Four legged animals found a good source of food in fresh water. Animals like Indohyus (below) would have paddled in shallow water foraging for fish and marine grubs. Rivers and lakes were full of food, and these animals spent most of their time in water.
Over time, water-foragers ventured further and further from land. They made the move from fresh water towards the sea, foraging now in brackish water. The next stage of whale evolution came in the form of Ambulocetus, five million years down the line. It has much shorter legs and a powerful tail to help it swim, and even evolved to have fat surrounding the jaw to help it hear under water.
Dorudon lived 40 million years ago. It resembled modern whales and dolphins, and developed tail flukes and a dorsal fin to stabilise itself while swimming. Dorudon’s nostrils had shifted from the end of the snout to the top of the head, forming the blowhole we recognise in modern whales and dolphins.
Over the following 40 million years, dorudon adapted to different food sources and environments. The 87 species of whale and dolphin arose from these early animals, and their skeletons give us clues about their origins. Most whales and dolphins have lingering leg bones, like those in the sperm whale skeleton below.
Image from flickr.com/photos/rjshade