Why do whale species move their tails up and down, while fish tails flip from side to side

A. The answer lies in their different evolutionary paths. Whales and dolphins are mammals - they evolved from an animal that once lived and walked on land. Their tails, and the underlying skeletal structure that supports them, developed from this ancestor's hind legs; those limbs, which join the hip bone at each side, rather than at the top and bottom, presumably originally moved in an up and down direction.

By contrast, modern-day fish have evolved from animals that have always lived in water. Fishes' tails have never had to support the body out of water, and simply follow on from their spines and propel their owners through the water using sideways movements.

Q. What evidence is there that whales evolved from land mammals

A. Basing their theories on genetic and fossil evidence, scientists have long known that cetaceans (whales, dolphins and porpoises), are descended from land mammals. Recently discovered fossils reveal a creature from a group called the pakicetids that, for the first time, displays features found in both whales and land mammals: sheep-like ankle bones but with ear bones unique to whales.

Q. What did pakicetids look like

A. They were predators, so were well adapted for running. They were about the same size as wolves.

Q. When and where did pakicetids live

A. Around 50 million years ago. The latest fossil finds are from contemporary Pakistan.

Q. Why did they move into the water

A. Scientists believe the animals became fond of fish, developed a swimming technique and eventually took to the water altogether. Over millions of years they became fully adapted to living in the water. But they're still mammals and so have to surface to breathe.

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by Lisa Cardy