Whales evolved from a small, deer-like land mammal about 50 million years ago.

Modern whales belong to the order Cetacea, but their journey began 50 million years ago with the Indohyus, a fox-sized mammal discovered in the Himalayas. Research by paleontologist Hans Thewissen in 2007 revealed that this creature shared a unique thickened ear bone with modern whales. This bone, called the involucrum, is a signature trait only found in cetaceans and their direct ancestors.The transition from land to sea is documented through a series of fossil 'links' like Pakicetus and Ambulocetus. Pakicetus lived in freshwater environments and had legs designed for walking, yet its teeth and ears were already evolving for aquatic life. By the time Basilosaurus appeared 35 to 40 million years ago, whale ancestors were fully aquatic but still possessed tiny, useless hind legs.One of the most compelling pieces of evidence for this evolution is the pentadactyl limb structure found in whale flippers. X-rays show that beneath the skin, whales have the same five-fingered bone structure as humans, dogs, and their deer-like ancestors. This homologous structure proves they descended from land-dwelling tetrapods rather than evolving from fish.Genetic sequencing has further confirmed these fossil findings by identifying hippopotamuses as the closest living relatives to whales. Both groups share a common ancestor that lived roughly 55 million years ago. This evolutionary path highlights how extreme environmental pressures can completely transform a species' anatomy over millions of years.