It is a common assumption that hippopotamuses and elephants are closely related, likely due to their massive size, thick gray skin, and shared African habitats. Despite these superficial resemblances, the two species are not close relatives. Modern genetics and the fossil record reveal a surprising evolutionary distance between the two largest land mammals. This separation highlights how appearances can be deceiving in the study of animal taxonomy.
The Hippo’s Closest Living Relatives
The hippopotamus belongs to the order Artiodactyla, traditionally known as the even-toed ungulates, which includes deer, pigs, and cattle. Molecular phylogeny—the study of evolutionary relationships using genetic data—has revealed a much more unexpected connection. Hippos are the closest living relative to the entire group of whales, dolphins, and porpoises, collectively known as Cetacea.
This evolutionary pair is grouped into a clade called Cetancodonta, or the suborder Whippomorpha. The divergence between the ancestors of hippos and whales occurred approximately 50 to 60 million years ago. The shared lineage is supported by molecular evidence and by the fact that the earliest fossil whales were semi-aquatic mammals that descended from a terrestrial ancestor within the Artiodactyla order.
While hippos are semi-aquatic and whales are fully marine, their shared ancestor was likely a wolf-like terrestrial mammal. The aquatic traits seen in both groups, such as a lack of oil-producing glands and a dense layer of subcutaneous fat, may have evolved independently as they adapted to water. The common ancestor of hippos and whales was already distinct from the lineage that gave rise to the elephant.
The Elephant’s Distinct Evolutionary Path
Elephants follow an entirely separate and ancient path of evolution from the hippopotamus and the Artiodactyla. Elephants belong to the order Proboscidea and are the sole living members of the superorder Afrotheria. This superorder is an ancient branch of placental mammals that originated in Africa when the continent was isolated.
The elephant’s closest living relatives are not other massive land animals, but rather the small, herbivorous Hyraxes and the fully aquatic Sirenians (manatees and dugongs). Together with elephants, these groups form the clade Paenungulata within Afrotheria. The distinct African origin of Afrotheria separates elephants from the Artiodactyls and Cetaceans.
Members of the Proboscidea are defined by unique characteristics, including the elongated, prehensile trunk (a fusion of the nose and upper lip). Their tusks are specialized, elongated incisor teeth. Their massive size and specialized molar morphology are adaptations for consuming large amounts of tough vegetation. The earliest known ancestor of the elephant, Moeritherium, was a pig-sized creature that lived a semi-aquatic life approximately 37 million years ago.
Superficial Similarities and Convergent Evolution
The visual resemblance between hippos and elephants is an example of convergent evolution, where unrelated species develop similar traits due to similar environmental pressures. Both animals evolved to occupy the niche of a massive herbivore in a hot climate with access to water. This ecological similarity drove the independent development of similar body plans.
The most striking shared trait is their thick, nearly hairless skin, which is highly sensitive to the sun and prone to desiccation. Both species require constant access to water or mud to regulate body temperature and protect their skin from the harsh African sun. This need for a water-dependent lifestyle resulted in massive, barrel-shaped bodies and short legs, necessary to support enormous weight while moving through soft mud or water.
Despite the superficial similarities, their dental structures highlight their deep evolutionary separation. The hippo’s enormous tusks are canine and incisor teeth used primarily for fighting and display, while their molars are adapted for grinding grass and aquatic plants. Conversely, the elephant’s tusks are elongated incisors. Their molars are designed for a continuous, conveyor-belt-like replacement pattern, allowing them to grind abrasive grasses and tree bark throughout their long lives.