Hippos and dolphins, despite their vastly different appearances and habitats, share a surprising evolutionary connection. Science has uncovered that these two groups of mammals have a common ancestry.
The Surprising Evolutionary Link
Modern scientific research, particularly genetic studies, reveals that hippopotamuses are the closest living relatives to cetaceans, which include whales, dolphins, and porpoises. Genetic evidence overwhelmingly supports this evolutionary relationship, grouping them together in a suborder called Whippomorpha or Cetancodonta. (1, 2, 8) This connection is considered one of the most surprising animal family trees in nature. (26)
This close kinship is supported by analyses of shared DNA segments present only in whales and hippos, indicating a common ancestor not shared by other animals. (2, 10) Both hippos and cetaceans possess shared traits related to aquatic life, such as a dense layer of subcutaneous fat and largely hairless bodies. (1) They also exhibit amphibious and aquatic behaviors and have similar auditory structures, hinting at their shared past. (1)
Tracing the Shared Ancestry
The scientific evidence for the relationship between hippos and dolphins comes from both molecular analysis and the fossil record. Molecular studies, comparing DNA sequences, have placed hippos within the order Artiodactyla (even-toed ungulates) and confirmed their status as the closest relatives to cetaceans. (1, 10) Researchers have identified nearly 11,000 orthologous genes between cetaceans and hippopotamuses, strongly indicating a shared evolutionary history. (1, 12)
Fossil discoveries bridge the gap between their ancient common ancestor and modern forms. The ancestors of both hippos and cetaceans were semi-aquatic, four-limbed mammals that lived around 50 to 60 million years ago. (3, 4, 5, 17) A key transitional fossil for cetaceans is Pakicetus, an extinct wolf-like mammal from about 50 million years ago. (30, 32) It possessed an ankle bone linking it to artiodactyls and an ear bone characteristic of whales, providing a crucial link between land mammals and whales. (30, 33) The common ancestor of hippos and whales is thought to be an extinct group of pig-like, semi-aquatic mammals called anthracotheres, which flourished for millions of years. (3, 4, 7, 11, 13) These findings reconcile molecular data with paleontological evidence, confirming anthracotheres as the link between hippos and cetaceans. (3, 11, 22)
Divergence and Adaptation
Despite their shared ancestry, hippos and dolphins exhibit significant differences in their current forms due to divergent evolution. This process occurs when two species evolve from a common ancestor but adapt to different environments and lifestyles, resulting in distinct physical and behavioral traits. The common ancestor, likely a semi-aquatic creature, gave rise to two lineages that followed separate evolutionary paths over millions of years. (4, 17)
Dolphins evolved for a fully marine, predatory life, developing specialized adaptations. Their bodies became highly streamlined to minimize water resistance, with front limbs transforming into flippers and hind limbs disappearing entirely. (16, 19) They developed powerful tail flukes for propulsion and a blowhole on top of their heads for efficient surface breathing. (15, 16) Dolphins also possess a thick layer of blubber for insulation and advanced echolocation systems for navigation and hunting. (16, 20, 21)
In contrast, hippos adapted to a semi-aquatic freshwater existence, spending their days submerged in rivers and lakes to protect their sensitive skin from the sun. (14, 23, 25) Their large, barrel-shaped bodies and short, stout legs allow them to walk or run along riverbeds. (14, 24) Hippos have eyes, ears, and nostrils positioned on top of their heads, enabling them to remain mostly submerged while sensing their surroundings. (23, 24) Their thick, hairless skin secretes a reddish substance that acts as a natural sunscreen and moisturizer, and they have teeth adapted for grazing on land at night. (14, 23) These distinct adaptations reflect the different selective pressures faced by each lineage after diverging from their common ancestor. (12, 28)