The hippopotamus and the pig are two large mammals, sharing a bulky physique and a familiar, somewhat porcine appearance. Many people compare the two based on their general shape, wondering if they are direct evolutionary relatives. Scientific inquiry, driven by physical evidence and modern genetic analysis, has definitively explored this question. This research reveals that while a distant relationship exists, the hippo’s true evolutionary kinship is far more surprising than a simple connection to the pig.
The Common Misconception
The widespread assumption that hippos and pigs are closely related stems from superficial, observable similarities. Both animals possess thick, sparsely haired skin that is sensitive to the sun and often appears gray or brown. They also share a robust, barrel-shaped body set upon relatively short, stout legs.
Pigs and hippos both exhibit large snouts and have four toes on each foot, placing them among the even-toed mammals. They also share a similar dietary approach, as neither animal is a true ruminant like a cow or a deer. This combination of build, thick skin, foot structure, and non-ruminating digestion historically led people to group them together.
Modern Scientific Classification
Modern biology places both the hippopotamus and the pig within the Order Artiodactyla, the group of even-toed ungulates. This classification confirms that the two animals share a distant common ancestor. The Order Artiodactyla is now often referred to as Cetartiodactyla, reflecting the inclusion of whales within this group.
Within this broad order, their immediate family connections diverge significantly. Pigs belong to the family Suidae, which includes warthogs and wild boars. Hippos belong to their own distinct family, Hippopotamidae. The pig lineage, Suina, split off from the main Artiodactyla evolutionary tree much earlier than the lineage that led to hippos.
This taxonomic distance means the pig is not the hippo’s closest living relative, despite both being even-toed ungulates. Shared characteristics like their non-ruminating stomachs or general shape result from ancient ancestry and similar physical demands, not a recent shared history. Genetic evidence supports that the two families separated deep in the past.
The Surprising Closest Relative
Molecular and genetic studies show the closest living relatives to the hippopotamus are not pigs or other land ungulates, but the Cetaceans—whales, dolphins, and porpoises. This unexpected pairing is supported by DNA, leading scientists to create the clade Whippomorpha to unite the Hippopotamidae and Cetacea families.
Genetic Markers
The confirmation of this relationship comes from specific genetic markers, such as the analysis of Short Interspersed Nuclear Elements (SINEs). SINEs are DNA sequences that insert themselves randomly into an organism’s genome. Identical SINE insertions in both hippo and whale DNA, but not in other Artiodactyls, serve as a powerful molecular signature of shared ancestry. This evidence is more reliable than comparisons of physical traits, which can be misleading due to convergent evolution.
Shared Aquatic Adaptations
Beyond genetics, hippos and whales share unusual adaptations related to aquatic life not found in pigs. Both groups lack a coat of hair and possess specialized subcutaneous fat layers. Furthermore, they are among the few mammals that nurse their young underwater, a behavior pointing to a shared heritage with an early water-adapted ancestor.
Evolutionary Journey
The evolutionary path connecting the semi-aquatic hippo to the fully marine whale involves a crucial extinct link. The common ancestor of hippos and whales branched off from other even-toed ungulates approximately 60 million years ago. This ancestor was likely a semi-aquatic mammal from the group Anthracotheriidae, sometimes called “coal beasts.”
These pig-like anthracotheres flourished globally for about 40 million years, exhibiting adaptations to life near water. The lineage that became the whales diverged from the anthracothere-hippo line around 55 million years ago, committing fully to the ocean. The hippopotamus is the sole surviving descendant of this once-diverse group of anthracotheres.
Fossils like Indohyus, a small, fox-sized animal, are considered a primitive relative of the hippo-whale lineage. This ancient link provides an example of how different environments can shape two related groups, leading to the physical differences seen in the river-dwelling hippo and the ocean-faring whale today.