Man: The Aquatic Primate

Brandt Rakowski

“ An aquatic Ape is a likely ancestor of humans in terms of primate behaviour, marine ecosystems and geophysical timing." - Prof. Derek Ellis,


One of the prime factors that has allowed human beings to achieve as much as we have in the past 200,000 years is our unique physiology – our bipedalism, our capacity to grasp objects with an opposable thumb, and our ability to create and interpret complex aural symbols. These traits coincide with several other aspects of our phenotype that are radically different from our closest cousins in the evolutionary tree. What reasons could there be for evolution to select for human nakedness? What about sweat and tears, which are conspicuously absent in other great apes? In my opinion, these questions and several more about the uniqueness of the human race can be addressed by the still somewhat radical hypothesis that proposes that our early ancestors were semi-aquatic creatures, evolutionarily poised to become full-time marine mammals.

This so-called “aquatic ape” theory was first proposed in 1946 by Max Westenhofer, and has since been steadily gaining credence within the scientific community. Modern research in geology and behavioral biology has been steadily adding to the body of evidence that supports this theory, even though the concept is still poorly regarded in mainstream anthropology. The purpose of this essay is to outline and critique the basic arguments that support aquatic ape theory, or AAT.

Human nakedness is a trait that starkly distinguishes us from all other primates. In general, only three environments have been known to select for nakedness in mammals: a large size in a tropical environment (elephants, rhinos, etc.), a subterranean lifestyle (naked mole rats), or an aquatic or semi-aquatic existence (hippos, whales, etc.) Clearly, early humans lacked the tools and sophistication to forge an underground lifestyle, and we are not large enough to get by in the tropics without hair – orangutans and gorillas are larger in size than humans and live exclusively in the tropics, but they are both hirsute in nature. Furthermore, human hair is arranged in such a manner so that it matches fluid lines over the skin while swimming. Just beneath the skin there exists another clue to our aquatic heritage: fat, and lots of it. Humans have about ten times as much body fat as similarly sized terrestrial mammals, which we retain throughout the year regardless of season. Human babies are especially fat when compared to other primates, and all of our fat is attached to the skin of our central body parts. These are traits that are only shared by medium-sized aquatic mammals – terrestrial mammals have their fat layer attached instead to the muscle.

True bipedalism has only evolved three times in all of evolution: in dinosaurs, kangaroos, and humans. Clearly, special circumstances are required to make the leap from quadrupedalism to standing on two legs. However, gorillas, chimpanzees, and certain species of monkeys have been observed wading through water on two legs. Much of the strain on the vertebral column is relieved when the body is supported by water, thus allowing the gradual shift to two legs provided long enough exposure to an aqueous environment. Bipedalism may have been the most important factor in human evolution, as it allowed us to grasp and carry objects even while in motion.

Many aspects of human reproduction can also be traced to an aquatic background. The “missionary position” has long been the favored method of human copulation, but is rarely observed in terrestrial animals. However, all aquatic mammals mate in this fashion. The human penis is longer than that of most primates, comparable only to the bonobo, which live in partially flooded forest ecosystems. This may have been an evolutionary adaptation to compensate for a less accessible vagina, designed to keep out water. Human females have permanently enlarged breasts, which are mainly composed of fatty tissue. While some biologists argue that this is because sexual selection has favored this trait (for various reasons), larger breasts are also more buoyant in water, allowing easier nursing in an aquatic environment. Finally, even human birth shares many traits with the live birth of many aquatic, air-breathing mammals. Human infants are born encased in a waterproof coating called the vernix caseosa, while they continue to receive oxygen from their mother through the umbilical cord. Babies can swim from birth, and have a natural instinct to hold their breath. Such traits have allowed a recent resurgence in water births, which are hailed as being safer for both the mother and child. Additionally, a water birth allows the infant’s head to be supported by water throughout the birth, possibly allowing for the safe and gradual expansion of the cranium, which in turn is responsible for the extraordinary capacity for abstract and symbolic thought found only in humans.

Sweating and crying are both traits that are commonplace among humans, but not observed in other primates. Both of these mechanisms may have originally evolved as a method of maintaining the proper balance of salinity in our bodies, effectively removing excess salt after a swim in the ocean. The crying of saline tears is only observed in other mammals that are thought to have descended from aquatic ancestors, such as elephants. The ability for humans to control their respiration may also be traced to swimming: most land animals have no conscious control over their breath. However, human breath control can be compared to other diving mammals, and may very well have been an important precursor to speech. It is noteworthy that other diving mammals, such as dolphins and whales, have likewise adopted complex verbal communication schemata.

Finally, modern human nutrition provides us with vital clues about our ancestors. Human brain tissue is more dependent on omega-3 fatty acids, the HDL cholesterol-promoting compounds abundant in seafood, than most terrestrial creatures. Additionally, humans lack the digestive enzyme used by land predators to break down terrestrial fatty acids, thus avoiding the negative effects of “bad” cholesterol. This rings true in the common vernacular today: fish is still regarded as healthy and bacon is not. It is worth noting here that as most animals adapt to savannah life, their brain size decreases; whereas as animals adapt to an aquatic lifestyle the trend is for brain size to increase, possibly due to a diet rich in omega-3 fatty acids.

AAT is by no means to be regarded as anything more than a theory at this point in the debate. Critics have pointed out several flaws in the theory, including counterexamples of apes walking on two legs out of water, terrestrial mammals with similar fat arrangements to humans, and conscious control of breathing in some land mammals. There are also many features of human beings that are not usually seen in aquatic mammals: long limbs, long hair top of the head, and large ears. However, many champions of AAT have pointed out that while these features are not best adapted for a completely aquatic lifestyle, they serve well as tools for a species that spends a majority of its time wading.

So, if humans have developed all of these adaptations for a marine lifestyle, why is it that we all live on land today? There is nothing in the fossil record that can answer this question; we are relegated to the land to speculation and supposition. My own theory is that several key adaptations for the life aquatic were the proximate cause for the “Great Leap Forward” in human history – the explosion of tools and culture that shows up in the fossil record about 50,000 years ago. The bipedalism and larger brain size that developed out of our aquatic lifestyles were the major changes that needed to happen to propel human thought and dexterity towards the point where we could think in the abstract and create specialized tools. In turn, the advantages of these tools and of bipedalism could only best be realized on land, where humans were able to advance themselves up the food chain because of their ability to hunt. This great revolution in human evolution interrupted the path to an aquatic lifestyle, but not before we were left with both the useful and coincidental physical traits of aquatic mammals.




Hardy, A.C. "Was man more aquatic in the past?” New Scientist , 7,642-645 (1960)


Kordos, László, Begun, David R (2001). "Primates from Rudabánya: allocation of specimens to individuals, sex and age categories". Journal of Human Evolution40 (1): 17-40. 


Kuliukas, A., "Wading for Food: The Driving Force of the Evolution of Bipedalism." Nutrition and Health, 16(4), 267-290, (2002).


Morgan, Elaine (1997). The Aquatic Ape Hypothesis. Souvenir Press.

Return to ENVS2 homepage

Send message to Swarthmore College Environmental Studies

last updated 2/13/07