Were we once aquatic apes?

Dr Darren Naish examines a debate that's lasted 50 years.

15th June 2012
The Aquatic Ape Hypothesis

In 1960, the marine biologist Sir Alister Hardy proposed that humans had gone through an aquatic phase in their history. Hardy pointed to human swimming abilities, hairlessness, hair tracts patterns and the presence of subcutaneous fat in support of his idea. Hardy’s hypothesis was mentioned by Desmond Morris in his 1967 book The Naked Ape, but it was television script-writer Elaine Morgan who made the Aquatic Ape Hypothesis (AAH) famous thanks to her several books, especially The Aquatic Ape (published in 1982). Morgan expanded on Hardy’s idea, arguing that many additional peculiarities of human biology and anatomy pointed to an aquatic phase in human ancestry. A 1987 conference devoted to the AAH resulted in a technical volume containing 22 contributions, the more scholarly of which argued against the AAH.

Image credit: Robin Boyden

It has been argued that German anatomist Max Westenhöfer supported the AAH in his work of the 1920s and 30s. Westenhöfer interpreted an aquatic ancestry for humans within his ‘initial bipedalism’ hypothesis. According to this idea, humans are the most structurally primitive of primates and the ancestors of all other primate groups. This work is generally regarded as ‘fringe’ science today.

Today, the AAH has mostly failed to win adherents and is not considered seriously by mainstream primatologists or anthropologists. There are several reasons for this. One is that the basic arguments are erroneous. That is, the supposedly ‘aquatic’ features of humans are either not unique among primates, or are not like those of aquatic mammals as the AAH argues. Some monkeys are capable swimmers and divers, for example. The ‘diving reflex’ is present in other primates and indeed in other mammals, the babies of just about all mammals behave the same when placed in water, and there are non-human primates that sweat as much as humans do. The descended human larynx is not unique to aquatic species, nor are humans unique in their hair tract patterns, in possessing hymens, in the position of their fat deposits, and so on.

Fossil evidence also fails to support the AAH. Morgan and her supporters argued against the idea that humans owed their anatomy to a life in dry, hot grasslands, but this is a straw-man argument since early fossil members of the human lineage were woodland- or forest-adapted animals. Furthermore, fossil hominids lack adaptations special to mammals that regularly swim or forage in water. The supposed peculiarities of humans are well explained by adaptation to a lifestyle where sweating, complex formation of sounds in communication, and exploitation of scarce, fatty foods were important.

Some modern supporters of the AAH argue that great apes went through an ‘aquarboreal’ phase in which they waded and swam in swampy forests, feeding on molluscs and fruit. Members of the human lineage, they argue, became bipedal in this context and remained tied to waterside resources as coastal foragers. It is possible that at least some fossil humans and human relatives foraged on shorelines or in mangroves, waded in shallows, or ate aquatic foods like crabs, stranded fish and shellfish. However, there are no indications that such a lifestyle, if it existed, left any obvious mark on human anatomy, so this watered-down idea of a link with aquatic environments cannot be considered in the same ballpark as the AAH.

Despite a huge number of recent fossil hominid discoveries, fossil evidence that might support the AAH has not appeared. The evidence from primate physiology, behaviour and anatomy also fails to support it. Overall, the AAH remains a historical curiosity, but not an acceptable explanation for human evolution. It is, many argue, dead in the water.

Darren Naish is a palaeozoologist at the University of Southampton

5 issues for £5 when you subscribe to BBC Focus Magazine