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What is the Aquatic Ape Hypothesis?

Michael Anissimov

Last Modified Date: February 01, 2024

The Aquatic Ape Hypothesis, sometimes referred to as the Aquatic Ape Theory or AAT, is a questionable theory from paleoanthropology that enjoyed popularity in the 80s and early 90s. The basic idea is that human evolution was heavily influenced by the presence of bodies of water, and many of our signature characteristics and differences from other primates, such as hairlessness and bipedalism, can be explained by reference to this aquatic habitat. The Aquatic Ape Hypothesis' most vocal proponent is Elaine Morgan, a television playwright and feminist writer. Although the theory was ultimately dismissed by the paleoanthropological community, being aware of it and the reasons for why it was refuted can help us learn more about the nature of the evolutionary process.

The first argument for the Aquatic Ape idea comes from hairlessness. Getting rid of our thick primate hair makes it easier to swim and faster to dry off when exiting a body of water. The next argument comes from bipedalism. It is argued that the buoyant properties of water would have made the incremental evolution from quadrupedalism to bipedalism easier. Another argument comes from control over our breathing. We can deliberately control our breathing like many aquatic and semi-aquatic creatures, but unlike other land creatures.

Dolphins are sometimes included in arguments for the Aquatic Ape Hypothesis.

There are many other anecdotal arguments for the Aquatic Ape hypothesis. A few are our fat surplus, perpendicular nostrils, the ability of infants to hold their breath and swim from birth, the greater nutrition of fish relative to land animals, and face-to-face sex, like in dolphins, are all cited as possible evidence for the influence of aquatic environments over our evolution.

There are many arguments against the Aquatic Ape Hypothesis. The most obvious is that arguments in its favor tend to be vague, offer few testable predictions, and change their assumptions based on what trait it is they are trying to argue is related to an aquatic past. The premises of the theory have not changed substantially since the 50s, when the theory was originally introduced.

Another argument is that most of the bodily features attributed to water evolution by the Aquatic Ape enthusiasts are either not truly exclusive to aquatic animals or their evolution can be explained by other means. For example, many species of non-aquatic apes are capable of walking bipedally, at least temporarily, which places doubt on the idea that water was necessary to facilitate permanent bipedalism. Our hairlessness is probably a result of walking longer distances and their corresponding need to dissipate heat more effectively. Our fat surplus is common among all animals with no natural predators and substantial amounts of food. The Aquatic Ape hypothesis is not necessary to explain any of this.

Sometimes theories teach us even more about science when they're wrong than when they're right. The Aquatic Ape hypothesis is frequently studied by paleoanthropologists as a way of how evolution theories should be falsifiable and as amenable to scientific testing as possible.

Michael is a longtime AllTheScience contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

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Discussion Comments

anon218548

September 29, 2011

Nice to see AAT discussed.

Of course the term "aquatic ape" is an unfortunate misnomer. It's not about apes or australopiths (only about Homo), and it's not about having been aquatic (a better term is "littoral"), but – however one wants to name it – the Hardy–Morgan theory is beyond doubt. It's obvious that Pleistocene Homo populations dispersed along coasts and rivers; how else could they have reached Flores? Why else are all archaic Homo fossils found next to edible shellfish (work of J. Joordens, of S. Munro, and others), all over the Old World, from the Cape to Eritrea to Boxgrove to Dmanisi to Mojokerto, from at least 1.8 Ma until 125 ka?

The only "problem," in my opinion, is that anti-AAT people attack their own idea of what they believe AAT is (eg, dolphin-like ancestors). Their "critiques" are nearly invariably irrelevant, misunderstood misrepresented, obsolete, non-essential (attacking a possible sub-hypotheses), unrealistic and/or illogical ("crocodiles would have killed aquatic apes").

In my opinion, we have to discern two theories: The literal theory of Homo (AAT s.s.): Pleistocene diaspora of human ancestors along coasts and rivers, beach-combing, wading and diving for waterside and aquatic foods and the aquarboreal theory of apes: Mio-Pliocene hominid adaptations (eg, vertical branch-hanging and wading) in flooded forests (mangrove, gallery, swamp forests).

Do some research on econiche Homo on human evolution,

aquarboreal on hominid evolution or read our forthcoming book "Was Man More Aquatic in the Past? Fifty Years after Alister Hardy: Waterside Hypotheses of Human Evolution," by M. Vaneechoutte, A. Kuliukas and M. Verhaegen eds 2011 Bentham Science Publishers, with contributions from Elaine Morgan, Phillip Tobias, Michel Odent, Anna Gislén and others.

Or see our recent paper "Patch Osteosclerosis suggests archaic Homo frequently collected sessile littoral foods" in HOMO J. compar. hum. Biol. 62:237-247, 2011.

Marc V.

anon140818

January 8, 2011

If these are the arguments against AAH then I am still a firm supporter of this thesis. Although the theory of evolution is not difficult to test and 'prove', much of evolutionary adaptation is difficult to prove. For example, how has it been proven that hairlessness is a result of walking longer distances and their corresponding need to dissipate heat more effectively? (you said probably). What it comes down to, is that there is far more evidence for the AAH, than for competing theories. This doesn't mean its true, but it means that we should be investigating it more thoroughly.

And you are suggesting that animals that walk long distances are hairless? What examples are there?

1. Which other terrestrial mammals don't have body hair? They are moles (that live underground) and the rhino, and the elephant (scientists agree both had an aquatic ancestor.

2. Other apes can walk bipedally --yeah, like when they are wading.

I agree that the AAH is not necessary to explain many of the adaptations, but it seems more probable. A problem is that there is insufficient investigation into AHH, so its difficult to prove something if you don't investigate it. Also, early human migration seems to be along coastal regions.. suggesting the sea was a resource. I expect that we swam and waded to obtain food.

anon36445

July 12, 2009

"the theory was ultimately dismissed by the paleoanthropological community"

When, and by whom?

anon28163

March 12, 2009

I think Elaine Morgan does an excellent job of presenting the substantial arguments for the hypothesis. And refuting the often lame 'conventional' theories for bipedalism, hair reduction, brain development etc.

I recommend any of her books on the subject.