Wednesday, January 30, 2013

The aquatic ape hypothesis is rubbish

I mentioned the aquatic ape hypothesis in a recent post on wrinkly fingers improving grip when handling submerged objects. As if on cue, I then received this email:

Dear colleague,
Humans are very different from other primates, and many evolutionary hypotheses have been proposed to explain this remarkable fact. Is there scientific consensus on which of those hypotheses are most substantiated? If not, do opinions differ among researchers with different backgrounds?
Please help to establish the state of the art by answering a survey on this topic. You receive this invitation because you have recently published in a scientific journal covering paleoanthropology, paleontology or (human) biology, which indicates that you have expertise in a relevant field. 
A link to the survey is provided in the end of this message. 
Your responses will be anonymous. If you wish to be informed about the outcome of the survey, you can add your e-mail address to the mailing list after having completed the main survey. The present mailing list will only be used to send the invitations. My apologies if you receive this message more than once. 
Thank you for participating! 
Yours sincerely,
[Name and contact details redacted]

The first couple of pages were reasonable enough, containing questions on a variety of theories for how human traits evolved. Which is as you would expect from a survey that is trying to "establish the state of the art" on human evolution. But, the survey then degenerated into asking questions solely about the aquatic ape hypothesis. A quick bit of googling later and I find that the sender is a proponent of the hypothesis.

I am baffled by the persistence of the aquatic ape hypothesis. It has received almost no attention in the scientific literature because on more then superficial examination it fails to provide a parsimonious explanation of human evolution. In fact, to accept the aquatic ape hypothesis you would not only have to find its scant evidence convincing, but assiduously ignore the contradictory evidence.

Take the claim that bipedalism evolved in our ancestors as an adaptation to an aquatic conditions, from a situation much like how gorillas and chimpanzees walk when wading. It's a fine and very plausible claim, but it doesn't fit with what we know from the fossil record. All of the putative human ancestors, such as Australopithecus and Orrorin, retain features that show they were still capable tree climbers when they first started walking upright. For the aquatic ape hypothesis to be true, they would have had to come to the ground first then moved to water before becoming bipedal. There is nothing to support this in the fossil record.

Another problem that the human fossil record poses for aquatic ape proponents is that several of the traits claimed to be adaptations to aquatic conditions appeared separately. For instance, it's claimed that only a seafood diet rich in omega-3 fatty acids could support the expansion of brain size in the human lineage. Brain expansion, however, came after bipedal locomotion, indicating that the association with water had to have lasted several million years for both of these claims to be true.

Hairlessness is one of the traits that is central to the aquatic ape hypothesis. Again it is a fine and plausible claim to make, but hairlessness is poorly associated with an aquatic lifestyle. There are some notably hairless aquatic mammals, such as whales, dolphins and walruses, but there are many more aquatic animals that retain hair. Moreover, there are some hairless mammals that are clearly not aquatic, such as naked mole rats, rhinoceros[1] and elephants. Hairlessness is clearly neither necessary for an aquatic life nor is it uniquely associated with aquatic mammals.

A feature that is far more commonly associated with aquatic mammals is that they are testicond, that is they have internal testicles (at least the males do!). In fact, I know of no aquatic mammal that is not testicond. There are some mammals that are not aquatic and are also testicond, like our friends the rhinoceros and elephant[2], but the far more common condition for terrestrial mammals is to have external testicles. So, the external testicles of human males suggests that humans are typical terrestrial mammals and not aquatic apes.

There are a whole host of other features that aquatic ape proponents cite as evidence consistent with aquatic ancestry and it would take an entire website to debunk them all (like this one). Individually, none of these traits are very compelling evidence for their hypothesis, and taken together they're a complete mish-mash. While some are highly derived features, like hairlessness, others have barely changed from the ancestral condition, like webbing between the fingers.

More parsimonious explanations for the evolution of human traits involve multiple causes, not a single cause as proposed by the aquatic ape hypothesis. The aquatic ape hypothesis is neither simple[3], nor does it fit well with the available evidence. Proponents of the aquatic ape hypothesis need to bend the evidence to fit their hypothesis and resort to special pleading to explain away the inconsistencies in their arguments. It's a textbook example of adaptationist just-so storytelling.

[1] An interesting aside is that the species of rhinoceros that is most closely associated with water is often called the hairy rhinoceros.

[2] Another interesting aside is that both the rhinoceros and elephant are thought to have aquatic ancestors. The support for this is much stronger for the elephants, which are part of a clade (the Tethytheria) who's other members are all aquatic (the extant Sirenia and the extinct Desmostylia). This may explain why they and rhinoceroses are testicond. It does not explain their hairlessness, as we know that as recently as 10,000 years ago there were a number of very hairy rhinos and elephants, indicating that the common ancestors of modern rhinos and elephants were also hairy.

[3] The transition from arboreal to terrestrial to aquatic back to terrestrial is not a simple evolutionary scenario. A simpler assumption would be that human ancestors were all terrestrial after descending from the trees.


  1. The aquatic ape hypothesis was first conceived by marine biologist, Sir Alister Hardy, back in the 1920s. But he didn't reveal his hypothesis to the public until 1960 during a lecture and then in an article in the journal New Scientist.

    Elaine Morgan first encountered the hypothesis after she read a synopsis of it in the Desmond Morris book, the Naked Ape. Then she wrote about it in her own book, the Descent of Woman in the early 1970s.

    Basically, Hardy's argument was that humans became bipeds and developed a thick subcutaneous fat because they needed to wade into shallow water in order to get access to shellfish. Such food gathering behavior is actually common in many tribal populations even today.

    The smoking gun, IMO, is the fact that humans are the only primate that has kidneys with medullas that are normally lobulated in their morphology. This is a characteristic that is universal in marine mammals. Lobulation of the medulla in kidneys increases the surface area between the medulla and the surrounding cortex, enhancing the rate in which high concentrations of ingested salt can be excreted from the body.

    Marcel F. Williams

    1. A quick look at the literature shows that bipedalism in archosaurs evolved several times and all of the hypotheses I read about do not link its evolution to water. If you have a reference that does, I would be happy to read it and let you know what I think.

      A quick look at the literature on Oreopithecus shows that it is generally accepted that it probably evolved bipedalism to increase the efficiency of locomotion. Again, if you have an article that contradicts this, I would be happy to read it and let you know what I think.

      Your discussion of the kidney serves to highlight a point I made in the post. If humans are descended from ancestors that had such a strong association with salt water that they evolved a specialised kidney, why do many other traits show a lack of specialisation to aquatic environments? For instance, why are we not testicond given that every other aquatic mammal is?

      It is not obvious that our kidneys, or any of our traits, evolved because of an association with water. The evidence for the aquatic ape hypothesis is pathetic and sometimes even pseudo-scientific. You need to do better than provide circumstantial evidence. You need to provide evidence that is not only consistent with the aquatic ape hypothesis, but also rules out the currently better supported hypotheses.

  2. To be honest, I haven't really followed the aquatic ape argument very closely, but I seem to recall that it concerned the ancestor of humans growing up in close proximity TO water, and not IN water. As far as I know, no-one can seriously be claiming that our early primate ancestor was actually an aquatic mammal like an otter or whatever.

    As far as I recall, the argument was based on the fact that, in order for the brain to grow, essntial fatty acids are required. Such fatty acids are found in fish, but not in the diet of apes living in the jungle or the savannah. And this was the original case put forward for humans descending from apes that lived close to lakes and the sea.

    So what is all the brouhaha about?

    1. The arguments are all over the place. But the original one and the one that Elaine Morgan is pushing, is definitely about the ancestor being in water. Hence it's the aquatic ape hypothesis, not the littoral ape hypothesis. Although, I think some proponents are pushing a semi-aquatic, close to the water and sometimes in it hypothesis.

      The fatty acid argument is popular among AAH proponents, but it is not true. While DHA fatty acids are high in seafoods, there are sufficient DHA fatty acids in other foods for brain development.

  3. This comment has been removed by a blog administrator.

  4. Marc, usually I wouldn't delete comments, but you where using this blog for self promotion.


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