The idea of the AAH is that at some stage in our evolution, well before modern humans emerged, we foraged main in water for all or part of the year, but also spend time on land, and presumably in trees. Spending a lot of time wading and diving in water over tens of thousands or even hundreds of thousands of years led to partial adaptations to that environment that make humans superficially unlike any other great ape, for example we are upright, bipedal, and relatively hairless (hence we are the "naked ape"), except on our heads where we grow extraordinarily long hair.
The mainstream view was that we were semi-arboreal like chimps are today, but that the climate changed and left us living on the savannah that now covers eastern, sub-Saharan Africa. Why we did not do what other species of apes did and move with the trees, is not clear. Put a chimp on the savannah and it has no food and no protection from group predators like hyenas and lions. Chimps can defend against their main predators, leopards, because they are solitary hunters who can be driven off by a concerted group effort. Not so hyenas!
According to this hypothesis, life on the savannah encouraged us to walk upright and left our hands free for other tasks, though whether this was tools, weapons, or babies is unclear. But if the chimp is badly adapted to finding a living on the savannah, make them stand upright to move about and they are suddenly visible to predators from far off and too slow to run away from most predators because four-legged gaits are must faster!
Losing our body hair is said to be an adaptation to heat loss, but we are curiously the only African animal of that area that lost our body hair. If it heat loss was the driver, then their ought to have been parallel evolution of other naked animals. But there was not, except for the hippopotamus which spends most of its time in the water!
But it has recently emerged that in fact there was no savannah where we evolved when we evolved. The area was woodland and wetlands. The savannah hypothesis idea is definitely defunct. Which leaves the field open and the aquatic ape hypothesis ought to be a contender. Some of the key points follow.
Adaptations to a Semi-Aquatic Lifestyle.
Like other aquatic mammals we are relatively hairless and have a layer of subcutaneous fat (or blubber). Land mammals don't need the buoyancy and insulation that blubber provides; and where they do need insulation, they tend to opt for fur instead. Except in the semi-aquatic hippo.
We also share a curious feature with seals that we share with no terrestrial mammal, i.e. vernix - the (water-proof) waxy coating that covers new born babies. In fact it was a prediction of the hypothesis that aquatic mammals might share this feature with us, and the discovery of vernix in seals is a confirmed prediction of the hypothesis.
The AAH argues that wading was the origin of bipedalism. Other normally quadrupedal modern apes, i.e. chimps and baboons, switch to bipedalism when wading. Land-based bipedalism would have left us with a major a disadvantage in terms of speed before we were fully adapted to running. Chimps for example are very much slower and less manoeuvrable on two legs than on four. But equally any adaptations towards bipedalism would have been a major disadvantage to arboreal life. Loosing the ability to grip with feet, for example, makes life in the trees much less viable. In other words, the early trait, which had to have lasted millennia would have not survived natural selection if we were solely land-based because disadvantages outweighed advantages. Bipedalism on land would have killed us off. Only once we were fully upright and agile could be have started running on two legs. You have to walk before you can run.
There is also good evidence for food collection from water dating well before anatomically modern humans (going back 2 million years in fact). Especially good evidence for catching and consumption of-large fresh water fish that were rich in omega 3 fatty acids and thus probably contributed to our growing brains. Add to this the recent observation of chimps using long sticks to "fish" for pond algae, also high in protein and fatty acids. These foods are available in the dry season, precisely at the time when fat content of land-based prey animals is at its lowest, which (in a confirmation of recent changes in dietary advice) make it a poor dietary choice, especially for nursing mothers.
Lastly, communities of women who dive in shallow water for a living in Korea and Japan remain fit for much longer than land based workers. There is no deterioration of their ability to work until around age 75. Women frequently continue to productively dive for a living into their 90s. People who do this or spend a lot of time in cold water get a bony growth in their ear canal called "Surfer's Ear". Identical growths are found in Homo erectus (ca 2 million years before present) and in both anatomically modern humans and Neanderthals, indicating a semi-aquatic lifestyle for all three.
The route modern humans took out of Africa ca 100,000 ybp followed the coastline and rivers, suggesting that water continued to be an important source of food, as it does right up the present day for many communities.
So the Aquatic Ape Hypothesis is now mainstream or rapidly becoming mainstream.
But predictably, despite spelling out the evidence and giving an example of a testable hypothesis that has been tested and shown to be accurate, some scientists are still saying that the aquatic ape hypothesis is flatly wrong. It seems that no matter how much evidence accumulates, there is no evidence whatever. This kind conservatism and resistance to paradigm changes in science has been noted by historians of science. Max Planck once quipped that progress in his field occurred one funeral at a time, i.e. only as the old guard died out and were replaced by youngsters with nimbler minds.
Observations in want of an explanation that the Aquatic Ape Hypothesis explains.
- Lack of body hair when other great apes are hairy, and many aquatic or semi-aquatic mammals, like hippos, are not.
- Subcutaneous fat layer (blubber) when no other ape or land mammal has it, but marine mammals do.
- Presence of exostosis (aka "surfer's ear") in fossils. It occurs with frequent immersion in cold water.
- Bipedalism. When on land any early attempts at bipedalism carry significant evolutionary disadvantage (on two legs our ancestors were slower, less manoeuvrable, and more visible to predators). Watch a chimp on it's back legs and you see what the problem is - they are fucking awkward! The advantages of carrying stuff in our hands was minimal at first. Also other apes are routinely bipedal when wading in water.
- Vernix (waxy substance covering newborns). Also found in seals, but in no other land mammal.
- Stone tool cut marks on fish bones ca 2 million year bp, and other fossil evidence showing a substantial part of our ancestors diet was from the water
- Diving reflex in infants newborn to 6 months
- Breath control. No other land based mammal can hold its breath!
Boesch, C., et al. (2016), Chimpanzees routinely fish for algae with tools during the dry season in Bakoun, Guinea. American Journal of Primatology. doi:10.1002/ajp.22613
See also the crab eating Macaque via BBC's Planet Earth documentary series.
Crab eating macaques also use simple tools.
And there is another amphibious monkey, the proboscis monkey (Nasalis larvatus) of Borneo (via New Scientist).
FYI, an update of the "littoral dispersal model" (Munro 2010), a more correct term than "aquatic ape". Paleo-environmental & comparative fossil data suggest 3 overlapping theories:
- Mio-Pliocene hominoids incl. australopithecines did NOT live in dry savannas, but in wetlands, wading & climbing vertically in above-swamp branches – aqu-arboreal theory,
- Pleistocene Homo did NOT endurance-run, but followed African & Eurasian coasts & rivers, beach-combing, diving & wading for littoral, shallow aquatic & waterside foods incl. shellfish – littoral theory,
- late-Pleistocene H. sapiens reduced diving, and mostly waded & walked bipedally, fishing & collecting with long straight legs & complex tools – wading theory.
Pliocene Epoch 5.333 million to 2.58 million years BP.
Pleistocene Epoch 2,588,000 to 11,700 years ago
Some relevant recent papers:
J.Joordens, S.Munro cs 2014 Homo erectus at Trinil on Java used shells for tool production and engraving, Nature doi 10.1038/nature13962
M.Verhaegen, S.Munro 2011 Pachyosteosclerosis suggests archaic Homo frequently collected sessile littoral foods, HOMO J.compar.hum.Biol.62:237-247
S.Munro 2010 Molluscs as ecological indicators in palaeoanthropological contexts, PhD thesis Univ.Canberra
J.Joordens cs 2009 Relevance of aquatic environments for hominins: a case study from Trinil (Java, Indonesia), J.hum.Evol.57:656-671
M.Gutierrez cs 2001 Exploitation d’un grand cétacé au Paléolithique ancien: le site de Dungo V à Baia Farta (Benguela, Angola), Compt.Rend.Acad.Sci.332:357-362
K.Choi, D.Driwantoro 2007 Shell tool use by early members of Homo erectus in Sangiran, central Java, Indonesia: cut mark evidence, J.archaeol.Sci.34:48-58
S.Cunnane 2005 Survival of the fattest: the key to human brain evolution, World Scient.Publ.Comp.
M.Vaneechoutte cs eds 2011 Was Man more aquatic in the past? eBook Bentham Sci.Publ.
P.Rhys Evans cs eds 2013-2014 Human Evolution conference London May 2013 proceedings, special editions Hum.Evol.28 & 29
M.Verhaegen 2013 The aquatic ape evolves: common misconceptions and unproven assumptions about the so-called Aquatic Ape Hypothesis, Hum.Evol.28:237-266,