It is one of the most unusual evolutionary ideas yet proposed: humans are amphibious apes who lost their fur, started to walk upright and developed big brains because they took to living by the water's edge.
Although treated with derision by some academics over the past 50 years, the aquatic ape theory is still backed by a small group of scientists.
They are about to hold a London conference where several speakers, including British naturalist and broadcaster David Attenborough, will voice support for the theory.
"Humans are very different from other apes," said Peter Rhys Evans, an organiser of Human Evolution: Past, Present and Future. "We lack fur, walk upright, have big brains and subcutaneous fat and have a descended larynx, a feature common among aquatic animals but not apes."
Standard evolutionary models suggest these features appeared at separate times and for different reasons.
The aquatic ape theory argues they all occurred because our ancestors decided to live in or near water for hundreds of thousands or possibly millions of years.
The theory was first proposed in 1960 by British biologist Sir Alister Hardy, who believed apes descended from the trees to live, not on the savannah as is usually supposed, but in flooded creeks, river banks and sea shores, some of the richest sources of food. To keep their heads above water, they evolved an upright stance, freeing their hands to make tools to crack open shellfish. Then they lost their body hair and instead developed a thick layer of subcutaneous fat to keep warm in the water.
Others dismiss parts of the theory. One or two human features could have arisen because our ancestors picked homes near the sea, they say, but the entire package is just too much.
"I think that wading in a watery environment is as good an explanation, at the moment, for our upright gait as any other theory for human bipedalism," said Professor Chris Stringer of the Natural History Museum, central London.
"But the whole aquatic ape package includes attributes that appeared at very different times in our evolution. If they were all the result of our lives in watery environments, we would have to have spent millions of years there and there is evidence for this - not to mention like crocodiles and other creatures would have made the water a very dangerous place."
The aquatic ape hypothesis
Originally outlined by biologist Alister Hardy, the aquatic ape hypothesis achieved prominence when the theory was taken up by the Welsh writer Elaine Morgan in the early 1970s. According to prevailing ideas, human males lost their body hair when they took up hunting and needed to sweat profusely in the African heat. No explanation was given to account for loss of female body hair, Morgan said. She believed the aquatic ape theory provided a more balanced vision of human evolution. Morgan wrote a popular account of the theory, The Descent of Women.