Controversial theory that seeks to explain one of the great leaps of human evolution finds new support but still divides scientists

It is one of the most unusual evolutionary ideas ever proposed: humans are amphibious apes who lost their fur, started to walk upright and developed big brains because they took to living the good life by the water's edge.

This is the aquatic ape theory and although treated with derision by some academics over the past 50 years, it is still backed by a small, but committed group of scientists. Next week they will hold a major London conference when several speakers, including 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 different 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 Earth's 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.

Scientists have since added other human attributes of claimed aquatic origin – a recent addition being the sinus, said Rhys Evans, an expert on head and neck physiology at the Royal Marsden hospital, London.

"Humans have particularly large sinuses, spaces in the skull between our cheeks, noses and foreheads," he added. "But why do we have empty spaces in our heads? It makes no sense until we consider the evolutionary perspective. Then it becomes clear: our sinuses acted as buoyancy aids that helped keep our heads above water."

Other palaeontologists dismiss parts of the theory. One or two human features could have arisen because our ancestors picked homes near the sea but the entire package of attributes – lack of fur, upright posture, big brains, sinuses and others – is just too much, they add.

"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, 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 no evidence for this - not to mention like crocodiles and other creatures would have made the water a very dangerous place."

It is not just human physiology that reveals our aquatic past, argue the theory's supporters. Our brain biochemistry is also revealing. "Docosahexaenoic acid (DHA) is an omega-3 fatty acid that is found in large amounts in seafood," said Dr Michael Crawford, of Imperial College London.

"It boosts brain growth in mammals. That is why a dolphin has a much bigger brain than a zebra, though they have roughly the same body sizes. The dolphin has a diet rich in DHA. The crucial point is that without a high DHA diet from seafood we could not have developed our big brains. We got smart from eating fish and living in water.

"More to the point, we now face a world in which sources of DHA – our fish stocks – are threatened. That has crucial consequences for our species. Without plentiful DHA, we face a future of increased mental illness and intellectual deterioration. We need to face up to that urgently. That is the real lesson of the aquatic ape theory."