Elaine Morgan, feminist author of The Descent of Woman, has long been ridiculed for proposing that early humans evolved along lake and estuary shores. Establishment theory instead pictured "Man the Mighty Hunter" striding out from the forest into the "hot, dry savanna". Labour Left Briefing invited Elaine to write for us in the light of recent 3.5 million year-old fossil discoveries by Philip Tobias, the 'grand old man' of South African anthropology and co-discoverer of Homo habilis. In a dramatic public announcement, Tobias has admitted that the first hominids loved shaded, well-watered environments and that the "savanna hypothesis" is now dead.

The aquatic ape theory (AAT) posits a semi-aquatic phase in early human evolution. For thirty years, this idea has been likened to stories about Loch Ness monsters, despite the impeccable standing of scientists such as the great marine biologist Sir Alister Hardy, who was among the first to propound the theory.

Several enigmatic features found in Homo, while unique among primates, find close parallels in the physiology of certain aquatic or semi-aquatic mammals. Examples include 'nakedness' (loss of functional body hair), a layer of fat bonded to the skin, a descended larynx, and voluntary breath control - an essential precursor of speech.

Walking on two legs has no mammalian parallel, but Hardy noted that a wading ape venturing into deeper water would simply have to walk upright to breathe!

My critics said AAT was unnecessary. They claimed that the scenario for the ape/hominid split was well understood: one population of the ancestral apes moved from forest out into the "hot, dry savanna". There, they became two-legged so as to "run faster" and carry weapons, while the torrid heat caused them to sweat profusely and shed their body hair. AAT referred to all this as "the savanna theory". In different versions, it reigned supreme for over fifty years.

In what for me has been a remarkable and exciting development, the experts have in recent months suddenly started to abandon this whole idea. Detailed studies of the African paleoenvironment show that savanna conditions evolved in Africa much later than had previously been imagined. The habitat of Lucy (Australopithecus afarensis) is now agreed to have been "lush and well watered" (her bones lay among crocodile and turtle eggs and crab claws).

Australopithecus ramidus, a million years earlier still, lived in woodland. Richard Leakey wrote in 1992: "The great plains and the immense herds on them are... much more recent than the origin of the human family". And now Professor Phillip Tobias, lecturing in London on his recent fossil discovery "Little Foot", has announced that "the savanna hypothesis" (of which he had been one of the most illustrious supporters) "is washed out".

Is this the beginnings of a paradigm shift completely revolutionising our understanding of human origins? I think it could be. The strength of savanna theory lay in its contention that the hominids' habitat was radically different from that of the apes. This claim is severely undermined if the protohominids were largely tree-dwelling, as is now agreed, only crossing open spaces in occasional transit from one patch of forest to the next.

Meanwhile, AAT is waiting in the wings. The arguments against it look increasingly precarious. Nearly all the Rift Valley fossils were recovered from lake or river sediments. Unfortunately, this can never prove that the entire species lived by the water, since only bones deposited in watery sediments get preserved. On the other hand, neither can it disprove the idea. If the first hominids were indeed largely tree-dwelling, why did they and no other apes become bipedal on the ground? A flooded forest offers a possible answer: there is geological evidence of extensive flooding in the areas where the oldest hominids are found.

The strongest argument for AAT is the number and variety of Homo's unique features, for some of which the aquatic explanation is the only one on offer. These anomalies are not a common subject of research. Many specialists still find them simply distracting, and sometimes respond "We may never know the reason" or even "There may not be a reason".

But the questions will not go away. Professor Tobias enjoys being back at square one. "A change of paradigm", he says, "shakes us up; it rejuvenates us; and this above all, it prevents mental fossilisation - and that is good for all of us".

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