Testing the water

Imagine our earliest ancestor. Probably you see him amid parched scrub, maybe picking over a carcass cooking in the sun, gazing wistfully at a herd of antelopes on the horizon. Now try this instead: he is standing breast-deep in a cool lake enjoying a breakfast of nutritious fresh fish.

I had better be honest and confess to having paid scant attention to this possibility and its various rebuttals since The Aquatic Ape was published 15 years ago, but no matter, since the dustjacket of The Aquatic Ape Hypothesis claims that it is "virtually a new case''. Here are some highlights.

Bipedalism. Elaine Morgan dismantles all the favourite theories by either of two means. Theories of bipedalism that rest on savannah adaptation are out, since the move to two legs is now believed to have occurred before the onset of savannah conditions in Africa. Theories that survive this turnabout mostly fall foul of "the teleological trap'', ever ready to catch those who lose concentration. The trap is to assume that the benefits attached to a trait are sufficient to explain its evolution. They are not: current benefits have nothing to say about a trait's evolution, since when the trait first started to evolve, these benefits were in the future. Only benefits immediately accruing to a trait in its fledgling state explain anything at all. Thus bipedalism is usually explained as being the most efficient method of long-distance locomotion. But this cannot be why it evolved. The half-baked bipedalism hominids must have suffered for thousands of generations would be a great step backwards. Quadrupedalism is one adaptive peak. Bipedalism is apparently another. How did we get between them, that is, under what circumstances would it benefit a quadrupedal animal to occasionally walk upright? Morgan's answer is that it would confer immediate advantages when the water was lapping around your chin. Seasonal flooding would have required hominids to wade from one food source to another, just as the normally quadrupedal proboscis monkey does today. This is a good explanation, not because the evidence of watery migrations is particularly great, but because it proposes evolutionarily fertile moments of duress recurring seasonally, in which a quadruped dabbling in bipedalism could have an advantage. If our ancestors found themselves occupying two different environments each year, then it was better they dealt with both badly than only one superbly. So half-baked bipedalism evolved in the water and only later did we find other uses for it out on the savannah where it became fully baked.

Fat and nakedness. The only other hairless mammals are whales and seals and pachyderms, which likely had semi-aquatic ancestors. A naked primate on land would overheat; in water it would overcool without an unusual amount of fat, which is exactly what we have. We also have sufficient fat that most of us float when we inhale and can submerge when we exhale. Suspicious, eh?

Descended larynx. This allowed us to speak once we had invented language but what was its original function? Mouth breathing, says Morgan; we can gulp air before diving. A chimpanzee cannot alter its breath rate, though a walrus can.

The nose. Human nostrils point downwards, unlike those of other primates, which is great for keeping water out when diving for food. Also, for Morgan's correspondents capable of the extraordinary feat of sealing their nostrils with the philtrum, that groove above the middle of the upper lip moulds perfectly into the bridge between the nostrils, completing the seal.

There is more depth to these arguments than this synopsis provides and much more to provoke. But Morgan's account contains a large gap nevertheless. What on earth were the aquatic apes eating? The primary adaptation of any animal is its diet and the means by which it obtains it, and yet Morgan consigns this to three pages largely concerned with the evidence that the human brain develops best on a diet of fish. If we ate fish how did we catch it? The human eye needs air to focus; that is why divers wear masks. Assuming we did not pass through an echolocatory stage or develop sensitive whiskers like the otter, or a luminous lure like the angler fish, the human eye must have evolved to focus in water and then lost this ability, leaving no vestige yet discovered. Otherwise we are left with the occasional diet of primates bordering water: crabs, turtle eggs, limpets and dead fish gathered from the water's edge. For a large mammal these would be lean pickings in a restricted habitat that would not in any case require aquatic adaptations. A "weak aquatic hypothesis" in which we waded between terrestrial food sources is compatible with known primate diets - fruits, seeds, tubers and small game. A "strong aquatic hypothesis" in which we made our living in the water requires us to come up with a new feeding ecology. Morgan's account of bipedalism entails only the weak version but her account of most other human features entails the strong version.

What then is the status of the aquatic ape? The burden of proof would seem to remain firmly with Morgan, for since we, and all our relatives, are non-aquatic it would be most parsimonious to assume we have always been so. On the other hand, humans are an odd lot as apes go, so perhaps the radical rather than the parsimonious is required here. Morgan brings an unparalleled breadth of evidence to bear, much of which may be misapplied. But scientific knowledge advances on too many fronts now for anyone credibly to claim a watertight case from evidence alone. Morgan's virtue is to do what ought to be standard practice: to show from any given evidence what must follow, what might follow and what is entirely irrelevant. Along the way she exposes the points of her hypothesis where new evidence could refute it. Perhaps this shows great scientific integrity or perhaps just that when one's problem is exclusion from the main current of debate there is little to lose by inviting refutation. We would progress much faster and more amicably if everyone behaved thus. Morgan keeps her options open, hypothesising only that "our unusual evolutionary path had something to do with water''. So dive in or muck about in the shallows but at least dip your toe into an excellent critique of previous attempts to explain ourselves which offers one intriguing alternative.

Tom Sambrook is honorary research fellow, department of anthropology, University of Durham.