Science writer Carl Zimmer has a blog dealing, mainly, with topics in evolutionary biology. His latest post is about mammals returning to the water, and in particular aquatic sloths. Aquatic sloths? Yep, aquatic sloths.
[The piece beneath, on voles and mating behaviour, is interesting too. No, really.]
The idea of mammals returning to the water makes me think of my favourite left-field scientific theory: the aquatic ape hypothesis.
Back in 1960, biologist Alister Hardy suggested as an explanation for some of the pronounced morphological differences between man and his nearest ape relatives - notably lack of body hair and upright gait - that at some time in our evolutionary past we'd reverted to the water. It's a plausible idea. Aquatic mammals in cold climates might, like otters, keep their hair as insulation, but in Africa that's not an issue and loss of hair would improve the dynamics of swimming. Walking on all fours is clearly not recommended if you're fishing around in the shallows. There are other quirks of human biology which could be explained by a recent aquatic spell: our subcutaneous layer of fat, our copious sweating in the heat, and not least our voluntary breath control and descended larynx, a rare feature that would seem to provide all sorts of problems with oesophagous and windpipe so close that we easily choke, but may have been useful for gulping in air, as we hit the surface in desperate need of some oxygen. That feature also turned out to be vital for speech. And we shouldn't forget our species' love of water, whether swimming, diving, or just lounging around on the beach.
Why would we have plunged back into the water? Well, like the sloths of Peru, if the pickings on land are pretty meagre, there may not have been much choice. The area of East Africa round the Rift Valley, especially the Danakil Depression in Ethiopia near the Red Sea, has been in the past connected to the ocean, and it's easy to imagine that at some point, as ice ages came and went and the sea level rose and subsided, a group of hominids would have found themselves isolated on an island cut off from their normal sources of food. Such a situation is ideal for evolutionary change: a small subset of a species, suddenly isolated by environmental change, in a harsh ecology where the usual life-style is no longer appropriate. It's adapt or die. With no land animals to hunt, few berries or nuts to gather, the only option would have been to take the plunge and exploit the shallow waters all around.
There are of course lots of details which are hazy. How aquatic is aquatic? We're not talking dolphins or whales here, where a species has made a decisive return to the water over millions of years. More like spending considerable time hunting for food by wading around in the shallows, with some swimming and diving. And when are we talking? Even more hazy. We split off from the apes about 7 million years ago, and modern man by our latest estimates headed out from Africa around 100,000 years ago, having out-performed loads of dead-end hominid relatives, some of whom (of which?) like the Neanderthals headed out of Africa as well. So anytime in between those dates would be possible. For how long? Clearly this beach existence would have needed to have lasted a good length of time to have an effect, though when you get these sudden environmental changes it's surprising how swiftly evolution can occur. [Punctuated equilibrium was the theory proposed by Niles Eldredge and Stephen Jay Gould arguing that evolution occurred in short sharp bursts against a backdrop of stasis: a position they pushed as being a radical break from Darwinian orthodoxy, but which is in fact entirely consistent with Darwinian orthodoxy.]
One last killer point here: these evolutionary changes would not necessarily have required separate changes to different groups of genes, ie changes to our hairiness genes, then changes to our brain-size genes, and so on. One of our most distinguishing features is that we humans look like baby apes, with our baldness and our big heads. This retention of juvenile traits into adulthood is known as neoteny. I'm no geneticist, but it would seem plausible at least to assume that the rate of developmental change could be varied by a mutation in a relatively few, if not just the one, gene. So if we imagine life for our unfortunate ancestors stuck on their little island, there would have been a strong selective pressure in favour of those with slower rates of individual development, in terms of reduced body hair and increased subcutaneous fat, like babies, for better swimming. There would have been side-effects too: neoteny would also tend to increase brain size (babies have massive heads compared to body size), a key element in our evolution, and as recent research shows a diet of fish and shellfish is ideal for growing brains.
This is all very speculative, but alternative theories about how we came to differ from other apes are hardly compelling, concentrating as they mostly do on the spread of savannah-like ecologies as the African forests started to shrink, necessitating an erect gait so we could look around, so we only needed hair on the top of our heads to protect us from the sun. Something like that. Similar to all those other bald savannah dwellers like, um, well, ....baboons? It's not an area where I've kept up to date by any means, and there could be startling new developments which I've missed, but this is not something on which science seems as yet to have provided definitive answers. So why isn't the aquatic ape hypothesis better known? The suspicion has to be that after its initial lukewarm reception it's become something of an embarrassment to professional biologists because its main if not its only champion is a rather wonderful Welsh lady - a total amateur - called Elaine Morgan, who has written a few books on the subject including "The Aquatic Ape Hypothesis." So the whole thing has rather a cultish, eccentric air about it. I still believe in it however, and will be extremely disappointed if it turns out not to be true.