A Staff Report from the Straight Dope Science Advisory Board

Did humans descend from "aquatic apes"?

January 22, 2002

Dear Straight Dope:

A few years ago, Elaine Morgan published a book called The Aquatic Ape that publicized a theory that humans, at some time in their evolution, had partially adapted to a marine environment. This theory had first been proposed by an English marine biologist named Alister Hardy (if I remember correctly) and was held to explain a host of differences between Homo sapiens and the rest of the great apes, among them: relative hairlessness, subcutaneous body fat, bipedality (to make swimming and wading more efficient), a "diving reflex" to prevent drowning in infants, our horrendously inefficient water management system, our lack of fear of the water, the webbing some people have between their fingers and toes, and so on. The theory seemed reasonable enough to me, but every time I've heard paleontologists refer to it, they seem to be rolling their eyes the way archeologists do when you mention James Churchward to them. What do these guys know that Hardy and Morgan and I don't?

Say AAH. That stands for the "Aquatic Ape Hypothesis." Now turn your head and cough. Just checking to make sure you were paying attention. The AAH is a fascinating, thought-provoking, and attractive idea. The only problem is that it's JPW ("Just Plain Wrong").

Alister Hardy--wasn't he the guy who wondered about the role of telepathy in evolution? As a matter of fact he was. Hardy was a respected marine zoologist for about as long as Fred Hoyle was a respected astronomer: that is, for as long as he didn't advocate insupportable speculations. Hardy wrote three short pieces on the aquatic ape, but they didn't exactly light a fire under the feet of paleoanthropologists. Elaine Morgan, a television writer, then took up the cause. She didn't simply write a book about aquatic apes, she made a second career out of them. There are at least five popular books by her on the subject (of which I have read two). They haven't taken the discipline by storm either. Why don't most paleoanthropologists take the AAH seriously enough to refute it? How would you feel if the anthropology department at the local state U. spent your tax dollars studying, say, the idea that humans are descended from extraterrestrial reptiles? (Some people actually believe that one.) It turns out that the AAH--sometimes known as the AAT, for Aquatic Ape Theory--has only slightly more evidence in its favor.

The burden of proof for a new hypothesis is always on its supporters, but there are two ways skeptics can shoot it down. They can either refute the assumptions it is based on, or they can show that known facts are inconsistent with it. The AAH fails on both counts. Let's look point-by-point at the facts that suggested the hypothesis, as Hardy first published it in April 1960 (quotations are from his article "Was Man More Aquatic in the Past?" in New Scientist, volume 7, reprinted in Morgan's The Aquatic Ape): ". . . [T]he exceptional ability of Man to swim . . ." Not very exceptional. Hardy admits "many" animals can swim on the surface. In fact, almost all terrestrial mammals can. With very few exceptions, adult mammals, when introduced to the water for the first time, can swim without any previous training. This is largely because they tend to float horizontally and are able to keep their nostrils above water. Apes and humans, on the other hand, tend to float vertically with their nostrils submerged. Humans (and at least some apes) can learn to swim, but it doesn't come naturally. Based on this point alone, hominoids would appear to be among the least likely mammals to return to the water.

". . . I have been told that babies put into water before they have learnt to walk will, in fact, go through the motions of swimming at once . . . " Partly true, but misleading. Babies, placed face down in the water, can hold their breath and rather inefficiently propel themselves through the water. Their motions are as much like crawling as they are like swimming. Babies cannot, however, lift their nostrils above water unassisted to breathe, which would seem to make their much-vaunted "swimming" ability worthless. Nor are human infants unique in being able to propel themselves through the water; the young of many, probably most, terrestrial mammal species can do the same.

"Does the idea perhaps explain the satisfaction that so many people feel in going to the seaside, in bathing, and in indulging in various forms of aquatic sport?" Uncertain, probably unverifiable, and more than a little silly. One of my neighbors had a trampoline in his back yard instead of a pool. Was he trying to recapture the days when our ancestors' bottoms were made out of springs? Are we Tiggers or are we men?

"Whilst not invariably so, the loss of hair is a characteristic of a number of aquatic mammals . . ." True, if you take "a number" to mean "a small number." Fur or hair is no great hindrance underwater. Fur seals, otters, beavers, and polar bears haven't lost theirs and they swim better than we do. Only some aquatic mammals have lost all or most of their hair, and they are almost invariably very large species weighing a ton or more, whose ancestors have been living in the water for tens of millions of years. Contrary to popular thought, fur remains an effective insulator even in water, because it traps a layer of stagnant water (or in the case of the sea otter, air) next to the skin. Further, our ape relatives generally have sparse hair, though not quite as sparse as ours. What do AAH supporters make of relatively hairless terrestrial species, such as the elephant, rhinoceros, and pig? They postulate an aquatic past for them as well. It just goes to show that you can explain away any inconvenient fact if you try hard enough. The real reason these species, including humans, lost their hair was to dissipate heat faster. If anything, we lose heat too fast when we're in the water (even tropical water), which should have made us retain our fur if we were really aquatic.

"All the curves of the human body have the beauty of a well-designed boat. Man is indeed streamlined." There's a big brown stain on this page of my copy of Morgan's book because when I first read this, I laughed out loud with a mouth full of coffee. Truly aquatic animals are shaped a lot like torpedoes. Let me know the next time you see a torpedo with long flowing hair, a slender neck, rounded shoulders, and enormous knockers. See if you can get her number for me first.

"The presence of . . . subcutaneous fat is a characteristic that distinguishes Man from the other primates."  It is true that many aquatic animals have a thick layer of subcutaneous fat, but not, as Hardy supposed, for insulation. Their subcutaneous fat is mostly for streamlining and energy storage. Fat is a much poorer insulator than it is popularly thought to be, and poorer than fur even underwater. Of course sedentary western humans tend to be fat, but they are not a fair representation of humans. Even so, it must be admitted that the human species, even hunter-gatherers, are probably quite fat compared to most other terrestrial species. Most likely this is due to self-domestication. Animals who no longer need to fear predators, including man and his domestic animals, have become much fatter than their wild relatives. The same is true of hedgehogs, whose natural protection renders running away unnecessary. Is the distribution of subcutaneous fat in humans somehow exceptional? Not at all. Sedentary zoo apes on a high-calorie diet accumulate subcutaneous fat stores in exactly the same places we do. What is exceptional is the difference between human and aquatic subcutaneous fat. Truly aquatic animals have thicker layers of fat surrounding the whole body. In humans, the fat layer is thinner and, on parts of the body, non-existent.

"It seems likely to me that Man first learnt to stand erect in the water . . ."  The idea here is that a primate tends to stand erect when wading in water, if it's deep enough. That's true, but it's not the whole story. Gorillas, bonobos, and chimpanzees all stand and walk on two legs occasionally when on dry ground. Their normal means of locomotion is knuckle-walking (i.e., walking on all fours with the knuckles of the hands on the ground rather than the palms). But the interesting part is the other apes: the orangutans and gibbons. Their normal means of locomotion is brachiating (swinging from branches). On those rare occasions when brachiators come down to the ground, they usually walk on two legs, like humans. There is a growing school of thought that the last common ancestor of humans and chimps was a brachiator and not a knuckle-walker as had been previously believed. This would require that knuckle-walking evolved twice (in gorillas and chimps) rather than once, an idea that not all experts accept. A fossil species called Oreopithecus ("swamp ape," not "creme-filled chocolate cookie ape") is cited by AAH supporters as an example of a primate that learned to walk upright by first wading in swamps. It is much more likely that Oreopithecus was a brachiator than a wader. It's worth noting that no aquatic species regularly walks on two legs when on land.

"Where are the fossil remains that linked the Hominidae with their more ape-like ancestors? . . . It is in the gap of some ten million years, or more, between Proconsul and Australopithecus that I suppose Man to have been cradled in the sea." It wasn't until recently that we knew how wrong Hardy was on this point. Since he wrote it in 1960, the gaps have been progressively filled in, most famously by Lucy in the 1970s. In late 2000 a specimen dubbed Millennium Man (Orriorin tugenensis) was discovered in Africa. It must have lived very close to the time when humans and chimpanzees diverged and fills in another important gap. It appears to have already developed upright walking, but also retained some climbing adaptations. The fossil gaps up to the last common ancestor of chimps and humans are now measured in hundreds of thousands of years rather than Hardy's tens of millions. Oddly enough, Morgan has used this point as an argument in favor of the AAH. She suggests that the gaps are so short that only something as revolutionary as an aquatic stage could account for the changes. OK, but when two contrary facts are both used in support of a hypothesis, alarms go off. Besides, the evolution of human features displayed by the fossils appears to be reasonably gradual, not abrupt. None of the fossils suggest an aquatic lifestyle. For more on the fossil record, see The Human Family Tree.

So much for Hardy's original hypothesis. Has there been anything new since 1960 that would support the AAH?

Morgan and other supporters have come up with a few new speculations, none very convincing. The presence of interdigital webbing in some people (syndactylism) is supposed to be an adaptation for swimming. In fact it is not an adaptation at all, but a birth defect, found in both humans and other apes. No one suggests that some people are born with two extra fingers as an adaptation to the base-twelve counting of our past. While doing research I came across a condition called "webbed penis," (or "penis palmatus") in which the penis is enclosed in the scrotum. God only knows what the AAH supporters would argue that is supposed to be an "adaptation" for.

Morgan makes much of face-to-face mating in humans. Dolphins, for example, do mate this way, but many aquatic species, like seals, do not. Among terrestrial mammals, our close relatives the orangutans usually mate this way, and bonobos and gorillas sometimes do.

The diving reflex (so-called) is found in all mammals to some degree, terrestrial and aquatic. It consists of a slowing of the heart rate and reduced blood flow to the extremities when the forehead is immersed in cold water. More than likely this is a protection against hypothermia, not drowning, since warm water doesn't have the same effect. At any rate, man is a poor diver even when compared to other terrestrial mammals. Untrained dogs, for example, can hold their breath and survive underwater for three minutes, compared to only about one minute for untrained humans.

Morgan claims sweat and tears may have evolved in man as a means of excreting excess salt. Even discounting the fact that excess sodium doesn't make us sweat or cry, it would be impossible, since these fluids are less salty than our inter-cellular plasma. The kidneys are the only means of removing excess sodium in aquatic and terrestrial mammals alike. Truly aquatic mammals have large and specialized kidneys that can excrete urine that is saltier than sea water. They can therefore safely drink sea water, but in practice they often get their water requirements from their food, which may be less salty than sea water. Human kidneys are of the terrestrial salt-conserving type and we die if we drink too much sea water.

Ignoring the evidence about kidneys is just one example of a common AAH tactic: Much is made of human characteristics that are supposedly aquatic, yet no mention is made of truly aquatic features we patently do not possess, such as small ears and short limbs.

There is no convincing evidence that any modern human trait evolved as a result of an aquatic past. I'm not suggesting that our ancestors never went near the water. They may have entered the water occasionally, as do many terrestrial animals. Reindeer swim across rivers, but that doesn't make them aquatic. Speaking of that, some supporters of the AAH (but not Hardy or Morgan as far as I know) suggest that tales of mermaids may constitute some sort of race memory of the time when we lived in the water. Yeah, right. And Comet and Cupid hearken back to the time when reindeer really knew how to fly.

The AAH has attracted a number of competitors, the most famous of which is the slightly less scientific Pliocene Pussy-Cat Theory (PPCT). But this and other such rejoinders leave me unsatisfied. I'm no great fan of cats, and unlike Hardy, I don't like beach holidays. I prefer to spend my vacations in the mountains, so I developed the MAH (Montane Ape Hypothesis), which is far superior to all competitors. For example, my hypothesis explains why human females have relatively large breasts. They obviously evolved as an adaptation for climbing. I dream of a return to the mountains. I dream of prehensile breasts.

FURTHER READING

The Aquatic Ape: Fact or Fiction, edited by Machteld Roede, et al.

Aquatic Ape Theory: Sink or Swim?

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