Not everyone is comfortable with the idea that humans are animals, or that we're apes. But the fact is, the cells that make up our bodies have nuclei and organelles such as mitochondria, we're capable of locomotion, and unlike plants we consume other organisms to survive. That's all it takes for an organism to be classified as an animal. And we're mammals, primates to be exact, with large brains and no external tails. That puts us in the class of hominids along with our closest cousins: the gorilla, chimpanzee, orangutan, and gibbon.
But humans do possess a number of unique attributes in our form and structure, the most obvious being that we are obligatory bipeds: We walk on two legs, and we don't have much choice about it. We're not the only large vertebrates to walk on two legs-dinosaurs, birds, and kangaroos are bipedal, but they're like teeter-totters, with their upper and lower bodies balanced over the fulcrum of their hips. Humans are like pogo sticks, with our heads balanced precariously atop a double-curved spine. This anatomy is unique in all the animal kingdom.
Our form of bipedalism comes with many drawbacks that four-legged animals don't suffer from. Fallen arches, shin splints, hernias, and back problems are all caused by walking upright. Given the high price we pay for walking on two legs, it's tough to imagine what original, critical advantage was gained by our proto-bipedal ancestors, whose bodies were even less adapted to the rigors of bipedal locomotion.
Why we became bipedal is mystery enough, yet other oddities are even harder to explain!
Our unique human qualities also include being bald and chubby. We are nearly hairless, and to the detriment of our self-esteem, we carry a high body-fat content compared to most mammals. Much of the fat is stored just under our skin. Also unusual is that humans can control breathing beyond the capabilities of most mammals.
Enter Elaine Morgan, a feisty Welsh feminist and writer. In the early 1970s, Morgan began to develop and promote a controversial hypothesis seeking to unite a number of human oddities within a single explanatory framework. Her hypothesis is that human ancestors lived in close proximity to water for extended periods, and spent so much time beach-combing, wading, and diving for foodstuffs that they evolved to suit their environment. We're not merely apes, we're Aquatic Apes!
The Aquatic Ape Hypothesis is astonishing, but Morgan makes some good points. For starters, take bipedalism: If a chimp tried to maintain an erect posture, the physiological consequences would not bode well for the animal. Over time it would incur problems keeping its blood pressure up, and suffer skeletal damage as it repeatedly moved from an upright to a reclined position. But if a chimp or a gorilla were wading on two legs and supported by water, those problems would be greatly reduced or eliminated. And there is an immediate survival benefit for a bipedal ape wading in three or four feet of water: The animal would have its head above the surface and be able to breathe! Given an immediate benefit, a new food supply to exploit, and the advantages of walking on two legs in water, natural selection would have a platform from which to work-and perhaps eventually craft apes that were obligatory bipeds.
The Aquatic Ape hypothesis explains our high body-fat content as more than energy storage: It represents critical insulation. Body hair prevents heat loss only when an animal is dry; however, a wet, furry mammal loses heat almost as fast as one with no hair. Among aquatic mammals-whales, walruses, and seals-hair is sparse, just as it is on our bodies. Most large mammals store subcutaneous fat for one of two purposes: seasonally for hibernation, or year-round because they're partially or fully aquatic.
Is the Aquatic Ape hypothesis valid? Any useful scientific hypothesis must make predictions that can be tested. If these predictions are validated through observation and experiment, then the hypothesis gradually becomes a scientific theory. The more data the theory unifies under a single coherent explanation, and the more successful its predictions, the stronger it becomes. If the evidence comes from independent sources that all interlock with the theory in a consistent manner, and this consistency keeps up as more and more information is discovered, then that theory will become part of the scientific consensus and you really have a winner.
Charles Darwin's original idea is an example of a hypothesis that became a theory. The evidence for common descent, one of the key predictions of the theory of natural selection, includes mountains of empirical data from the fossil record, molecular biology, and physiology. Common descent is so solidly supported by so many independent lines of evidence that it's considered an inferred fact by almost all scientists today.
To explain a few existing anatomical structures and physiological processes, the Aquatic Ape Theory is satisfactory. But we have little other evidence to support it.
In particular, the fossil record does not advance the Aquatic Ape conjecture. We have only a few fossil scraps of human ancestors from the time before bipedalism was well developed three million years ago, represented by the archetype A. afarensis, a.k.a. Lucy. And that's the critical period when an aquatic ancestral phase would have had to exist for it to explain the origin of bipedal locomotion. Even if we had a complete skeleton from the exact time and place required, how would we distinguish a partly aquatic hominid from a close relative that was not aquatic at all? It would be tough to peg a sea otter, a beaver, or a polar bear as partially aquatic from fossils, if we'd never seen such an animal in the flesh.
Some scientists think that the case Morgan makes has been overstated. Maybe there is a bit of elitism going on with a few of her critics; Elaine Morgan is not a paleontologist or an anthropologist by training. But some of the critics also put forth alternative explanations for the anatomical congruencies between humans and aquatic mammals. For example, people do store more fat than most of our land-dwelling mammalian relatives, but maybe that's because it's an effective reservoir of fluids, energy, and critical trace nutrients, all of which would be useful for a creature that moved from the steamy jungle to the arid plains.
For now, the Aquatic Ape scenario remains an intriguing hypothesis and not much else. But science moves in mysterious ways. One can never predict what will be found next.
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