The most common theory of human evolution suggests that humans evolved from an ape-like ancestor. But what if we evolved from something that was a little more like a seal or dolphin? Over the years, a handful of evolutionary biologists have proffered the weird theory that we evolved from "aquatic apes" who lived in the water for a few hundred thousand years. Let's take a dive into one of the worst ideas in the history of evolutionary theory.
Tucked away among the many theories on the quirks of evolution is the accurately-titled Aquatic Ape theory. Although it is what has to be one of the best B-Movie titles of all time, the aquatic ape theory has not gained much popularity over since its inception. Why do its proponents believe that our ancestors lived more like porpoises than chimps?
The Template for Going Back to the Sea
Humans wouldn't be the first mammals to wade back into the water when the going got tough on land. Seventy million years ago giant four-footed creatures shuffled back into the water, Their hind quarters turned into tails and they became whales. Fifty million years ago, big elephant-like creatures waded in. Their front legs became flippers and they became manatees. Twenty-five to thirty million years ago bear-like animals scrambled down the banks into the sea. Their limbs got modified and now they scramble in and out of the water as seals and sea lions. We were due for a dip.
And there are plenty of animals on land that hop in and out of the water. Beavers, hippopotamuses, water shrews, and many other kinds of mammals live their lives half-in, half-out of the water. If we had an aquatic ancestor, it would not be an anomaly, but part of a well-established model followed, to many different degrees, by many other mammals in evolutionary history.
Why Would Anyone Think We Went Aquatic?
Just because mammals can adapt to water doesn't mean that people actually did. Adding a sojourn in the water seems to needlessly complicate human evolution. What possible evidence is there for humans spending time in the water? One of the primary arguments for the theory is the puzzling fact of human hairlessness. (Or rather - hair thinness. We have the same amount of hair as apes. Ours is just so thin and short that it doesn't count.) Primates tend to have hair, especially the more human-like apes. Why did humans lose their coat? Each argument - parasites, neoteny, and sexual attractiveness - has its own down side. If parasites were the culprit, why did only humans shed their fur. Chimps and bonobos, so much like humans and living in the same environment, would surely have benefitted from less parasites as well. If neoteny - the retention of juvenile characteristics into adulthood - is the reason, why would human fetuses grow hair and then shed it? And although existing characteristics are exaggerated by virtue of sexual selection over generations, new characteristics are rarely founded in this way.
Take a dip in the water, though, and it's easy to see why humans may have lost their hair. Dolphins and whales have lost theirs to increase their streamlined silhouette and move through the water easier. Hippos elephants, and pigs, all well-known wallowers in swimming holes have lost their hair as well. These might have been hairy at one time, but spending a few generations with sopping fur would have selected the hair off of them quickly. As long as the hair doesn't provide insulation against the water itself - which might not be a great thing in a hot climate - it makes sense to get rid of it. It's also theorized that humans can grow the hair on their head long because mothers, wading in water, can breastfeed their child, who is clinging to their hair.
In conjunction with the loss of hair is the gain of fat. Specifically, fat that runs all up and down the body, under the skin. While plenty of land animals can grow fat, they generally store their fat around their abdomen. The rest of the body stays as a constant level of fat. Humans have cheeks and thighs and even ankles that get fatter as they gain weight. Biologists have argued that the loss of hair necessitated the gain of subcutaneous fat, but then why the loss of hair in the first place? Why lose a necessary and functional adaptation in the first place - if the only way to make up for it is to compensate with something else?
Bipedalism is also a unique characteristic of humans among primates, but explainable if early humans lived a significant portion of their lives in the water. When the few monkeys that do wade into the water go, they often wade in up to their waist. This leaves the head above water. The more comfortable an early human would be with bipedal motion, the farther into the water he or she could venture. And when aquatic animals swim, the position is the same as standing upright, with the head lined up with feet. It's just not as efficient to dog-paddle.
The move to water has been used, in part, to explain other disparate quirks. Our lack of powerful noses might be because it's hard to follow a scent trail in the water. The ability of babies to swim, or naturally hold their breath, so long as they are introduced to the water early indicates that maybe childbirth was done at sea. It's even thought that the loss of scent cues, and the loss of reliable lines of sight, might have caused humans to make sounds that could carry both underwater and above the sound of the waves. After all, whales sing and dolphins call each other by name. And for those who are sentimental, there's the fact that humans are the only primate that sheds tears, while sea birds and seals are known to weep at both physical and emotional stimulus.
When Did We Go Into the Water and When Did We Come Out?
About four million years ago, sea water started rising in the area now known as Somalia and the Arabian Peninsula. At the same time, the land dried up, pushing back the forests and shrinking the available resources. A group of early hominids, possibly Australopithecines, were stranded on islands. In the high peaks of those islands were apes who were competition. On the shores was little-to-no resources. Their only options were to take to the sea. Wading at first, the ocean became more and more a part of this ape's lifestyle, shaping its posture, its vocal cords, its senses, its child-rearing techniques, and its body hair. As the seas retreated, and land became more available, it used its ocean-acquired adaptations to spread along rivers and up onto land once more, displacing other early hominids, and becoming the first kind of animal that would come to be human.
Arguments Against the Aquatic Ape
Sure, it could have happened. But did it? There are several compelling reasons against the idea. The first is physiological. Humans don't just lose their hair to shed heat. They lose water to shed heat as well. That water gets shed through the skin. So far it seems that this would be as easily done in water as on land, but it's not quite the same principle. Everyone knows the blessedly cool feeling of lifting a sweaty body part up to a breeze. The evaporation is what chills us, and the evaporation happens in air, not in water. Humans might have lost hair like dolphins, but why would humans develop a mechanism that would be useless in the very medium that caused them to shed hair in the first place?
There's the fact that hairlessness is often seen in the rhino, and the rhino, unlike the hippo or even the elephant, isn't much of an aquatic animal. Sure, it's been know to roll around in mud, but that doesn't a swimmer make. In fact, the Sumatran rhino, the best swimmer of them all, which lives in marshes, has a fur coat of red-brown hair. Plenty of pigs, like wild boars, have hair as well.
The most compelling argument against the Aquatic Ape is a lack of evidence. Where are the intermediary stages of this Aquatic Ape? Where are the bones embedded in sea shores with shells? Where are the teeth? If the timeline of the theory is right, they were under water, but with the retreat of the oceans should be found along coastlines. Coasts can be boons for finding fossils, as bones or shells pressed into the sand can be slowly exposed by the waves. But no bones have been uncovered. All we have is behaviors that might be explained many other ways.
In the end, this lack of fossil evidence is why the Aquatic Ape theory isn't generally pursued by the scientific community at large. It can still remain a quirky theory though. And it certainly could still be a fantastic horror movie.
Top Image: Willyam Bradberry/Shutterstock
Hippo Image: Patrick Gijsbers
Sumatran Rhino Image: Ltshears