The Aquatic Ape Theory (AAT)

There is no doubt that Homo sapiens are much different than any other animals living on the Earth. We are a unique species in the animal kingdom. This is the reason we have always considered ourselves special and above other earthly beasts. For this reason, it is hard for people to understand evolution. Even with our closest ancestors the primates, they look very different in appearance. So what happened? What happened to our ancestors millions of years ago to set us on the course we are on today? Why do we walk upright and have relatively no hair? Why do humans get obese and sweat extensively? Why is our Larynx descended so low that we can choke on the very food that gives us life? These are some of the questions the Aquatic Ape Theory (AAT) tries to explain. Most of the classical theories about these anatomical anomalies have major flaws and problems. Even the classic “Savanna Theory” that had been accepted as fact has now been dismissed. The Aquatic Ape Theory gives us fresh perspective and new possibilities to some of the biggest mysteries about our origins.

The Aquatic Ape Theory states that approximately 6-7 million years ago, our ape ancestors were trapped in a semi-aquatic environment. This forced them to search for food in the water and become very comfortable in the water; over time altering their bodies to be more effective in the water. The theory says that a founder population of apes got isolated on an island-like setting when East Africa flooded. So if a population got isolated, they would need to take to the sea for survival. It took whales 50-60 million years to go from fully terrestrial animals to fully adapted aquatic animals. Skeptics of Aquatic Ape Theory insist that it would take 50-60 million years to see changes in physiology of a transition to becoming aquatic. The one thing that they do not understand is that AAT states that a small population of apes became isolated. Evolution acts much faster on small populations, so some changes could have easily taken place in a million years. That is not to say that we became mermaids with a whale-like tail, just that we started the transition to becoming aquatic. Some of those traits can still be seen today.

There are many facts that make this theory very plausible. We know that 6-7 million years ago East Africa flooded, creating many islands. This flooding lasted for around a million years and is the same part of Africa where famous fossils like “Lucy” have been found. We know that mutations in small groups of populations can greatly speed up the evolutionary process. This means that if a population got split off from the founding population it could evolve more rapidly. We know that as a result of becoming aquatic, terrestrial animals generally will lose their hair, become more streamlined, and retain blubber. Aquatic mammals all have blubber to help insulate and create buoyancy. For instance, whales were once terrestrial, but after millions of years they lost their legs became more streamlined, lost their hair and replaced it with blubber. Humans are the only terrestrial mammals that can consciously hold their breath. Not even chimps or other primates can do this. Having blubber or fat under your skin makes you more buoyant in water. These are some of the facts that AAT is based on.


Bipedalism has been a mystery plaguing us for a long time. It is very hard to understand why our ancestors started walking upright. Logically it makes no sense. Quadrupedal organisms are clearly faster at avoiding predators than bipeds. Our bodies are not made to walk upright. Humans have back problems are because we walk upright instead of on all fours. The spine is much better supported on all fours. So if our ancestors started walking upright because of chance mutations, nature did not select for them to die out. So it must have provided an advantage or been neutral possibly as an exadaptation (preadaptation).
Most conventional theories of why our ancestors starting walking upright include: the ability to stand up and scan the horizon for prey/predators; to carry things and use tools; and for feeding behavior. There is another mammal that stands up to look for danger and it is the meerkat. These little critters have their own show on Animal Planet. When you watch them they are constantly standing up on their hind legs to keep a lookout. However, when they see danger they drop down and run on all fours. In fact, no mammal that has adapted this behavior has ever taken more than a half dozen steps on two legs. If this theory was true it could account for an upright posture, not for locomotion. Walking and running upright is a lot different than just standing.

Carrying behavior was a very popular concept. This is an older theory that said we needed to use tools so our ancestors walked upright to free the hands. This has been disproved because our ancestors were walking upright before we ever used any tools. The other popular theory is that they were carrying food home to their family. This was a popular theory because people noticed that 1% of the time chimps walk upright to carry food. The question is what food were they carrying? Australopithecus afarensis diet consisted of nuts and berries. They would have to be carried by the handful and this would not have been worth all the trouble to carry home. Modern chimps, when carrying a larger meal, drag it behind them while walking on three legs. Some people speculate that our ancestors might have needed to carry water and food over long distances and it would be an advantage to walk upright. However, people would have needed some kind of container to carry water, and there is no evidence that they would have any idea about doing this.

The theory that our ancestors walked upright for feeding is a little outdated. Modern primates do not show any bipedal behavior for foraging. Scientist watched wild chimpanzees for over 700 hours. They noticed that 80% of the time that the chimps stood up was for feeding behavior. So it seemed the best theory. However, this fits in with the scanning the horizon theory. It might have been useful to stand to reach and get food, but it would not in any way lead to Bipedalism. No scientists agree solely on any one of these theories. Some people like to pick and mix from these theories. But the underlying fact is that something happened to our ancestors to make them start walking upright. No other primate uses this form of locomotion the way we do. The proboscis monkey lives in mangrove swamps and has been observed in the wild walking upright in the water from one feeding ground to another. This hasn’t led to the proboscis monkey becoming a biped, but is interesting nonetheless. Other apes, who are more closely related to us than monkeys, also walk upright when wading in water.

The Aquatic Ape Theory attempts to answer this question of Bipedalism. AAT says that our bodies started becoming more streamlined as a result of natural selection. This would have made our hips start to become narrower and our bodies more slender. If a population had evolved for swimming and diving, when the water receded, and they became more terrestrial, it would have been easier for them to walk upright than to go back to all fours. So AAT is suggesting that Bipedalism is an exadaptation from becoming a more streamlined swimmer.

Another of the main points of AAT is the fact that we are relatively naked mammals. The only other mammals that are naked are aquatic or underground, like whales, hippos, and naked mole rats. These mammals lost their hair for various reasons. Aquatic mammals lost their fur for insulation and gained blubber to keep them warm. Blubber is more effective in water than fur. Naked mole rats lost their fur as a result of living underground.
This is a question that people have been trying to figure out for quite some time. Most of the old theories have been dismissed, but AAT gives a reasonable explanation. Some of the old theories are: losing hair as a result of sexual selection, a result from wearing clothes, and a result of nature selecting animals that didn’t have hair where parasites could thrive.

The idea of wearing clothes to make us lose our fur is not very credible because the odds of humans wearing clothes in Africa where the temperature is very high, was not likely. Even if we were wearing clothes it would not likely lead to us losing our fur. Fur actually is a good adaptation to have in Africa. That is why primates and other animals still have fur. It is much more effective at cooling down the body than sweating with no fur. This has been tested by shaving animals and leaving them in the sun. They seemed to get hotter and sweat more. You can even look at people today; they sweat more when not wearing a shirt than when they wear one.

Sexual selection is another theory of why we might have lost our hair, however, it is quite unlikely. The concept is that chance mutations caused some early apes to lose their hair and then females selected to mate with them. In reality if this happened, they probably would not have mated with them because it would have not been normal. As to losing our fur because of parasites, you can look at modern primates. They groom each other to get rid of these parasites. Why would they not have evolved to lose their hair?

The obvious thing to do is to look at other hairless mammals and see why they lost their hair. Usually this is because of an environmental variable, like being in the water or subterranean. So what AAT states is that after a million years of selection for aquatic apes, they eventually lost most of their hair. This is the most reasonable explanation for why an organism would give up an advantageous trait like having fur for a less adaptable trait. People do have some hair but not fur. The hairs humans do have, has a very peculiar orientation. Unlike other primates, whose hairs all point down, the hairs on a human appear to follow the direction of water flowing over the body. Hair does seem to grow on other animals in relation to their life style. Most animals have downward facing hair, as if to assist in rain running off them. Animals like Camels that are not as likely to be caught in the rain, have hair that sticks out everywhere. Moles do not have hair because they need to be able to move backwards through their tunnels so dirt does not get engrained in the hair. So AAT provides a very good reason for why our ancestors lost their hair.


Another question that AAT answers beautifully is “Why are we so fat?” Of all the primates, we are certainly the fattest. This fatness is not just because we have McDonalds on ever corner. Some people are genetically predispositioned for obesity. When we are born, we are already chubby. In fact, it is our chubby cheeks that trigger parental protective processes. Other primates like chimpanzee’s infants are generally skinny. Rose Frisch stated that if a woman’s adipose tissue constitutes less than 17% of her body weight, she ceases to menstruate and cannot reproduce. Other mammals can reproduce with 2%. So there must have been a selection for this. In fact humans contain ten times as many adipocytes (fat cells) than would be expected in an average mammal of similar size.

Because we didn’t inherit these traits from primates, AAT looks at the only other animals that are liable to collect fat in this way. They are hibernating animals and aquatic animals. Nobody has yet to suggest that our ancestors slept off the winter in hibernation. It is interesting that terrestrial mammals tend to store fat around their internal organs whereas aquatic mammals store their fat on the outside, near their skin. Seals actually store 99% of their fat under the skin. In the 1950’s, P.F. Scholander reported, after studying which was a better insulator in water fur or fat, that fat was much more effective as an insulator in the water than fur. What is interesting is that humans have this same layer of subcutaneous fat. We store it just as aquatic mammals do. The question then, is if fur is a better insulator in air and fat is a better insulator in the water, why would we be naked with blubber? According to AAT this is simply because of our aquatic past.

Humans, unlike other mammals and primates, are missing the panniculus carnosus. This is a cutaneous muscle layer found throughout the skin in most mammals. It is the twitching muscle that terrestrial animals use to get rid of insects and dirt. All we have is a vestigial patch in the neck. Perhaps our skin had too much fat in it and the muscles could not work the same way and it gradually faded away. Or perhaps the fact that we could jump in the water to avoid the pests made it useless.

One of the best traits about humans is our ability to communicate verbally to each other. Because of this we became the species we are to day. We learned to teach, and record history. The reason why we have the ability to speak is because our larynx is descended past our esophagus and we can consciously control our breathing. Modern day chimpanzees, and most other mammals, cannot do this because their larynx is not descended; instead they can breathe while they eat. In fact, chimpanzees are mentally capable of speech, however they cannot do this because of their inability to consciously control their breath and they do not have a descended larynx. Their brains have a Broca's area that would enable them to speak, but they cannot physically use it.

There would not be a logical reason for our larynx to drop down. In fact it would be a disadvantage because of choking. We cannot swallow and breathe at the same time. So really the only explanation for it is for a need to speak. There are a few reasons that it is unlikely that nature selected for a descended larynx because of a need to communicate. You could ask the question why any other primates did not develop the same mechanism. Surely chimpanzees could benefit from being able to speak. The problem is, that is not how evolution works. It doesn’t try to find advantageous traits; it works through a process of trial and error. A lot of times things evolve as a result of other things and they are called exadaptations. A good example is spongy bones in birds. The small dinosaurs, that eventually gave rise to the birds, developed spongy bones as a result of running from predators. So the lightweight bones that helped the dinosaurs survive ended up being beneficial to birds for flight. They did not evolve for the purpose of flight, but they certainly helped it happen. AAT attempts to answer this anomaly in a similar fashion. They say that natural selection selected for these aquatic apes to be able to come up for air. Breathing in air through your mouth is much more effective than through your nose. So by being able to come up for a big gulp of air would be more efficient than trying to do this through the nose. The conscious control of breathing would have come from this diving behavior. Then millions of years later these seeming disadvantages on land enabled us to be able to speak.

The questionable problem about this idea is that there are so few animals that have a descended larynx. However, the ones that do have one are aquatic. The walrus and sea lion are two examples that have this trait. Not all aquatic animals have this function. Whales actually have a larynx that moved up instead of down. The reason that this is significant is because Homo sapiens are the only terrestrial animals that have a descended larynx. Also, there is no advantage to mouth breathing. In fact, it can lead to illness by not filtering air through mucus in the nose; breathing in pathogens that don’t get filtered out, and go right into the lungs. People are also the only animals that can choke on their own vomit. The descended larynx is also the cause of sleep apnea. So if there are so many disadvantages to have a descended larynx, why would nature not select for these animals that could choke on their food? AAT is the best theory for this. There is not any other theory that can stand up to it like AAT.


Another interesting aspect about our bodies that makes us unique in the terrestrial world is our sweat and tears. Humans, even with out exercise and in temperate climates, have to drink more water than any other terrestrial animal. Dehydration of 10% may be fatal to humans, whereas most animals can go up to 20%. So we cannot go too far from water in order to survive. Humans waste more water than any other land animals on earth.
All primates and most land animals pant as a result of thermal regulation. Humans are the only animals that seem to have lost this ability. We pant when we exercise, but this is to deliver oxygen to the lungs not to cool down. Our sweating is how we cool down. We are one of the few animals that use eccrine glands for cooling down. Other apes and animals have these glands; however, they do not use them for cooling down. In most mammals the eccrine glands are connected to hair follicles. Humans no longer have these glands connected to the hair follicles, however, we have many of them spread out through our skin and they secrete sweat directly onto the skin. We also have twice as many eccrine glands than other apes. One of the major downfalls of this type of cooling is that it takes up to 20 minutes to activate. It is also a wasteful type of sweating compared to the sweating of sheep and horses.

In it’s beginning AAT thought that perhaps these glands evolved as a result of being in salt water all the time and having excess salt in our bodies. This would sound logical to adapt this method of salt removal. However, it has been researched and we have determined that it is not possible that we used sweat and tears for salt removal. Sweating is essential to cooling our bodies, but our kidneys do much of the work for salt removal. But this does not mean that this excessive sweating matter didn’t arise because of our aquatic past. Basically, if AAT is true and it led to our Bipedalism and nakedness, when the water receded and we became more terrestrial we would need a new adaptation for cooling down our bodies. As a result, our bodies would need to start producing more eccrine glands to sweat and cool down. So indirectly we are such a leaky animal because of our aquatic pasts.

There are many other oddities with the human body that are interesting and worth mentioning. The Dive reflex is common in many animals. When animals are submerged in water, their heart rate slows down, and the blood supply to the skin lessens so that organs that need oxygen can have more available. This is fairly common in most animals from ducks to humans. It is basically a survival reflex that the body performs. In fact, sharks and fish do the same thing when you take them out of the water. Aquatic animals show this effect stronger than terrestrial animals. So over time the dive reflex evolved to be stronger. They decided to do a comparison test between humans and other animals. They concluded that a well trained diver had the same dive reflex of a semi-aquatic animal like a beaver or an otter.

Human noses are very interesting. It seems to be designed for a person to dive into the water or swim underwater without having water being forced into the nasal passages. We know that the nasal spine didn’t start showing in hominids until Home erectus about 2.5 million years ago. It could be argued that the downward pointing nose didn’t evolve until then. Fleshy appendages do not fossilize, so if you saw the fossilized skull of an elephant, there would be no way to tell that it had a trunk, unless you knew what one looked like today. The same thing could have happened with the nose. The aquatic environment could have selected for a downward facing nose for swimming through water, and then over time the boney structure formed for additional support.

Humans also have a strange embryological seam called a philtrum. This is the little channel that runs from your nose to your upper lip. No other primates have this strange feature. Some people are able to close their nostrils by pushing their upper lip up against their nose. It actually fits perfectly. This can give you an advantage underwater or when jumping into water to prevent water from going into the nose.

Human Blood is very interesting. It is known that aquatic mammals have a reduced number of blood cells per unit volume. Chimps have an average of 7.3 million red cells per cubic millimeter; gorillas have 6.3 million; and humans have 5.1 million. Also in regards to the hemoglobin that carry oxygen to the lungs, aquatics have a higher content of hemoglobin per cell. Chimps have on average 12.2 per cell; gorillas have 13.2 per cell; and humans have 18.6 on average. The increase in hemoglobin for aquatic animals is very advantageous for carrying more oxygen throughout the body. Perhaps the reason we have an increased number is because of an aquatic past.


The Aquatic Ape Theory attempts to answer a lot of unanswered questions that have plagued mankind about our heritage. The answers may be speculative, but no more than all of the other theories. The simple fact is that we love the water and feel comfortable in it. Some women even prefer to give birth in the water. We have swimming pools and Jacuzzis for recreation in the water. People have always been fascinated by the ocean and it has inspired some of the greatest stories and adventures. 90% of all life on earth evolved in the water, so is it unreasonable that our ancestors might go back to the most abundant food source in the world, the ocean? Something in our past changed us and put us on a course to what we are today. Our ancestors started walking upright on their hind legs becoming bipedal; the hair on their bodies changed direction and they became naked; the larynx descended; they became fatter; they forgot how to pant; they gained more eccrine glands for sweating; their nostrils started to point downward; and they learned to speak. The Aquatic Ape Theory explains how all of these things could have happened.

By Dan Blizniak