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Aquatic Ape Theory Overview & Hypothesis

Melissa Bialowas, Stephanie Gorski
  • Author
    Melissa Bialowas

    Melissa Bialowas has taught preschool through high school for over 20 years. She specializes in math, science, gifted and talented, and special education. She has a Master's Degree in Education from Western Governor's University and a Bachelor's Degree in Sociology from Southern Methodist University. She is a certified teacher in Texas as well as a trainer and mentor throughout the United States.

  • Instructor
    Stephanie Gorski

    Steph has a PhD in Entomology and teaches college biology and ecology.

Learn about the aquatic ape theory (AAT), also called the waterside hypothesis. Learn about Alister Hardy and his life and works in human evolutionary biology. Updated: 02/06/2022

Evolutionary Theory

Evolution is the scientific theory that all living things on Earth originated from other species and small changes and modifications in genetics are responsible for the large amount of diversity observed today. Charles Darwin, an English naturalist, agreed with evolution and provided scientific explanations. His explanation was based on the concept of natural selection. Natural selection happens when individuals have more useful traits, and the trait is selected for over multiple generations. This led to the modern theory of evolution. By the 1960s, molecular biology had advanced enough to investigate genes and compare them among species, which strengthened the theories of both natural selection and evolution.

Basic evolutionary theory says that animals with an advantage will be selected for in nature. For example, giraffes with a longer neck had more access to food than those with a shorter neck. The long neck giraffes would live longer or be healthier and more available to produce offspring. This trait would be selected for over a number of generations. Small changes, like neck length, add up over time. As groups of animals split into different geographic locations, they likely develop differences and eventually become different species. One example of this is the distinct similarities and differences between the African elephant, the Asian elephant, and the woolly mammoth.


Charles Darwin is well known for his research into evolution and natural selection.

Black and white portrait of Charles Darwin


Charles Darwin's research led him to believe four things about evolution.

  • More individuals are produced than can survive in any generation.
  • Variation within phenotypes exists among individuals and the variations are inheritable.
  • Individuals with traits better suited to the environment survive more than those without.
  • Reproductive isolation creates new species.

The Workings of Evolution

There are four basic mechanisms of evolution.

  • Natural Selection - This is what Darwin is famous for scientifically studying. Based on the environment, some traits are selected for and passed on to a greater percentage of the population.
  • Mutation - This is the change in the building blocks of DNA. They are usually small changes and rarely recognized from one generation to the next. This is what provides the individual variations within each population.
  • Genetic Drift - This is the chance of a trait being removed or greatly decreased in the population without relation to the helpfulness of the trait. For example, if the population with the traits best adapted to drought were all removed due to something unrelated, like a volcanic eruption, this would create genetic drift. Suddenly, the remaining population would not be as adapted to drought.
  • Gene Flow - This is the interaction and mixing of various populations. For example, when a seed travels by wind over a long distance, it has a high gene flow. When a seed is limited to only mixing on an island or other limited area, it would have a low gene flow.

This history of all species is a mixture of these basic factors. The movement of populations to new locations, the natural disasters that have taken out traits, the mutation of DNA, and of course, natural selection. Molecular biology and the study of how closely related DNA from various species are, allowed for a better understanding of the evolution of all living beings.

Blind Spots in Science

Scientists, at their core, are human. That isn't always a bad thing, but it means that sometimes the questions scientists choose to ask, or the places they choose to look for answers, are a result of their prejudices.

For example, Charles Darwin suggested that the earliest ancestors of humans probably lived in Africa. He was absolutely right, but racist attitudes of the time meant people were reluctant to look for our earliest ancestors there. Many people even fell for cheap pranks, such as the Piltdown man, a phony hominid fossil found in the United Kingdom. After decades of exhausting their search in Europe and the Americas, fossil-hunters chose Asia as the birthplace of humans. It was only in the late 1940s that scientists began accepting that humans evolved in Africa.

Alister Hardy

Alister Clavering Hardy was born in Nottingham, England, on February 10, 1896. After serving in WWI, he graduated with distinction in 1921 from Oxford, with a degree in zoology. While working he became fascinated by plankton. He became a naturalist studying the relationship between herring and plankton in the North Sea. Then he traveled to the Southern Ocean to study how plankton and krill affect the whales that eat them. During this time, he invented the Continuous Plankton Recorder (CPR). It allowed any shop to record plankton levels. This is still in use today recording the world's plankton distribution.

From 1928-1961 Hardy worked as a professor at three different universities. He retired in 1961. In March of 1960 he proposed that humans are different from other primates because humans evolved in water. He claimed the idea occurred to him three decades earlier, but he was afraid to publish it and ruin his career. After making headlines for his proposal, he quickly published an explanation, known as the aquatic ape theory, that was not accepted by the scientific community. It should also be noted that a very similar theory was proposed and published in 1942 by Max Westenhofer, which also was not accepted by the scientific community.

After retirement, Hardy focused his time on a study of religion. He was interested in the role religion has in the evolution and survival of humans. Alister Hardy passed away from a stroke in 1985.

Aquatic Ape Theory

Aquatic ape theory (AAT) started with Westenhofer in 1942, or Hardy in 1960, but it did not stop there. Elaine Morgan composed books and documentaries on the theory in the 1970s. By 2009, Richard Wrangham and colleagues wrote about an updated and revised version of aquatic ape theory in the American Journal of Physical Anthropology. While not accepted by the overall scientific community, there have been a continual small group of scientists supporting this theory.

The aquatic ape theory is based on the differences in the developments of humans versus other primates. Humans are bipedal, have subcutaneous fat, are relatively hairless, have a high brain to body size, and the location of the human larynx. The theory states that the ancestors to humans once moved into the water as a way to survive. It is easier to move in water on two legs rather than four. This developed larger leg muscles so that eventually humans could walk bipedally on land.

Humans have a layer of fat that is not found on other primates. This layer of fat is often found on aquatic mammals, though. This is believed to help humans handle a greater variety of temperatures. While other land-based mammals are covered in hair, humans have less hair and the hair follows the flow of water across the body. This pattern of hair helps increase swimming speeds and diving in aquatic animals.


The human brain looks similar to primate brains, but the ratio of brain size to body size is not as closely related as one might expect.

Human brain surrounded by four primate brains


Humans have a large ratio of their brain size to body size. After humans, aquatic mammals have the closest ratio. This is closely followed by apes, elephants, squid, and birds, though. In land mammals, the larynx is found in the nasal cavity, but in humans and some aquatic mammals it is found in the throat. This is believed to give more control over breathing. The sweat gland structure of humans is also different than those found in land-based mammals.

The aquatic ape hypothesis is that our ancestors, water apes, spent most of their life in water. They ate a great deal of aquatic food and developed physical characteristics more like aquatic mammals than the land-based mammals. After the Pleistocene, these ancestors returned to land as bipedal omnivores. This hypothesis is also supported by the fossil remains being found near water at a much higher rate than the fossils on land.

Man the Hunter?

If you've seen illustrations of 'the evolution of man,' you may have noticed that they almost always portray a man - specifically, a burly hunter. This is another prejudice based on early misunderstandings. There were some early ideas that humans developed bipedalism (the habit of walking on two feet), hairlessness, large brains, and other distinctly human features for hunting. The evidence simply has not panned out for this. But it's a compelling image for people who like to imagine their ancestors as early mammoth hunters. Besides physiological evidence and fossil evidence that this isn't accurate, these images forget fully half the population. What were women supposedly doing at that time?

What were the women doing all this time?
What were the women doing all this time?

Aquatic Apes

These cartoonish representations of macho 'cave men' were part of what attracted some feminists to the Aquatic Ape Theory, often abbreviated AAT, or the Aquatic Ape Hypothesis. But they were not the only reason.

Proponents of AAT suggest that our earliest ancestors lived around seashores and spent much of their time in shallow waters. Proponents of AAT suggest that many puzzling anatomical features of humans could be explained by the assumption that we evolved near the water.

For instance, bipedalism is difficult to understand because we're the only animals that have adopted it as a permanent lifestyle. You already know that bipedalism isn't an easy lifestyle, because you probably know quite a few people with back problems! The AAT suggests that bipedalism would be much easier to explain, and less punishing to the human body, if early humans spent a lot of time under water.

Humans also share several unusual features with aquatic mammals. Aquatic animals have extra fat cells and layers of fat that attach directly to the skin. So do humans, but most land mammals have much smaller fat deposits that are centered near the internal organs. Land mammals have thick coats of fur, unlike aquatic mammals - or humans. Humans also have unusual sweat gland physiology and waste a lot of water when they sweat, which would make sense if they lived in an environment where they did not have to conserve water.

Or Not?

The Aquatic Ape Theory interested marine biologist Alister Hardy since 1930, but he did not present it to the public until 1960. It was popularized by the author Elaine Morgan in the 1980s and 1990s. How has the idea developed since then? Not much at all.

Critics of AAT say that it does not read like a scientific proposal at all. Instead, it sounds like a really interesting story that is believed with religious fervor in its handful of adherents. In the decades since it was first suggested to the scientific community, the AAT has produced little in the way of fossil records, quantitative data, or any sort of hard evidence that would support it. It's telling that AAT's main proponent, Elaine Morgan, is a screenwriter rather than a trained scientist.

Critics also note that the weird examples cited by AAT have much more likely explanations. For instance, there is more evidence that humans evolved hairlessness to avoid parasites than that they evolved hairlessness for swimming. Fat distribution in humans, which AAT compares to fat distribution in aquatic animals, differs in important ways - shape, for instance, and how it is distributed over the lifetime of the organism. Fat distribution is more easily explained by sexual selection (some squishy parts look good) and by the sedentary lifestyle of many humans (monkeys get fat in captivity, too).

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Additional Info

Blind Spots in Science

Scientists, at their core, are human. That isn't always a bad thing, but it means that sometimes the questions scientists choose to ask, or the places they choose to look for answers, are a result of their prejudices.

For example, Charles Darwin suggested that the earliest ancestors of humans probably lived in Africa. He was absolutely right, but racist attitudes of the time meant people were reluctant to look for our earliest ancestors there. Many people even fell for cheap pranks, such as the Piltdown man, a phony hominid fossil found in the United Kingdom. After decades of exhausting their search in Europe and the Americas, fossil-hunters chose Asia as the birthplace of humans. It was only in the late 1940s that scientists began accepting that humans evolved in Africa.

Man the Hunter?

If you've seen illustrations of 'the evolution of man,' you may have noticed that they almost always portray a man - specifically, a burly hunter. This is another prejudice based on early misunderstandings. There were some early ideas that humans developed bipedalism (the habit of walking on two feet), hairlessness, large brains, and other distinctly human features for hunting. The evidence simply has not panned out for this. But it's a compelling image for people who like to imagine their ancestors as early mammoth hunters. Besides physiological evidence and fossil evidence that this isn't accurate, these images forget fully half the population. What were women supposedly doing at that time?

What were the women doing all this time?
What were the women doing all this time?

Aquatic Apes

These cartoonish representations of macho 'cave men' were part of what attracted some feminists to the Aquatic Ape Theory, often abbreviated AAT, or the Aquatic Ape Hypothesis. But they were not the only reason.

Proponents of AAT suggest that our earliest ancestors lived around seashores and spent much of their time in shallow waters. Proponents of AAT suggest that many puzzling anatomical features of humans could be explained by the assumption that we evolved near the water.

For instance, bipedalism is difficult to understand because we're the only animals that have adopted it as a permanent lifestyle. You already know that bipedalism isn't an easy lifestyle, because you probably know quite a few people with back problems! The AAT suggests that bipedalism would be much easier to explain, and less punishing to the human body, if early humans spent a lot of time under water.

Humans also share several unusual features with aquatic mammals. Aquatic animals have extra fat cells and layers of fat that attach directly to the skin. So do humans, but most land mammals have much smaller fat deposits that are centered near the internal organs. Land mammals have thick coats of fur, unlike aquatic mammals - or humans. Humans also have unusual sweat gland physiology and waste a lot of water when they sweat, which would make sense if they lived in an environment where they did not have to conserve water.

Or Not?

The Aquatic Ape Theory interested marine biologist Alister Hardy since 1930, but he did not present it to the public until 1960. It was popularized by the author Elaine Morgan in the 1980s and 1990s. How has the idea developed since then? Not much at all.

Critics of AAT say that it does not read like a scientific proposal at all. Instead, it sounds like a really interesting story that is believed with religious fervor in its handful of adherents. In the decades since it was first suggested to the scientific community, the AAT has produced little in the way of fossil records, quantitative data, or any sort of hard evidence that would support it. It's telling that AAT's main proponent, Elaine Morgan, is a screenwriter rather than a trained scientist.

Critics also note that the weird examples cited by AAT have much more likely explanations. For instance, there is more evidence that humans evolved hairlessness to avoid parasites than that they evolved hairlessness for swimming. Fat distribution in humans, which AAT compares to fat distribution in aquatic animals, differs in important ways - shape, for instance, and how it is distributed over the lifetime of the organism. Fat distribution is more easily explained by sexual selection (some squishy parts look good) and by the sedentary lifestyle of many humans (monkeys get fat in captivity, too).

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Frequently Asked Questions

What does the aquatic ape theory state?

The aquatic ape theory states that human ancestors previously lived in an aquatic environment. This opposes the more common belief that human ancestors evolved from land based mammals.

Is the aquatic ape hypothesis true?

The aquatic ape hypothesis is supported by a small number of scientists. It is not accepted by the scientific community overall.

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