What is the Aquatic Ape Hypothesis?

The Aquatic Ape Hypothesis (AAH) is a speculative theory about human evolution, which suggests that a stage of human ancestry involved an adaptation to a semi-aquatic or aquatic lifestyle. This hypothesis was primarily popularized by Elaine Morgan, a Welsh writer, in several books starting from the 1970s. The key arguments of the AAH are based on certain anatomical features in humans which are atypical of terrestrial mammals but common in aquatic ones. These features include:

1. Bipedalism: The hypothesis posits that wading in water led to bipedal locomotion, as moving on two legs is more efficient in shallow water. This contrasts with the traditional view that bipedalism evolved for savannah living.

2. Subcutaneous Fat: Humans have a layer of subcutaneous fat, similar to marine mammals, which is not found in other primates. Proponents of AAH argue this fat layer evolved for insulation in water.

3. Loss of Body Hair: The AAH suggests that humans lost their body hair to streamline for swimming, much like other aquatic mammals. This is in contrast to the conventional view that hair loss was an adaptation to heat regulation in savannah environments.

4. Nasal Conformation: The shape of the human nose, which helps to prevent water from entering the lungs, is also cited as evidence of an aquatic past.

5. Voluntary Breath Control: Humans, unlike most terrestrial animals, have voluntary control over breathing, a trait shared with aquatic mammals, which is necessary for speech but also beneficial for diving and swimming.

6. Omega-3 and Omega-6 Fatty Acids: The importance of these fatty acids, which are abundant in aquatic food sources, in human brain development is also used to support the hypothesis.

7. Descended Larynx: The descended larynx in humans, unusual among primates, is similar to aquatic mammals, and it's proposed to have developed for air retention during diving.

Despite these arguments, the Aquatic Ape Hypothesis is not widely accepted in the scientific community. Mainstream anthropologists and evolutionary biologists often criticize it for lacking empirical evidence and for being inconsistent with the fossil record. They argue that most of the anatomical features cited by the hypothesis can be explained through other evolutionary pressures, such as environmental changes or social adaptations.

Furthermore, the hypothesis doesn't align well with the known timeline of human evolution, which is well-documented through a robust fossil record. While the hypothesis raises interesting questions about certain human traits, it remains a fringe theory due to its speculative nature and lack of substantial supporting evidence.