Why do humans differ from other primates? What do those differences tell us about human evolution? Elaine Morgan gives a revolutionary hypothesis that explains our anatomic anomalies—why we walk on two legs, why we are covered in fat, why we can control our rate of breathing. The answers point to one conclusion: millions of years ago our ancestors were trapped in a semi-aquatic environment. In presenting her case Elaine Morgan forces scientists to question accepted theories of human evolution, while expressing complex ideas for the general reader in a clear and accessible style. A documentary by Sir David Attenborough, The Waterside Ape, is based on Morgan's Aquatic Ape Hypothesis.