Millions of years ago, something happened to our ape ancestors that did not happen to the forebears of gorillas and chimpanzees, something that made them walk on two legs, lose their fur, sweat, develop larger brains, and learn to speak. While scientists have visited many a dig and studied many a fossil for clues, Elaine Morgan argues that all of the facts about our mysterious origins are right in front of us--in the form of fundamental flaws in the human design. Our propensity to suffer from lower back pain, obesity, varicose veins, acne, even infant death syndrome, is essentially the result of a cataclysmic event in our distant past.

Scientists have long observed that our spines were not made for upright walking. Yet natural selection--the basic tenet of evolutionary theory--dictates that enduring changes to a species occur because of the need to adapt to changes in the environment. While thousands of working hours are lost each year to "bad backs," at some point long ago it must have been an advantage to walk on two legs. The most common theory is that we became bipedal while hunting on the African savannah, needing our arms free for weapons, using an upright stance to see enemies from afar. But as Morgan points out, animals need more speed on the savannah, both for pursuit and flight, than two legs can offer. Her explanation: bipedalism emerged from life in an aquatic environment due to the flooding of the African rift valley millenia ago. The apes that suddenly found themselves stranded in swamp land (a swamp that remained for thousands of years) had to walk upright to keep from drowning. The human tendency toward obesity was once not an unsightly health problem, but rather a lifesaving form of insulation, one present in all aquatic mammals. And as Morgan carefully considers all of our other uniquely human traits--our relative hairlessness, our ability to control our breathing, our inability to maintain proper salt levels--a compelling case emerges for our human origins in a watery environment.