Bipedalism is a quintessentially human trait that distinguishes us from other primates. But in some members of one Turkish family, that trait is strangely missing: Five mentally retarded siblings, aged 19 to 35, walk almost exclusively on all fours. Neurophysiologist Uner Tan of Cukurova University in Turkey documented their primitive language, limited intelligence, and quadrupedal gait. Even more bizarre than the family itself is Tan's interpretation. He argues that a freak reverse mutation "de-evolved" the siblings into a more primitive human state.

Tan invited Nicholas Humphrey, a London School of Economics psychologist, to conduct his own study. Humphrey derided Tan's claims as "just plain wrong," but then introduced a controversial idea of his own. The true importance of the Turkish family, he says, is that the siblings move relatively well on their feet and their palms, which suggests that early humans also walked that way. That conclusion clashes with the prevailing view that our ancestors leaned on their knuckles, like modern-day apes.

Much of the newspaper coverage about the hand walkers focused on the sensational side. "Their affliction is grotesque, disturbing, and like something out of a Victorian freak show," wrote the Daily Mail, a British tabloid. Even less dramatic reports generally accepted Tan's and Humphrey's explanations at face value. "All [the scientists] agree that the family's walk, described as a 'bear crawl,' may offer invaluable information on how our apelike ancestors moved," wrote The Times of London.