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The Dikika fossil also hints that brain development may already have started to take longer, a change that prolonged the dependence of human young on their parents. From the Dikika baby's teeth, the team estimated her age at three years; her brain, preserved as a sandstone cast inside the skull, had a volume of about 330 cc—roughly the same as a small three-year-old chimpanzee's. This could mean her brain was growing no faster than a chimp's, so it might have taken longer to reach its adult size, slightly larger in an australopith than in a chimp.

During human evolution, ever longer brain growth led to the extended period of dependence we call childhood. In most mammals, including other primates, the young move on to forage for themselves after they finish nursing. In the Dikika baby, Zeresenay already sees hints of this uniquely human life stage. "This is extraordinary," he says. "We've captured a moment in time for an individual, but also a moment in the life history of a species."

A cascade of other changes may have begun around the same time. "It's no good growing a big brain if you don't have a long life span," says Holly Smith, an expert on hominin development at the University of Michigan. "You need that for the investment in a big brain to pay a return." She sees the beginning of a longer childhood as a sign that human ancestors were also living longer than their ape cousins, a trend that ultimately led to humans outliving other apes by decades.

Growing bigger brains had other consequences. Gray matter is the gas-hog of our bodies. A fifth of the calories you consume go to fuel your brain. Within a million years of the Dikika baby our ancestors learned to supplement the mostly vegetarian diet of Lucy and her kin with nutrient-packed meat, devising stone tools to strip flesh and crack bones for the protein-rich marrow. Good nutrition made even bigger brains possible. And that led to more inventions, and then bigger brains. The rest is history.

The Dikika baby's biography is short, but the evolutionary steps she embodied have had profound and enduring effects. Although bipedalism and big brains carried a high cost, particularly for the mothers of our lineage, these traits ultimately combined to produce smarter babies who would eventually be able to master technologies, build civilizations, and, yes, explore their own origins.

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