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He doesn't know how the Dikika baby died, but the river must have rapidly buried the body in pebbles and sand, protecting it from scavengers and weather before gradually hardening into rock. While most hominin fossils have to be glued together from hundreds of fragments, Zeresenay faced the opposite challenge. He had to etch away hard sandstone with a dentist's drill, navigating between tiny vertebrae and ribs so anatomical details could be seen. "I cleaned it grain by grain," he says. "You don't want to destroy it by rushing." The task has taken five years so far.

The payoff: details rarely seen in a fossil australopith, among them a full set of both milk teeth and unerupted adult teeth. All of her tiny ribs were positioned, as in life, along a sinuous spinal column. Several fingers were still curled in a tiny grasp, and where her throat once was, Zeresenay found a rare example of a hyoid bone, a bone that later became crucial to human speech. The discovery offers an early glimpse of the evolution of the human voice box, says Fred Spoor of University College London, another member of the study team.

From the waist down the Dikika baby looked like us. One of her humanlike knees was complete with a kneecap no bigger than a dried pea. But her upper body, like Lucy's, had many apelike features. Her brain was small, her nose flat like a chimpanzee's, and her face long and projecting. Her finger bones were curved and almost as long as a chimp's. Her two complete shoulder blades, the first ever found from an australopith, were similar to those of a young gorilla—a shape that might have made it easy for her to climb. A. afarensis walked on two feet, but some scientists think this species also spent time in trees.

Either way, the Dikika baby was a distinctly different creature from the apes that her ancestors had diverged from several million years earlier. The differences rippled through later human evolution, affecting everything from family ties to the origin of speech.

As apelike feet evolved to support and propel an upright body, they could no longer grasp objects with a thumb-like big toe, as the feet of chimps and other apes can. For hominin mothers and infants, the consequences were momentous: While chimp babies cling to their mothers' hair with muscular hands and grasping toes, a baby hominin probably had to be carried, limiting the mother's ability to provide for herself. She may have had to depend on her mate and the larger group—which may have strengthened social bonds and could help explain why humans are largely monogamous, unlike most apes. Brain evolution expert Dean Falk speculates that the helplessness of baby hominins could even lie at the root of speech, which could have evolved from "motherese," the sounds a mother makes to comfort her baby when she has to set it down.

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