Zeresenay Alemseged has two babies. One is Alula, who spends most of his time in his mother's arms in a cozy bungalow in Addis Ababa, Ethiopia's capital. The other is a little girl of three, who spent 3.3 million years locked in sandstone, until the Ethiopian scientist and his team discovered her remains and painstakingly teased them out of the rock. It was a long, slow second birth for a baby from the dawn of humanity.
Until now all fossils of babies this ancient could have fit in a diaper. This new arrival is not only the most complete ancient infant but arguably the best fossil of her species, Australopithecus afarensis. That's the same species as the superstar fossil called Lucy, a 3.2-million-year-old adult female found in 1974. Unlike Lucy, the baby has fingers, a foot, and a complete torso. "But the most impressive difference between them," says Zeresenay (Ethiopians' first names are their formal ones), "is that this baby has a face."
No bigger than a cantaloupe, the little bundle of bones may also bear witness to a key event in the evolution of hominins, as humans and their ancestors are known: the beginning of our long, dependent childhood, when we grow our large brains. "Outside of its completeness, the major importance of this find is the light it will shed on how this species lived and grew," says Bill Kimbel, an expert on A. afarensis and a member of the study team. "Now we can begin to read its biography."
It is a curious coincidence that the world's oldest baby, who died while still of nursing age, lived her short life in a region named Dikika—"nipple" in the local Afar language, after a distinctly shaped hill. The hill is just across the winding Awash River from Hadar, the site in Ethiopia's Rift Valley where Lucy and the fossils of many other hominins have been found. The region is plagued by extreme heat, flash floods, malaria, and occasional shoot-outs between rival ethnic groups, not to mention lions, hyenas, and other uninvited nocturnal guests. It is one of the most difficult places on Earth to hunt for fossils—and one of the most fruitful.
For decades the low-lying northern end of Africa's Great Rift Valley, the Afar depression, has been the domain of foreign-led expeditions. Zeresenay, one of a new generation of Ethiopian paleoanthropologists, changed that in 1999 when he led a band of Ethiopian fossil hunters into the Afar badlands.
By December 2000, the search had turned up plenty of fossil mammals, such as elephants, hippopotamuses, rhinoceroses, and antelopes, but no hominins. Yet Zeresenay, who is based at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, knew his team was looking in the right place. These animals would have thrived in the gallery forest that flanked the ancestral Awash River. Early hominins would have lived in these shady woodlands as well.
The prehistoric forests of Dikika are long gone, and there was no shade on December 10, when team members forced themselves out into the hot sun to look again. Tilahun Gebreselassie was the first to see the Dikika baby's tiny face peering out from a dusty slope. It was no bigger than a monkey's, but a smooth brow and short canine teeth told Zeresenay right away that this was a small hominin. His team had struck fossil gold, for not only was the baby's skull in perfect shape, but tucked beneath the head in a hard ball of sandstone were many bones of the upper body as well. "This is something you find once in a lifetime," Zeresenay says.
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.
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.