By Ann Gibbons
In 1974, the world was stunned by the discovery of “Lucy,” the partial skeleton of a human ancestor that walked upright—and still spent time in the trees—3.2 million years ago. Later discoveries revealed her species, scattered throughout eastern Africa, had brains bigger than chimpanzees. But a new study of an ancient toddler finds that the brains of Lucy’s kind were organized less like those of humans and more like those of chimps. That suggests the brains of our ancestors expanded before they reorganized in the ways that let us engage in more complex mental behaviors such as making tools and developing language. The remains also suggest Lucy’s species had a relatively long childhood—similar to modern humans—and that they would have needed parenting longer than their chimp relatives.
Anthropologists have made much of the fact that adult members of Lucy’s species—Australopithecus afarensis—had skulls 20% larger than a chimpanzee’s. Researchers have long debated what this meant for their brain power. Had the brains of these early hominins, or members of the human family, already reorganized by the time their kind was walking upright in Africa and—perhaps—hafting sharp stone tools 2.9 million to 3.9 million years ago? “There’s been a big debate about when the reorganization of the brain took place in the hominin lineage,” says University of Chicago paleoanthropologist Zeresenay Alemseged.
To test this idea, an international team of paleoanthropologists used a synchrotron in Grenoble, France, to take super–high-resolution images of the deformed skull and teeth of an A. afarensis toddler, known as the Dikika child, which Alemseged discovered in Ethiopia in 2000.
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