Lucy’s kind had small, chimplike brains that, nevertheless, grew at a slow, humanlike pace.
This discovery, reported April 1 in Science Advances, shows for the first time that prolonged brain growth in hominid youngsters wasn’t a by-product of having unusually large brains. An influential idea over the last 20 years has held that extended brain development after birth originated in the Homo genus around 2.5 million years ago, so that mothers — whose pelvic bones and birth canal had narrowed to enable efficient upright walking — could safely deliver babies.
But Australopithecus afarensis, an East African hominid species best known for Lucy’s partial skeleton, also had slow-developing brains that reached only about one-third the volume of present-day human brains, say paleoanthropologist Philipp Gunz of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues. And A. afarensis is roughly 3 million to 4 million years old, meaning slow brain growth after birth developed before members of the Homo genus appeared, perhaps as early as 2.8 million years ago (SN: 3/4/15).
Too few A. afarensis infants have been studied to calculate the age at which this species attained adult-sized brains, Gunz cautions. The brains of human infants today reach adult sizes by close to age 5, versus an age of around 2 or 3 for both chimps and gorillas.
In the new study, Gunz and colleagues estimated brain volumes for six A. afarensis adults and two children, estimated to have been about 2 years and 5 months old. The kids had brains that were smaller than adult A. afarensis brain sizes in a proportion similar to human children’s brains at the same age relative to adult humans.
The new data suggest that, for Lucy’s species, “infant brain size [relative to that of an average adult] may have been proportionally even smaller than in human infants,” says biological anthropologist Zachary Cofran of Vassar College in Poughkeepsie, N.Y., who did not participate in the new study. If so, that pattern would strongly point to an extended period of brain growth for A. afarensis.
Gunz suggests the extended post-birth brain growth among A. afarensis may have eased the physical and nutritional burden on mothers caring for infants, especially if food was scarce. It also “likely provided a foundation for the evolution of long childhoods in the human lineage,” he says.
His group used high-resolution CT scans to study fossilized braincases from an infant A. afarensis and six adults, including Lucy, all found at Ethiopia’s Hadar site. A second A. afarensis infant braincase came from Ethiopia’s Dikika site (SN: 9/20/06). The scans helped the researchers create 3-D digital reconstructions, or endocasts, of impressions made by the brain on the skull’s inner surface. Endocasts display signature folds and creases in brain tissue typical of humans or chimps, although preservation of these neural landmarks varies.
CT scans also let the researchers determine the infants’ ages by revealing microscopic layers of dental enamel that form daily during childhood, which can be counted like tree rings.
The Dikika child’s well-preserved endocast retained a crease and a set of grooves toward the back of the brain that are found in chimps, but not in humans. These impressions mark a prominent neural area involved in vision. Human brain surfaces lack these markings due to expanded neural tissue that integrates visual and sensory information. A South African Australopithecus skull was previously revealed to have signs of a chimplike visual area in the brain (SN: 1/10/19).
Neither of the A. afarensis infants showed evidence of humanlike frontal brain organization, as has been reported for an approximately 300,000-year-old South African hominid, Homo naledi (SN: 4/25/17).
Anthropologist and neuroscientist Todd Preuss of Emory University in Atlanta agrees. Endocasts of the Hadar A. afarensis skulls contain many preserved features from the brain’s visual area that resemble those on an Australopithecus child’s skull from South Africa that dates to about 2.8 million years ago, says Preuss, who was not part of Gunz’s team.
Much remains to be learned about the pace of brain growth in A. afarensis, says paleoanthropologist Aida Gomez-Robles of University College London. Researchers can’t track brain sizes of A. afarensis individuals from infancy into adulthood, so the new results don’t conclusively determine growth rates, she notes. And earlier interpretations of hominid brain organization based on endocasts have sparked frequent debate, leading Gomez-Robles to withhold judgment on the brains of Lucy’s kind.