Human brains are three times larger, are organized differently, and mature for a longer period of time than those of our closest living relatives, the chimpanzees. Together, these characteristics are important for human cognition and social behavior, but their evolutionary origins remain unclear. To study brain growth and organization in the hominin species Australopithecus afarensis (famous for ‘Lucy’ and ‘Selam’ from Ethiopia’s Afar region) more than 3 million years ago, an international team of researchers scanned eight fossil skulls using conventional and synchrotron computed tomography. Published in the journal Science Advances, the findings show that while Australopithecus afarensis had an ape-like brain structure, the brain took longer to reach adult size, suggesting that infants may have had a longer dependence on caregivers, a human-like trait.
Forensic facial reconstruction of Australopithecus afarensis. Image credit: Cicero Moraes / CC BY-SA 3.0.
Australopithecus afarensis inhabited eastern Africa more than 3 million years ago — Lucy herself is estimated to be 3.2 million years old — and occupies a key position in the hominin family tree, as it is widely accepted to be ancestral to all later hominins, including the lineage leading to modern humans.
“Lucy and her kin provide important evidence about early hominin behavior — -they walked upright, had brains that were around 20% larger than those of chimpanzees, and may have used sharp stone tools,” said Dr. Zeresenay Alemseged, director of the Dikika field project and researcher at the University of Chicago.
The scientists produced high-resolution digital endocasts of the interior of Australopithecus afarensis’ skulls, where the anatomical structure of the brains could be visualized and analyzed. Based on these endocasts, they could measure brain volume and infer key aspects of cerebral organization from impressions of the brain’s structure.
A key difference between apes and humans involves the organization of the brain’s parietal lobe — important in the integration and processing of sensory information — and occipital lobe in the visual center at the rear of the brain.
The exceptionally preserved endocast of Selam, a skull and associated skeleton of an Australopithecus afarensis infant found at Dikika in 2000, has an unambiguous impression of the lunate sulcus — a fissure in the occipital lobe marking the boundary of the visual area that is more prominent and located more forward in apes than in humans — in an ape-like position.
The scan of the endocranial imprint of an adult Australopithecus afarensis fossil from Hadar (A.L. 162-28) reveals a previously undetected impression of the lunate sulcus, which is also in an ape-like position.
Some scientists had conjectured that human-like brain reorganization in australopiths was linked to behaviors that were more complex than those of their great ape relatives. Unfortunately, the lunate sulcus typically does not reproduce well on endocasts, so there was unresolved controversy about its position in Australopithecus.
“A highlight of our work is how cutting-edge technology can clear up long-standing debates about these 3-million-year-old fossils,” said Dr. William Kimbel, a paleoanthropologist in the Institute of Human Origins at Arizona State University.
“Our ability to peer into the hidden details of bone and tooth structure with CT scans has truly revolutionized the science of our origins.”
A comparison of infant and adult endocranial volumes also indicates more human-like protracted brain growth in Australopithecus afarensis, likely critical for the evolution of a long period of childhood learning in hominins.
The pace of dental development of the Dikika infant was broadly comparable to that of chimpanzees and therefore faster than in modern humans. But given that the brains of Australopithecus afarensis adults were roughly 20% larger than those of chimpanzees, the Dikika child’s small endocranial volume suggests a prolonged period of brain development relative to chimpanzees.
Brain imprints in fossil skulls of Australopithecus afarensis (famous for Lucy and the Dikika child from Ethiopia pictured here) shed new light on the evolution of brain growth and organization. The exceptionally preserved endocranial imprint of the Dikika child reveals an ape-like brain organization, and no features derived towards humans. Image credit: Philipp Gunz, MPI EVA Leipzig.
“The combination of apelike brain structure and humanlike protracted brain growth in Lucy’s species was unexpected,” Dr. Kimbel said.
“That finding supports the idea that human brain evolution was very much a piecemeal affair, with extended brain growth appearing before the origin of our own genus, Homo.”
Among primates, different rates of growth and maturation are associated with different infant-care strategies, suggesting that the extended period of brain growth in Australopithecus afarensis may have been linked to a long dependence on caregivers.
Alternatively, slow brain growth could also primarily represent a way to spread the energetic requirements of dependent offspring over many years in environments where food is not always abundant.
In either case, protracted brain growth in Australopithecus afarensis provided the basis for subsequent evolution of the brain and social behavior in hominins and was likely critical for the evolution of a long period of childhood learning.
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Philipp Gunz et al. 2020. Australopithecus afarensis endocasts suggest ape-like brain organization and prolonged brain growth. Science Advances 6 (14): eaaz4729; doi: 10.1126/sciadv.aaz4729
