Lucy had an ape-like brain, but prolonged brain growth like humans

02-04-2020

A study led by the Max Planck Institute for Evolutionary Anthropology reveals that Lucy’s species, Australopithecus afarensis, had an ape-like brain. However, the protracted brain growth suggests that infants may have had a long dependence on caregivers, as in humans. The study, in collaboration with the ESRF, is published in Science Advances.

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The species Australopithecus afarensis, well-known as Lucy’s species, inhabited East Africa more than three million years ago, and occupies a key position in the hominin family tree.. “Lucy and her kind provide important evidence about early hominin behavior. They walked upright, had brains that were around 20 percent larger than those of chimpanzees and may have used sharp stone tools,” explains senior author Zeresenay Alemseged from the University of Chicago, who directs the Dikika field project in Ethiopia, where the skeleton of an Australopithecus afarensis child, known as Dikika child and nicknamed Selam, was found in the year 2000. “Our new results show how their brains developed, and how they were organized,” adds Alemseged.

In 2009, scientists brought the complete skeleton of the Dikika child to the ESRF to be scanned using synchrotron microtomography. These data have been used for many different research projects on this exceptional fossil, but one of the main goal was to study general development as well as brain growth and organization in Australopithecus afarensis,. In addition to high precision anatomical investigations, special attention was given to the teeth, as they permanently record the development of an individual from before birth to death. With the help of this state-of the-art technology, researchers can reveal the age at death with a precision of a few weeks without having to cut the teeth to access this record. Several years of painstaking fossil reconstruction, and counting of dental growth lines, yielded an exceptionally preserved brain imprint of the Dikika child, and a precise age at death.

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Brain imprints in fossil skulls of the speciesAustralopithecus afarensis(famous for “Lucy” and the “Dikika child” from Ethiopia pictured here) shed new lighton the evolution of brain growth and organization. The exceptionally preservedendocranial imprint of the Dikika child reveals an ape-likebrain organization, and nofeatures derived towards humans.Credit: Philipp Gunz, MPI EVA Leipzig.

In parallel, seven other well-preserved fossil crania, from the Ethiopian sites Dikika and Hadar, were scanned using conventional microtomography. These data shed new light on two questions that have been controversial: Is there evidence for human-like brain reorganization in Australopithecus afarensis? Was the pattern of brain growth in A. afarensis more similar to that of chimpanzees or that of humans?

Extended childhood

Contrary to previous claims, the endocranial imprints of Australopithecus afarensis reveal an ape-like brain organization, and no evidence of the beginning of an anatomical evolution towards humans. However, a comparison of infant and adult endocranial volumes indicates more human-like protracted brain growth in Australopithecus afarensis. This is likely critical for the evolution of a long period of childhood learning in hominins.

The brains of modern humans are not only much larger than those of our closest living ape relatives, but they are also organized differently and take longer to grow and mature. For example, compared with chimpanzees, modern human infants learn longer at the expense of being entirely dependent on parental care for longer periods of time. Together, these characteristics are important for human cognition and social behavior, but their evolutionary origins remain unclear. Brains do not fossilize, but as the brain grows and expands before and after birth, the tissues surrounding its outer layer leave an imprint in the bony braincase. Based on these endocasts, the researchers could measure endocranial volume and infer key aspects of brain organization from impressions of brain convolutions in the skull. When combined with the time information derived from the teeth in infants and juveniles, it becomes possible to relate the brain growth to the age, and then to investigate the evolution of life history in fossil hominins.

Teeth determine Dikika child’s age at death

In infants and juveniles, synchrotron computed tomographic scans pushed to sub-micron resolution make it possible to determine an individual’s age at death by counting growth lines in teeth. Similar to the growth rings of a tree, but with a precision reaching one day, virtual sections of a tooth reveal incremental growth lines reflecting the body’s internal rhythm. “When studying the endocast, the resolution we used was 50 microns (the diameter of a human hair). In order to access the time record preserved in the non-erupted permanent teeth, we had to scan all of them at 5 microns, and few spots down to 0.7 microns. It corresponds to what we can do with a microscope, but instead of having to physically cut the teeth, everything was done non-destructively. Thanks to the high quality of these data, we were able to calculate an age at death of 2.4 years, with a precision of few weeks”, explains Paul Tafforeau, scientist at the ESRF in charge of paleontology and specialist of dental development in hominids.

“After seven years of work, we finally had all the puzzle pieces to study the evolution of brain growth: the age at death of the Dikika child and its endocranial volume, the endocranial volumes of the best-preserved adult Australopithecus afarensis fossils, and comparative data of more than 1600 modern humans and chimpanzees.” says lead author Philipp Gunz, paleoanthropologist from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.  

“Our data show that Australopithecus afarensis had an ape-like brain organization, but suggest that these brains developed over a longer period of time than in chimpanzees,” concludes Philipp Gunz. Among primates in general, 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 abundant. In either case, the protracted brain growth in Australopithecus afarensis provided a 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.

Reference:

Gunz, P., et al, Science Advances  01 Apr 2020: Vol. 6, no. 14, eaaz4729. DOI: 10.1126/sciadv.aaz4729