New research from a team of anthropologists has found that a species widely accepted to be an ancestor to humans had a brain with characteristics of apes.
The research, led by a team of scientists from Max Planck Institute in Germany that included Florida State Professor of Anthropology Dean Falk, is published in Science Advances.
The team found that the brain of this early species was organized like that of an ape, but it grew at a rate more comparable to humans.
"Our analysis shows that the small ape-sized brains of these relatives looked apelike in the pattern of their cerebral convolutions, rather than humanlike as some have argued," said Falk, a specialist in brain evolution. "My position has always been that the brains appeared apelike. This paper moves the field forward by definitively resolving the debate - the cerebral cortex of A. afarensis was apelike, not humanlike."
Researchers examined fossil skulls of the species Australopithecus afarensis - a type of primate that walked upright, had a brain 20 percent larger than chimpanzees and had a mixture of ape-like and human-like facial features. The fossils are more than 3 million years old, and one of the fossils was a well-preserved child, known as the Dikika child.
The researchers scanned the Dikika child using synchrotron microtomography at the European Synchrotron Radiation Facility in France to examine the cranial anatomy and teeth histology, which can reveal the age of death. Researchers also scanned seven other well-preserved fossil crania of the same species from Ethiopian archaeological sites.
Several years of painstaking fossil reconstruction and counting of dental growth lines yielded an exceptionally preserved brain imprint of the Dikika child, a precise age at death, new endocranial volume estimates and previously undetected endocranial features.
In the Dikika child's endocast, researchers observed an impression of a lunate sulcus, a fissure that borders the occipital lobe. In humans, the lunate sulcus is usually missing but, on those rare occasions when it is present, is farther back in the brain. It has a more frontal location in chimpanzees.
The preserved endocast of the Dikika child has an unambiguous impression of a lunate sulcus in an apelike position.
Similarly, the computed tomographic scans revealed a previously undetected impression of an apelike lunate sulcus in a well-known fossil of an adult A. afarensis skull from another archaeological site in Ethiopia.
While the organization of the brain appeared to be apelike, the data gleaned from these scans show that the brain's development was slowed down, more comparable to humans.
"Our data show that Australopithecus afarensis had an apelike brain organization, but suggest that these brains developed over a longer period of time than in chimpanzees," said lead author Philipp Gunz from the Max Planck Institute for Evolutionary Anthropology.
Modern human brains are three to four times larger than those of apes, organized differently and take longer to grow and mature. For example, modern human infants learn longer at the expense of being entirely dependent on parental care for longer periods of time, characteristics that are important for human cognition and social behavior.
"After seven years of work, we finally had all the puzzle pieces to study the evolution of brain growth," Gunz said.
Other researchers contributing to this work are Paul Tafforeau from the European Synchrotron Radiation Facility, Adeline Le Cabec from the Max Planck Institute for Evolutionary Anthropology, Tanya Smith from the Australian Research Centre for Human Evolution, William Kimbel from Arizona State University, Fred Spoor from the Max Planck Institute for Evolutionary Anthropology and Zeresenay Alemseged from the University of Chicago.
