Lucy had a brain with ape-like organization but a prolonged maturation

Lucy is widely accepted as the predecessor of modern humans, but scientists have been trying to determine if key differences in the human brain occurred after divergence of Homo from Lucy’s species Australopithecus afrensis or if the differences evolved within her genus.

A new study published in “Science Advances” on April 1, reported that A. afrensis’ brains are organized more similar to ape brains than human brains, but appear to have prolonged maturation.

Researchers Philipp Gunz, Simon Neubauer, Dean Falk, Paul Tafforeau, Adeline Le Cabe, Tanya M. Smith, William H. Kimbel, Fred Spoor, and Zeresenay Alemseged studied brain organization and maturation in the A. afrensis by scanning eight fossil crania with conventional and synchrotron computed tomography. During the research project, they examined and compared the cranial volume of both infant and adults of the species.

“Some have argued that structural changes of the brain resulted in a more posterior (human-like) placement of the lunate sulcus on endocasts of australopiths and eventually to the disappearance of a clear endocranial impression in humans,” said study authors. 

The results of this study, however, contradicted those claims, showing that Lucy and others of her species had an ape-like brain organization. This work also resolved a debate about the position of the lunate sulcus in Lucy’s species.

 “Contrary to previous claims, we find no unambiguous indication of brain reorganization in any A. afarensis endocast that preserves detailed sulcal impressions,” the study notes.

The size of the human brain and birth of offspring that require prolonged care –unique characteristics of Homo– led to the obstetric dilemma hypothesis. This hypothesis proposed that human brain size might necessitate an earlier birth to allow the skull to fit through constrained pelvic dimensions due to walking upright.

However with the discovery that Lucy had a prolonged brain maturation, these researchers suggest rethinking the obstetric hypothesis. "Our findings, therefore, challenge the central tenet of the obstetric dilemma hypothesis and suggest that obstetric constraints are not the proximate cause of the origins of prolonged brain growth in hominins."

"The fact that protracted brain growth emerged in hominins as early as 3.3 Ma ago could suggest that it characterized all of subsequent hominin evolutionary history."