Groove patterns on the surface of modern chimpanzee brains throw a monkey wrench into proposals that some ancient southern African hominids evolved humanlike brain characteristics, a new study suggests.
MRIs of eight living chimps reveal substantial variability in the shape and location of certain features on the brain surface. Some of these brains showed surface creases similar to ones that were thought to have signaled a turn toward humanlike brain organization in ancient hominids hundreds of thousands, if not millions, of years ago. Paleoanthropologist Dean Falk of Florida State University in Tallahassee and colleagues report their findings online March 13 in Brain, Behavior and Evolution.
The study casts doubt on a 2014 paper by Falk that was based on casts of the inside of fossil braincases, called endocasts, which preserve impressions of these surface features. At the time, Falk argued that four endocasts from southern African hominids — three Australopithecus africanus and one Australopithecus sediba — showed folding patterns that suggested that brain reorganization was underway as early as 3 million years ago in a frontal area involved in human speech production.
But MRIs of three of the chimp brains reveal comparable creases, the researchers found. Two other chimps display other frontal tissue furrows that Falk had also previously described as distinctly humanlike.
MRI scans of eight chimp brains (one at right) provide new detailed looks at surface folds used to interpret brain impressions on ancient hominid skulls. Until now, some of the best evidence of these folds came from five line drawings (one at left) published nearly 70 years ago.
“I was really wrong about the handful of Australopithecus endocasts,” Falk says. The endocasts were made from A. africanus and A. sediba fossils dating to between roughly 2 million to 3 million years ago (SN: 8/10/13, p. 26).
And in one chimp, the new study finds, a pair of grooves correspond with those on a Homo naledi endocast that were described in 2017 as humanlike (SN Online: 4/25/17). H. naledi, a small-brained species with many humanlike skeletal features, inhabited southern Africa close to 300,000 years ago (SN: 6/10/17, p. 6).
Still, researchers have spent decades debating the implications of partially preserved brain surface features on hominid endocasts. And the new findings based on MRIs are also controversial.
Nothing in the new chimp study undermines the original finding of humanlike folds in H. naledi’s frontal brain, says biological anthropologist Shawn Hurst of Indiana University Bloomington. The frontal brain grooves on a H. naledi endocast, like those in modern humans, lie farther back than the grooves seen in the chimp MRI scan, Hurst contends. Expanded tissue folds around those grooves also follow a distinctly humanlike pattern not observed chimps, he argues. Those features indicate H. naledi had a humanlike capacity for pride and other complex social emotions and possibly verbal communication of some type, Hurst says. What’s more, the new study fails to consider that the A. sediba endocast shows furrows and folding patterns found in humans but not chimps, he says.
Endocast researchers need to study the range of brain surface characteristics in a larger sample of living chimps and other apes to make more accurate comparisons, Falk says. Until now, line drawings published in 1950 of only five chimp brain hemispheres, she notes, have provided the most accurate and comprehensive look at furrow patterns on chimps’ brains.
How specific folds and creases on the brain’s surface relate to inner structures, such as those involved in speech and language, also remains poorly understood, Falk says.