2925. Tiny Brains of Extinct Human Relative Had Complex Features

By Nicholas St. Fleur, The New York Times, May 14, 2018

Researchers say that the skull of Homo naledi, a hominin that lived 236,000 to 335,000 years ago, contained a brain the size of an orange that had a similar shape and structure with that of modern humans.  Photo: John Hawks.

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What makes humans so smart? For a long time the answer was simple: our big brains.

But new research into the tiny noggins of a recently discovered human relative called Homo naledi may challenge that notion. The findings, published Monday, suggest that when it comes to developing complex brains, size isn’t all that matters.

In 2013 scientists excavating a cave in South Africa found remains of Homo naledi, an extinct hominin now thought to have lived  236,000 to 335,000 years ago. Based on the cranial remains, the researchers concluded it had a small brain only about the size of an orange or your fist. Recently, they took another look at the skull fragments and found imprints left behind by the brain. The impressions suggest that despite its tiny size, Homo naledi’s brain shared a similar shape and structure with that of modern human brains, which are three times as large.

“We’ve now seen that you can package the complexity of a large brain in a tiny packet,” said Lee Berger, a paleoanthropologist at Wits University in South Africa and an author of the paper published in the journal Proceedings of the National Academy of Sciences. “Almost in one fell swoop we slayed the sacred cow that complexity in the hominid brain was directly associated with increasing brain size.”

Not every scientist agrees with their interpretation.

Since its remains were first retrieved, Homo naledi has puzzled scientists. From head to toe the ancient hominin displays a medley of primitive, apelike features and more advanced, humanlike characteristics.

“It’s this mosaic that is unlike anything we have seen or expected,” said Dr. Berger who first discovered Homo naledi in the Dinaledi Chamber in South Africa’s Rising Star cave system. So far, researchers have found more than 2,000 fossils belonging to the human relatives which have provided a portrait of what the species once looked like.

Homo naledi had small teeth like a human’s, but their shape more closely resembles that of an ape’s. Its shoulders are also apelike, but the arms, wrists and hands more humanlike. The fingers, though, were long and curved, and the thumbs appeared particularly strong. The spine was a combination of primitive and Neanderthal-like, the pelvis resembled that of another more distant human relative, Australopithecus afarensis (dubbed Lucy), and the thighs also looked primitive. But below the knee, the legs were long and thin like a human’s and the feet were nearly identical to our own.

Now, researchers have found that Homo naledi’s similarities with modern humans extend into the brain. After examining the imprints, or endocasts, from five Homo naledi skull fragments, the team found that the species had a frontal lobe that was very similar to that of modern humans and unlike that of an ape’s. The scientists also found that Homo naledi had an asymmetrical brain, with the left brain appearing more forward than the right, which is also seen in humans. Asymmetry in the brain is associated with higher levels of behavioral complexity, the team said.

Based on the regions of the brain that Homo naledi shared with modern humans, the authors suggested that it may have exhibited complex behavior. But what they did not say was what those behaviors may have been, said John Hawks, an paleoanthropologist at the University of Wisconsin, Madison, and an author on the paper.

“Is that aspect of the brain evolution central to talking or stone tool making? We don’t know enough to say that,” he said. He added that the finding does not mean that brain size is not important to creating a complex brain — it is. Rather, size alone does not tell the whole story. “There’s something about shape that actually matters too,” he said.

“These new fossil hominids show that the evolution of hominins is much more complex than we thought before,” said Ralph Holloway, a paleoanthropologist at Columbia University and lead author of the paper.  

Emiliano Bruner, a paleoneurobiologist at Centro Nacional de Investigación sobre la Evolución Humana in Spain, said the finding still needs to be confirmed from a wider fossil record of the species that comes from outside the Dinaledi Chamber fossil site. That would help with understanding any variation that appears within the species.

Simon Neubauer, a physical anthropologist at the Max Planck Institute for Evolutionary Anthropology in Germany, said the finding did support the idea that both brain size and brain organization are important to human evolution. But he added that because of its age, the tiny-brained Homo naledi might be an outlier in a general hominin trend toward increasing brain sizes.

Dr. Bruner agreed.

“It is not reasonable to forget all that evidence because of a couple of outliers,” he said. “Exceptions are, as always, expected. But this does not break the rule.”