Monday, May 15, 2017
Mystery human species Homo naledi had tiny but advanced brain
It’s not the size of your brain, it’s how you organise it. The most recently discovered species of early human had a skull only slightly larger than a chimpanzee’s, but its brain looked surprisingly like our own – particularly in an area of the frontal lobe with links to language.
This could back suggestions that these mysterious early humans showed advanced behaviours, such as teamwork and burial, even though we still don’t know exactly when they lived.
In 2013, news broke of an extraordinary discovery in a chamber deep inside a South African cave. Researchers led by Lee Berger at the University of the Witwatersrand in Johannesburg had discovered thousands of ancient human fossils – comfortably the largest cache of its kind ever found in Africa.
The first official scientific reports were published in 2015, and they painted a confusing picture. The bones belonged to a never-before-seen early human, which was named Homo naledi.
It had a peculiar mix of anatomical features, which is part of what makes it hard to tell when the species lived. But what really set tongues wagging was the suggestion by Berger and his colleagues that H. naledi had deliberately disposed of its dead in this deep, dark, difficult-to-reach cave chamber full of remains.
Such an endeavour probably required emotional sophistication, not to mention teamwork, to carry out the task, but H. naledi’s skull was less than half the size of our own. Could its tiny brain have powered such advanced behaviour?
Berger and the other members of the H. naledi research team think it could. Using pieces of fossil skull, the group has now produced casts of parts of H. naledi’s small brain. The pattern of ridges and troughs (called gyri and sulci) on the surface of the casts offers hints about the way the brain was organised.
“Some of the casts we are working on are the most extraordinarily preserved I’ve ever seen,” says John Hawks at the University of Wisconsin-Madison. “The detail is just pristine.”
What excites the team most is a region on the side of H. naledi’s frontal lobe called Brodmann area 45, part of Broca’s area, which in modern humans has links to speech production. In this part of our brains, the pattern of gyri and sulci is very different from that seen in chimpanzees. H. naledi seems to have had our pattern, even though as an adult its BA45 was not much larger than that of a chimpanzee.
“You look at the naledi cast and you think – holy crap this is just a tiny human,” says Hawks.
Team member Shawn Hurst of Indiana University in Bloomington discussed the findings at a meeting of the American Association of Physical Anthropologists in New Orleans last week. “I would think the implication is that [H. naledi] was moving strongly towards enhanced communication,” he says.
Hurst adds that there is also evidence for a general expansion of the bottom surface of the frontal lobes – a region associated with higher emotions like empathy. Together, these observations might help to explain why groups of the small-brained hominin could have become interested in careful disposal of their dead, and how they could work together to transport bodies through the narrow and pitch-black cave system that led to the burial chamber.
Dean Falk at Florida State University in Tallahassee was also at last week’s meeting, and had an opportunity to look at the H. naledi brain casts and discuss them with Hurst. “We agreed on most of the interpretations,” she says – but not on the presence of a modern BA45. “This is just my initial reaction, but I’m not seeing BA45,” says Falk. “To me the general shape of the region looks ape-like.”
Hurst isn’t surprised by Falk’s conclusion. “My first reaction was the same,” he says. It was only after hours spent carefully comparing the H. naledi brain cast with the casts of other hominin and ape brains that he and his colleagues became convinced that it had a modern configuration. When the research is officially published, Falk and other researchers will have a better opportunity – and more time – to properly assess the claim.
Other regions of the H. naledi brain tell a similar story. Ralph Holloway at Columbia University in New York also gave a talk at the New Orleans meeting, focusing on casts of the rear part of the H. naledi brain.
Holloway looked at a sulcus here that he says separates the visual cortex at the very rear of the brain from the parietal and temporal lobes that lie slightly further forward. In humans, the sulcus is smaller than in chimpanzees, reducing the size of the visual cortex and increasing the size of the parietal and temporal lobes. In H. naledi, the sulcus seems to have begun shifting into a modern-human-like configuration along some of its length,.
“The significance is that the visual cortex is purely sensory,” says Holloway. “But the parietal and temporal lobes right adjacent to it are very important for complex social behaviour.”
Again, it seems that H. naledi was more socially sophisticated than the small size of its brain might suggest.
“In our field, there is this dispute about whether the important thing in human brains is their size or the way they are organised,” says Hawks. H. naledi seems to suggest organisation is more critical.
Simon Neubauer at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, says the work supports the idea that parts of the brain became modern in their configuration before they grew large.
But he adds that we won’t know how significant the new findings are until we have some idea of how old the H. naledi fossils are.