Many believe that our particularly large brain makes us human – but is there more to it? The shape of the brain, as well as the shapes of its component parts (lobes) can also be important.
Findings from a study we published in Nature Ecology & Evolution on Thursday (January 5). (opens in new tab) show that the way the different parts of the human brain evolved sets us apart from our primate relatives. In a way, our brains never grow up. We only share this “Peter Pan syndrome” with one other primate – the Neanderthal.
Our findings provide insight into what makes us human, but also further narrow any distinction between us and our extinct, lumbering cousins.
On the trail of brain development
The mammalian brain has four distinct regions, or lobes, each with distinct functions. The frontal lobe is associated with reasoning and abstract thinking, the temporal lobe with memory preservation, the occipital lobe with vision, and the parietal lobe helps integrate sensory input.
We examined whether the brain lobes evolved independently, or whether evolutionary change in one lobe appears to be necessarily associated with changes in others—that is, evidence that lobe evolution is ‘integrated’.
In particular, we wanted to know how human brains might differ from other primates in this regard.
One way to answer this question is to look at how the different lobes have changed over time in different species and measure how strongly the shape change in each lobe correlates with the shape change in others.
Alternatively, we can measure the degree to which the brain lobes are integrated with each other as an animal grows through different stages of its life cycle.
Does a shape change in one part of the growing brain correlate with a change in other parts? This can be instructive as evolutionary steps can often be traced through the development of an animal. A common example is the brief appearance of gill slits in early human embryos, reflecting the fact that we can trace our evolution to fish.
We used both methods. Our initial analysis included 3D brain models of hundreds of living and fossil primates (monkeys and apes, as well as humans and our close fossil relatives). This allowed us to map the development of the brain over time.
Our other digital brain dataset consisted of live monkey species and humans at different growth stages, allowing us to depict the integration of brain parts in different species as they matured. Our brain models were based on CT scans of skulls. By digitally filling the brain cavities, you can get a good approximation of the shape of the brain.
A surprising result
The results of our analyzes surprised us. In tracking changes over time in dozens of primate species, we found that humans had particularly high levels of brain integration, particularly between the parietal and frontal lobes.
But we also realized that we are not unique. The integration between these lobes was also similarly high in Neanderthals.
Looking at shape changes through growth showed that in monkeys like the chimpanzee, the integration between the brain lobes until puberty is reached is comparable to that in humans.
At this point, integration slows down quickly in apes, but continues well into adulthood in humans.
Neanderthals were highly evolved humans
What does it all mean? Our result suggests that it’s not just our brain that differentiates us from other primates greater. The development of the different parts of our brain is more deeply integrated, and unlike all other living primates, we retain this into adulthood.
Greater learning ability is typically associated with adolescent life stages. We suspect that this Peter Pan syndrome played an important role in the development of human intelligence.
Continue reading: When did humans first start speaking? How the language developed in Africa (opens in new tab)
There is another important implication. It’s becoming increasingly clear that Neanderthals, long labeled as brutish fools, were adaptable, capable, and highly evolved humans.
Archaeological finds continue to support their development of sophisticated technologies, from the earliest known evidence of string to the manufacture of tar (opens in new tab). Neanderthal cave art shows that they indulged in complex symbolic thought (opens in new tab).
us and them
Our findings continue to blur any dividing line between us and them. Despite this, many remain convinced that an innate superior intellectual quality gave us humans a competitive edge that enabled us to drive our “inferior” cousins to extinction.
There are many reasons why one group of people can dominate or even wipe out others. Early Western scientists attempted to identify skull features related to their own “greater intelligence” to explain European world domination. Of course, we now know that the skull shape had nothing to do with it.
We humans could have come perilously close to extinction even 70,000 years ago (opens in new tab).
If so, it’s not because we weren’t smart. If we had gone extinct, today’s Neanderthal descendants might be scratching their heads trying to figure out how their “superior” brains gave them the advantage.
Continue reading: The first-ever genetic analysis of a Neanderthal family paints a fascinating picture of a close-knit community (opens in new tab)
This article is republished by The conversation (opens in new tab) under a Creative Commons license. read this original article (opens in new tab).