How Is the Human Brain Different from the Brains of Other Primates?
There is no doubt that humans are the dominant species on Earth. While we are yet to fully subjugate the planet itself to our whims, with the occasional catastrophic earthquake or a similar natural disaster serving as an apt and humbling reminder that we are still at the mercy of nature, our place at the top of the proverbial food chain is undisputed by any other inhabitant of this planet. We occupy this lofty position solely thanks to our intellect, as we are otherwise not all that impressive as a species – we are not the strongest out there, we are not the fastest, and certainly not the biggest.
However, all those shining achievements of human ingenuity that we are rightly proud of cannot change the fact that we are, at the end of the day, primates and that we share common ancestry with the likes of gorillas or chimpanzees, for example. It may sound incredible, but the species which invented the Internet shares almost 99% of its DNA with bonobos (Pan paniscus) and the aforementioned chimps (Pan troglodytes), which happen to be our closest living relatives in the animal kingdom(1).
The Basic Facts
This, naturally, raises the question of what makes us unique and how we were able to rise so far above the creatures with which we share such a staggering percentage of our genetic makeup. It all boils down to the brain, of course, and we’ll start with some basic facts about the most complex organ in our body.
The size of the human brain increases drastically throughout childhood, but once it is fully developed, the weight of an adult’s brain is approximately 3 pounds (or 1,300 – 1,400 grams). On average, the brain is 140mm wide, 167mm long, and 93mm high(2). For a creature of our body size, that is enormous (though it’s not the biggest brain in the animal kingdom, as there are creatures much larger than us out there – for example, the brain of an elephant weighs around 13 pounds).
Now, all primates have large brains, but humans stand out even in this category. To name but one example, the brain of a chimpanzee is three times smaller than a human’s(3), despite the fact we are so close genetically. Aside from this size discrepancy, which is universally acknowledged, scientists struggle to come to a consensus regarding further differences between the brains of humans and other primates. The primary reason for this is the fact that we still haven’t fully unraveled all the mysteries behind the inner-workings of our central nervous system, and we are unlikely to for at least some time. But work is ongoing and comparing our brains to those of primates is a big part of this effort. That’s why we will provide you with several theories regarding these differences and that should, hopefully, help shed some light on this mystery that has occupied brain scientists for decades.
One theory puts forward the idea that the key difference may lie in the neurotransmitter dopamine. Research led by André M. Sousa(4) compared brain tissue samples from humans and various primates. They found that the one region of the brain which showed the greatest number of human-specific distinctions was the striatum, which is involved in the decision-making process, motor coordination, etc.
The researchers found a dopamine-producing set of interneurons which was enriched in this part of the human brain but was without a counterpart in the brains of gorillas, bonobos, or chimps. Strangely, these cells could be found in macaque brain samples, which are otherwise less similar to humans, indicating interesting evolutionary patterns.
Seeing as dopamine plays a vital role in many functions tied to behavior and cognitive skills, this could be a crucial element in our rise to the top as a species. Still, the researchers note that the way in which our brain is structured is not without its drawbacks and explain that its inherently increased size and connectivity compared to other primates leave it vulnerable not only to an increased number of neural problems, but also for a longer period of time.
More Communication between Parts of the Brain
A study(5) which compared humans and macaques during a visual attention test found that the human brain displayed increased levels of communication between the two hemispheres, and that certain regions of the human brain exhibited more activity. Finally, there was no activity in the temporoparietal junction of the macaque brain, unlike with the humans (in humans, this region is crucial when it comes to redirecting attention).
However, Gaurav Patel, a co-author of this study, says that this increased level of communication doesn’t automatically make our brains better, just different. The fact that information is shared between different parts of the brain may actually lower reaction speeds, but humans have gained heightened cognitive flexibility and better attention control in the trade-off.
There is a popular theory that our intellectual superiority over other primates is due to certain regions of the brain, such as the prefrontal cortex, being enlarged and taking up a bigger share of the overall brain. However, a study(6) claims to have disproven this notion by using the isotropic fractionator, a different method of quantifying neurons. These findings seem to indicate that the prefrontal cortices in the human brain and in the brains of seven other primates contain the same fraction (around 8%) of the total number of cortical neurons. The researchers go on to propose that the reason for our outstanding intellect is actually rather plain – our brains aren’t special, they’re simply bigger. The distribution of neurons is the same, we just have more of them.
There is a lot more that could be said on this topic and there are many ideas regarding what it is exactly that sets us apart from other primates. It could be the first two traits we mentioned, it could be the size of our brains, maybe some combination of the two, or perhaps something else entirely – this is an ever-developing branch of science and we’ll probably have to go through a lot more theories before we get close to a definitive answer. For now, just looking at the differences, but also the similarities, mentioned in this text should be enough to have us pause for a second and consider our place in this world.