The Evolution of the Human Brain
What really makes us uniquely human?
The question of what makes humans human has puzzled anthropologists, paleontologists, and even the general public for decades. Why are we able to harness fire, generate electricity, and have complex thoughts about our past and future?
Over the last million years, our brains have increased dramatically in size (European Research Council, 2018). Although it is generally accepted that our above-average brain size is responsible for most of our aforementioned traits, the answer actually lies in the structural complexity of the organ rather than its size.
When looking at the chimpanzee, one of our closest living relatives, we have much more white matter — made primarily of axons — in our brains (Smithsonian National Museum of Natural History, 2022). This means we have many more neural connections in the brain and much better processing efficiency. The graph below shows the size of various animals’ brains compared to the number of neurons present inside.
The size of a brain is not the only factor in the “intelligence” of a species. The number of neurons is much more telling, as well as the body’s ability to provide the necessary amount of fuel for the organ to function to its fullest potential (Hunt, 2022). Humans do have large brains, but we also have dense cortices and able bodies (Hunt, 2022). As an example, dolphins, like us, have extremely intelligent nervous systems, yet they lack the ability to manipulate their environment — hence they will never take over the world in the way Homo Sapiens have (Hunt, 2022).
The evolution of the brain, since before the “Brain”
Although many believe the brain to be the central driving force in most animal species, the enteric nervous system, or “gut brain,” is thought to have evolved before and independently of the central nervous system (Sense of Mind, 2022). Evidence of this can be found in cnidarians like Hydra, who have no brain in the head, but do have a net-like nervous system that closely resembles the one in our digestive system (Sense of Mind, 2022). Cnidarians’ simple nervous systems allow them to contract and digest foods, but lack the complexity of most animals’ central nervous systems (Sense of Mind, 2022).
Many have speculated that the human brain evolved in a way similar to Maslow’s hierarchy of needs — first taking care of physiological needs, such as food, water and maintaining homeostasis (likely beginning with the enteric nervous system), then safety needs, and finally evolving to complex thought (Sense of Mind, 2022). The neocortex, or new cortex, is the newest addition to our brain and is responsible for all executive functions, like making socially mindful decisions (Sense of Mind, 2022). The “old brain” includes the brainstem and limbic system, and is responsible for our more basic physiological functions, like breathing, motor responses, and some functions of feeding (University of Minnesota, 2015). Although all animals have a neocortex, some have a much larger one than others (Sense of Mind, 2022).
Let’s take a closer look…
Modern humans share about 99% of our genome with modern-day chimpanzees (European Research Council, 2018). We also have some of the same genetic information as other organisms, like chickens and fruit flies (European Research Council, 2018).
So, what makes us any different?
Many novel genes have arisen in the past few million years (Suzuki et al., 2018). Consequently, this is also when the cerebral cortex started to rapidly expand (Suzuki et al., 2018). Duplication of genes is the most common way novel genes are created — it leads to other related genes, called “paralog” genes (Suzuki et al., 2018). Pierre Vanderhaeghen, an EU-funded researcher, studied various human-specific genes in order to find links between genetic changes and the rapid development of the human brain (European Research Council, 2018).
“Developmental biologists usually look at changes in the regulation of genes to explain evolutionary differences, and not so much at genes themselves, since we share so many of our genes even with simple organisms such as worms. But gene duplication can lead to novel genes in a species, which could contribute to the rapid emergence of human-specific traits, like the increased size of the brain’s cortex” -Pierre Vanderhaeghen
Using tailored RNA sequencing, his research team found several human-specific genes involved with the development of the cerebral cortex (European Research Council, 2018).
One of these groups of genes are known as the NOTCH2NL genes — a group of genes that evolved from ancient NOTCH genes as a human-specific paralog (European Research Council, 2018). NOTCH genes control notch signaling, which is heavily involved in the development of various stem cells (European Research Council, 2018). The gene is located on the first chromosome, in a region that hosts many other genetic diseases related to brain size and structure, like microcephaly and macrocephaly (European Research Council, 2018). The Vanderhaeghen team first tested the NOTCH2NL gene in mouse embryos, and found a significant increase in their number of cortical stem cells (EurekAlert, 2018). They then conducted similar research on human stem cells and found a similar result — the NOTCH2NL gene resulted in an increased number of cortical stem cells, which in turn led to an increased number of neurons (EurekAlert, 2018).
“From one stem cell, you can either regenerate two progenitor cells, generate two neurons, or generate one progenitor stem cell and one neuron. And what NOTCH2NL does is bias that decision in a slight way towards regenerating progenitors, which can later go on to make more neurons. It’s a small early effect with large late consequences, as often happens with evolution,” -Pierre Vanderhaeghen
Neanderthals, one of the last hominins to live on the planet, had a very similar brain size to H. Sapiens (Hunt, 2022). Some research even suggests that their brains were slightly larger than ours (Hunt, 2022). Why are we able to speak or create advanced technology? How come Neanderthals went extinct, but we thrived?
What was the missing link?
A group of researchers led by Anneline Pinson from the Max Planck Institute of Molecular Cell Biology and Genetics have found a single amino acid difference — lysine to arginine — in the H. Sapiens version of the gene, hTKTL1, and the one in Neanderthals, aTKTL1 (Mathur, 2022). The researchers inserted the two different genes into various specimen, including mice embryos, ferret embryos, and brain tissue grown from human stem cells (organoids) (Hunt, 2022).
It was concluded that, although the brain size was similar, H. Sapiens had more neurons than Neanderthals, especially in the frontal cortex where the TKTL1 gene was most active (Hunt, 2022). The human variant, hTKTL1, produced more basal radial glia (bRG), which are progenitor cells that resulted in a much larger number of neurons in the cerebral cortex (Mathur, 2022). The archaic variant, aTKTL1, resulted in less production of bRG and overall less neuron generation (Mathur, 2022).
The FOXP2 gene, also known as the “language gene,” is not a human-specific gene — it is actually found in many vertebrates (Your Genome, n.d.). It is responsible for language, bird song, echolocation, and various other functions among different organisms (Your Genome, n.d.). The FOXP2 gene codes for the FOXP2 protein, which varies by 2 amino acids between humans and chimpanzees, and 3 amino acids between humans and mice (Your Genome, n.d.). When the human FOXP2 gene was inserted into mice, it resulted in changes in the frequency of their vocalization, as well as slightly different wiring of certain parts of their brain (Your Genome, n.d.). The research team concluded that the FOXP2 gene is responsible for different sequences of movements in different species — in humans, it is speech and language, but it may be different in other animals (Your Genome, n.d.).
Many human patients with speech and language difficulties were found to have a mutation in their FOXP2 gene (Your Genome, n.d.). Simon Fisher, Anthony Monaco, and their team concluded that the gene was essential for the development of language and speech in humans, and a mutation would result in an inability to verbally communicate (Your Genome, n.d.).
Neanderthals had the same FOXP2 variant as humans, so they may have had the same capacity for speech as modern humans (Your Genome, n.d.). However, some evidence suggests that Neanderthals required more of their brain to support their larger eyes, bodies, and other physiological functions (Your Genome, n.d.). Therefore, Neanderthals may not have had enough brain capacity to spare for sophisticated language and verbal communication (Your Genome, n.d.).
Genetics or Environment?
During times of extreme climate change, the human brain increased in size dramatically (Smithsonian National Museum of Natural History, 2022). The greatest instance of this was around 800,000–200,000 years ago (Smithsonian National Museum of Natural History, 2022). As can be seen in the graph below, the greatest increase in brain size occurred simultaneously with the greatest climate fluctuations.
Humans developed larger brains as they encountered new, different environments and climates. Their need for problem-solving skills increased and helped develop a more complex brain. Many other hominins encountered the same obstacles as Homo Sapiens, but did not manage to overcome the problems and develop bigger brains, hence becoming extinct in the face of severe climate (Smithsonian National Museum of Natural History, 2022).
In addition, the structure of the brain has evolved with the species in order to adapt to its lifestyle and needs. The olfactory bulb is relatively smaller, whereas the visual cortex and neocortex have increased in importance, and hence in size (Rosales-Reynoso et al., 2018). Older primates like prosimians, who are nocturnal, rely much more on olfaction for reproduction and communication, which is supported by the findings that their olfactory bulbs are relatively larger than those in modern humans (Rosales-Reynoso et al., 2018). Anthropoid primates like monkeys and modern humans rely much more on the neocortex (Rosales-Reynoso et al., 2018). They do not need to depend on olfaction, as they can process visual information in broad daylight (Rosales-Reynoso et al., 2018). Their visual acuity enables them to spot ripe fruit, locate distant objects, etc.
The larger brain resulted in a much larger need for fuel, as the brain uses about 20% of a human’s resting metabolism and 20% of the body’s blood, while it only makes up around 2% of the body’s weight (Smithsonian National Museum of Natural History, 2022). In 1995, anthropologist Leslie Aiello and evolutionary biologist Peter Wheeler hypothesized that in order to accommodate such a large fuel consumption, the body likely had to trade off by shrinking other organs and organ systems, like the small intestine and other parts of the gastrointestinal tract (Jabr, 2015).
Anthropologist Richard Wangham expanded on this idea and pointed out that cooked foods were much easier to digest than raw foods, which meant less work for the digestive system and possibly led to a smaller GI tract over time (Jabr, 2015). Other hypotheses have been proposed regarding the muscular system, especially given how much stronger our hominid relatives, like chimpanzees, are compared to us (Jabr, 2015).
A lack of adequate nutrition since the agricultural revolution around 10,000 years ago may have caused our brains to actually shrink in size (Scientific American, 2013). However, this trend may have rebounded in the last 100 years with the increase in healthcare and childhood nutrition (Scientific American, 2013).
Our brains are continuously evolving, and a few hundred years of trends does not dictate how our brains will continue to develop in the next several thousand years. The history of our brain’s evolution has been a topic of debate for several years, and continues to be a field full of new discoveries.
References available upon request.
