One of the major turning points in scientific knowledge of our own species took place in 2001. After a 13-year research project and five billion dollars, an international team of geneticists was able to announce that they had now mapped the entire human genome.
In one fell swoop, scientists now had the recipe for a human in their hands. Five years later, chimpanzee DNA was also mapped. Scientists were now ready to find crucial differences between the two species – and among other things explain why our brain is three times larger than a chimpanzee’s. But it turned out not to be as easy as one might think.
The genes of humans and chimpanzees are 99% the same – but that does not mean that all the differences are found in this one percent that separates them, because the differences can also be hidden in the parts of the genetic material that are outside the gene.
A small piece of DNA gives the brain growing pains
Even extremely small differences in human and chimpanzee DNA can have huge consequences. Just 16 variations in DNA bases in the more than 1,000 base pairs of DNA contribute to our having much larger brains than other apes.
The genes act as blueprints for the body’s building blocks, the proteins, while mapping the human DNA revealed that genes make up only 1.5% of the DNA. The rest was called DNA junk, because it didn’t seem to have any particular function in our bodies. This turned out to be a huge mistake. In 2012, scientists showed that at least 80% of all our DNA is biologically active.
A large part of it controls the activity of the genes – including the genes that control the development of the brain. Finding the difference that gives us big brains is a task that is far from over – even though scientists have already made many important discoveries.
But as scientists struggle to understand why our brains have grown so much, they have encountered another problem. It has been found that our brains are shrinking again.
Our genes give animals big brains
In 2015, American researchers inserted a human DNA sequence called HARE5 into mouse fetuses and compared brain development with other mouse fetuses that received the chimpanzee variant of the same sequence.
The HARE5 sequence, which was previously considered to belong to DNA debris, is 1,219 base pairs long, and the difference between ours and the chimpanzee type is only 16 base pairs. But the experiment showed how important this difference is. The fetuses with the human variant developed brains that were 12% larger than those with the chimpanzee variant.
In 2019, Chinese researchers inserted the human variant of the MCPH1 gene into 11 macaque fetuses, with the result that their brains continued to grow longer than otherwise. Five of the fetuses survived to birth, and when scientists then tested their cognitive abilities, it was found that their memory was better than that of macaque monkeys without the human gene.
If macaque monkeys can be born with human genes, so can chimpanzees.
There are many important ethical questions that arise from such experiments – especially in cases where genetically modified animals live beyond birth. The fact that Chinese scientists have allowed macaque monkeys to be born with human genes shows that something similar could happen with chimpanzees.
The result would then be a chimpanzee with a larger brain, thus reducing the difference between us and them and raising questions about the moral rights of such an animal.
The brain is a relative size
This new insight into the development of the brain’s genes is a major step forward, but it has not yet helped scientists understand what causes development.
In order to solve that riddle, they have instead turned their attention to the skulls of our ancestors. Previously, scientists did not have an overview of how and how fast the development of the brain took place, but this was corrected by a team of American and British scientists in 2019.
They compiled all the data they found about the brains of our ancestors over the past four million years. Based on the correlation between brain size and body weight in living primates, the researchers developed a baseline that expresses this correlation in the average primate.
And when they compared the numbers to our ancestors at baseline, a clear picture of brain development emerged. Our brains are now 238% larger than expected for primates of our body weight.
If we go back two million years to one of the first species in the human family, Homo erectus, the figure is 121%. And if we go back a few million years to the species Australopithecus afarensis, the figure is 59%. The figure for chimpanzees is also 59%.
The growth rate of the brain has increased
Both humans and chimpanzees have unusually large brains compared to other apes, but humans are in a completely different league than chimpanzees. Between four and two million years ago, our brains ran out in the race for size.
The results confirm the picture that our brains have grown over time; something that has not occurred to a similar extent in chimpanzees. But the researchers’ data also reveal surprising trends.
The development of a larger brain has previously been linked to the development of our mental qualities, so the experts expected that it was precisely these abilities that could be found in the outermost part of the cerebral cortex that had grown the most. But that is not the case. In us, the volume of the cerebral cortex is almost the same as in other monkeys.
Thus, researchers found no difference in the proportions of the brain between humans and chimpanzees. That means the basic design of our brains was already there when we diverged from chimpanzees.
The little brain is underrated
The proportions of our brain had not changed for at least 13 million years. He has certainly grown, but all parts of him seem to have done so equally. The key to this growth is therefore not to be found in any one place in the brain, but in all of it.
An upright gait freed the hands of our ancestors.
These truths have opened the eyes of scientists to parts of the brain that were previously not considered central to evolution. One of them is the cerebellum. It sits beneath the cerebrum and is best known for fine-tuning our movements to be fluid rather than jerky, just as it contributes to coordination, precision and timing.
But it’s also involved in mental issues like attention, language, and emotional control—all qualities our ancestors needed when they ventured out into the wild in the old days.
Walking upright alone has made great demands on balance, and when the hands no longer had to support walking on the ground, new possibilities opened up to use fine motor skills to create tools, among other things. Furthermore, the evolution of language and the ability to suppress violent emotional reactions has made it easier for us to live together in large groups.
All of these traits have probably played an important role in the development of the brain, but it is not yet clear in what order the needs and abilities have emerged or how the two have interacted.
The brain is everything to humans
The human brain is about 4% of body weight, but it absorbs a fifth of the energy we ingest. No other animal relies so heavily on its brain. Therefore, there must be some special reason why this can pay off for us to survive.
In one attempt to find this cause , English scientists created a mathematical model in which they set up different scenarios. The model is a kind of financial statement that shows what kind of problem can be solved by investing in a bigger brain.
In the model, a distinction is made between ecological problems created by the external environment and the social problems created by relationships between numerous individuals in a group. After running many scenarios, the researchers found a combination of these two factors that best matches the actual evolution of human brain size.
The model shows that the driving force behind brain development is 60% ecological problems, 30% social problems and 10% competition between individuals.
Our ancestors therefore primarily benefited from a larger brain in order to survive the challenges and accompanying dangers of the grasses. This large brain was used, among other things, to find food, preserve it until later and process it so that it would be easier to eat and digest.
This is where traits such as stalking prey, making tools and starting fire come into play. Social traits have also played a major role in brain size. They have made it possible for us to work together in larger groups, for example to bring down large prey or to attack larger groups and steal their food.
“If it weren’t for social challenges, our brains would have gotten even bigger.”
Mauricio González-Forero, marine biologist
But to the researchers’ surprise, their models showed that social problems alone do not lead to larger brains, but rather to shrinking brains.
“If there were no social challenges, our brains might have gotten even bigger, but definitely unfortunate for social life.” Bigger is therefore not necessarily better,” says one of the scientists behind the project, Mauricio Gonzáles-Forero.
The model thus explains not only why we have such large brains, but also why they are not yet larger. Similarly, the ancestors of chimpanzees must have faced other challenges that have resulted in smaller brains – for example, ecological problems were less important and social ones more important.
Our brains are shrinking again
A big brain costs the body a lot, and if it doesn’t increase our chances of survival and reproduction, then evolution will see to it that the brain shrinks. And that is exactly what has happened in the last 20,000 years.
Measurements of skulls show that the size of the brain fell from 1,500 cubic centimeters to 1,350 in men. It corresponds to one tennis ball. And women’s brains shrank correspondingly. Our brains probably reached their maximum size around 45,000 years ago with the so-called Cro-Magnon humans.
This was the first Homo sapiens species in Europe and is known for creating spectacular cave paintings and various types of tools, figurines and ornaments. Cro-Magnon man lived at the same time as Neanderthals who also had larger brains than we have today. But Neanderthals were also heavier than us, so proportionally their brains were smaller than ours.
Why our brains have shrunk over the past 20,000 years is still debated. Scientists have come up with many hypotheses and perhaps the truth lies in their mixture. One of them is that our bodies changed when the ice age ended.
In warmer climates, it was no longer necessary to rely on large body mass. We got leaner and lost muscle mass, so we could get by with smaller brains. Another theory says that the transition from a hunter-gatherer society to arable farming changed our diet, which became less protein, but it is an important building block for a large brain.
Finally, we can mention the third theory which simply says that we have become dumber because it no longer pays to be smart in a society where the strong oppress the weak.
One well-reasoned hypothesis came from the American scientists David Geary and Drew Bailey who work at the University of Missouri. In 2010, they studied the evolution of brain sizes during the period from 1.9 million years ago to 10,000 years ago.
The period thus covers the development all the way back to the appearance of Homo erectus until Homo sapiens had spread over the entire globe. The aim of the study was to reveal how social conditions have played a role in the size of the brain.
As a measure of the complexity of social conditions for our ancestors, the researchers estimated the population. There they found a clear correlation between population and brain size.
The more the ancestors were together in groups, the smaller the brain became. The biggest reduction in the brain occurred around 15,000 years ago when the brain shrank by 3 – 4%. This applies to human beings everywhere in the world.
The researchers believe that the reason may be that a densely populated society provides a better option for the division of labor. Instead of being millenarians who can solve all tasks, each one can specialize in his field. Each one may not be as smart but in the company of peers we become smarter.
Smaller brains can be better
Other researchers have found similar trends. American anthropologist John Hawks has studied skulls in Europe from the Bronze Age to the Middle Ages. During this period, the number of people increased significantly and the brain decreased accordingly.
If evolution continues, our brains could become the size of chimpanzees.
However, Hawks does not believe that evolution has necessarily made us dumber as individuals. He suggests that urbanization may have increased the rate of evolution, because then more mutations occur in large groups.
Furthermore, beneficial mutations spread more rapidly the closer we are to each other over several distances. New mutations may thus have made the brain more efficient, for example by improving connections between individual brain centers.
A bigger brain is thus not necessarily a better brain. And the trend towards smaller brains is probably not over.
If our brain continues to shrink at the rate it has for the last 20,000 years, in 50,000 years it will be the same size as the brain of Homo erectus was 2 million years ago. And in some 130,000 years, humans may have a brain the size of a chimpanzee.
