Brains of Neanderthals and us

Not only the shape of our skull distinguishes us from Neanderthals, but also the structure of our brain. Nevertheless, Neanderthals could have been capable of thinking similar to ours.

Neanderthals and humans – different biology, similar behavior.

Illustration: Charlotte Eckstein / NZZ

Modern man likes to feel like the crown of creation. He assumes that he is intellectually superior to his archaic relative, the Neanderthal. But this assumption could also be wrong. How else could it be explained that humans and Neanderthals lived side by side for thousands of years and even fathered children together? Scientists are beginning to understand how Neanderthal brains differed from ours.

It has long been known that the shape of the Neanderthal skull was different from that of our skull. Compared to modern humans, Neanderthals had a large face and a receding forehead. Nevertheless, the brains were about the same size. This was the result of anthropological measurements of the skull bones. Whether the brains were similar at the cellular level or not – for a long time there was no answer to this question.

This is slowly beginning to change. The discovery of a bone in 2008 gave the starting signal for the study of the Neanderthal brain. It had belonged to a Neanderthal who lived in what is now Croatia 38,000 years ago. Thanks to this find, researchers now know the complete Neanderthal genetic code.

Initial comparisons of the genome of our relatives with our genes confirmed what evolutionary biologists had long suspected: Neanderthals had many things in common with modern humans. Not only did they share common ancestors, they also fathered children together. All this emerged from the analysis of the genetic material. But the researchers also found more than a hundred genetic differences. Based on these differences, it is possible to reconstruct how the Neanderthal brain might have been structured.

Humans and Neanderthals have in common

In order to investigate the consequences of genetic differences on the brain development of Neanderthals and humans, researchers place a Neanderthal gene in human stem cells in a petri dish and observe how the growth of the cells changes. The effect of genes can also be studied in animals: for example, a gene from Neanderthals or modern humans can be implanted in the brain of a mouse embryo. How does the brain develop and how do animals behave?

20 weeks after conception: fewer cells in the brain

The results of these studies show that the brains of Neanderthals and modern humans differ in their structure. The Neanderthals had fewer nerve cells, so-called neurons, in their frontal lobes – the control center of the brain. This was reported by researchers at the Max Planck Institute for Molecular Cell Biology and Genetics in Dresden.

This could be shown by examining a gene called TKTL1, which is structured differently in Neanderthals than in humans. The amino acid lysine is found there in Neanderthals and not arginine as in humans.

The tiny difference – also known as a point mutation by experts – already has an effect in the first 22 weeks of brain development. Because then the fetus develops cells from which neurons later develop. And the more of these progenitor cells develop in the first few weeks, the more neurons develop in the course of later development in the frontal lobe.

The researchers found not only in the experiment in the petri dish, but also in the mouse embryo that fewer progenitor cells are formed in the gene variant of the Neanderthal compared to that of humans.

Differences in the brains of humans and Neanderthals

More nerve cells in the human forehead because an amino acid in the DNA was exchanged during development.

Differences in the brains of humans and Neanderthals - More nerve cells in the frontal lobe of humans because an amino acid in the DNA was swapped over the course of development.

Philipp Gunz, who researches at the Max Planck Institute for Evolutionary Anthropology in Leipzig, emphasizes how important these research results are. They clearly showed how important a small genetic difference can be for brain development.

“There are different ways to reach the goal”

So Neanderthals probably had fewer neurons in their frontal brain than modern humans. But what this difference meant for the way Neanderthals thought and behaved remains an open question. Especially in the early development phase, the brain is still very malleable. So it’s entirely possible that the Neanderthals were capable of similar thinking with fewer cells in their frontal lobes.

Or, as the neurobiologist Gunz puts it: “In neurobiology, there are different ways to reach the goal.” For example, the modern human brain can sometimes effortlessly compensate for the loss of brain cells or even entire brain regions.

This is illustrated by the case of a young woman. A stroke during childbirth destroyed a large part of her brain – the left temporal lobe – as an infant. This part of the brain normally processes spoken language. The doctors predicted that the child would never be able to speak. A severe handicap seemed inevitable. But nothing of the sort happened. The woman later studied neuroscience herself. Only recently did she an article about her career for the «New York Times» written.

So the human brain can potentially make up for the loss of an entire temporal lobe. It is quite possible that the Neanderthals, with their few neurons in the frontal lobe, got along quite well.

More neurons improve memory – in mice

Even if Neanderthals had few neurons in their frontal lobes, their entire brain was about the size of our brains and thus much larger than those of chimpanzees, gorillas and other great apes.

Researchers suspected that the cause of this was the so-called “human-specific gene ARHGAP11B” could be. Both Neanderthals and modern humans carry this gene in their genome.

In fact, using experiments in petri dishes and in mice, the researchers showed that the gene is the cause of the large brain. But when they studied this gene, they went one step further and studied the behavior of the mice. Not only did the mice develop more neural progenitor cells and more neurons in the brain, they also had better memories. Mice that had been implanted with the human-specific gene became a bit more intelligent than their conspecifics.

Mice with a large frontal brain will not exist

If you look at the whole brain, the number of neurons is characteristic of modern humans, says biologist Anneline Pinson, who was involved in the experiments. In addition, it is precisely the frontal lobe in which many human thought processes are controlled. Both suggest that the two genes and the resulting brain cells are important for human thinking.

But the researchers agree: the two genes alone do not make up the human brain and thus human thinking. How exactly the growth of the cells in the frontal lobe is related to the function of the brain – i.e. behavior – is what researchers now want to test in animals. However, mice are not suitable for this. Overall, they develop too few neurons in these experiments.

That’s why the researchers want to work with ferrets. It will be interesting to see whether these animals will suddenly become more intelligent if they receive the TKTL1 gene variant from Neanderthals or from modern humans.

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