Bigger Cerebellum Size May Have Helped Early Humans Thrive

Reconstructed Neanderthal brains. (a) Population-average. (b) Representative modern human subject. (c) The reconstructed brains with the neuroanatomical labels.

Source: Takanori Kochiyama et al. (2018)/Scientific Reports

In recent months, there’s been a groundswell of evidence showing that more volume in both the left and right hemispheres of the cerebellum (Latin for “little brain”) may be linked to Homo sapiens’ evolutionary success in comparison to Neanderthals, who inhabited Ice-Age Europe about 250,000 to 40,000 years ago.

Differences in brain shape between a present-day human (left, in blue) and a Neanderthal from La Chapelle-aux-Saints (right, in red).

Source: Max Planck Institute for Evolutionary Anthropology/Simon Neubauer, Jean-Jacques Hublin, Philipp Gunz (CC BY-NC)

In January 2018, researchers from the Department of Human Evolution at the Max Planck Institute in Leipzig reported that the bulging of the cerebellar hemispheres played a significant role in giving present-day human brains a more globular shape in comparison to Neanderthals, who had a more elongated endocranial shape. This paper, "The Evolution of Modern Human Brain Shape," was published in Scientific Advances.

Now, a new paper, “Reconstructing the Neanderthal Brain Using Computational Anatomy,” published on April 26 in the journal Scientific Reports, also shows that early Homo sapiens had bigger cerebellar hemispheres than Neanderthals. The multidisciplinary team of researchers speculates that the extinction of Neanderthals and the expansion of Homo sapiens may be tied to cerebellum size.

For the latest study on cerebellum volume, an international team led by researchers in Japan used state-of-the-art computational neuroanatomy to reconstruct 3-D maps of the entire brain from three cohorts: (1) Neanderthals, (2) early Homo sapiens, and (3) modern human subjects. (These images are at the top of the page.) The authors sum up the main takeaway from their findings:

In conclusion, we found that Neanderthals had significantly relatively smaller cerebellar hemispheres than Homo sapiens, particularly on the right side. Larger cerebellar hemispheres were related to higher cognitive and social functions including executive functions, language processing and episodic and working memory capacity.

Such a neuroanatomical difference in the cerebellum may have caused important differences in cognitive and social abilities between the two species and might have contributed to the replacement of Neanderthals by early Homo sapiens.

Left and right cerebellar hemispheres in red. "Cerebellar" is the sister word to "cerebral" and means, 'located in or related to the cerebellum.'

Source: Life Sciences Database/Wikimedia Commons

Interestingly, when the researchers set out to reconstruct these ancient brains using computational anatomy, they expected to find that early Homo sapiens had bigger frontal lobes in the cerebrum (Latin for "brain"), because this cerebral region is considered the seat of higher-order cognitive functions. However, to their surprise, Kochiyama et al. found that the brain volume in the frontal lobes of Neanderthals and early Homo sapiens was basically the same. On the flip side, the cerebellar hemispheres, which are neatly tucked beneath the left brain-right brain of the cerebrum, stood out as being much smaller in Neanderthals.

Until recently, most experts believed that the cerebellum was not involved in higher-level cognitive functions. The general belief was that the sole job of the “non-thinking” cerebellum was to coordinate muscle movements and maintain balance. That being said, it was widely accepted that the right cerebellar hemisphere worked in conjunction with the motor cortex in the left cerebral hemisphere to fine-tune coordination on the right side of the body in a lateralized fashion. Conversely, the left cerebellar hemisphere coordinates with the right brain to fine-tune movements on the left side of the body.

This “motor-coordination-only” concept of the cerebellum began to shift in the late-20th century, when Jeremy Schmahmann of Harvard Medical School published his landmark “Dysmetria of Thought" (1998) hypothesis. Based on his extensive clinical and bedside observation of ataxia patients at Massachusetts General Hospital, Schmahmann realized that specific regions in each hemisphere of the cerebellum appeared to work in conjunction with specific regions in each cerebral hemisphere to coordinate our movements and our thoughts. This was a radical and revolutionary concept.

Kochiyama et al. describe the current 21st-century understanding of cerebro-cerebellar lateralization in their recent 2018 paper:

The functions of the cerebellar hemispheres differ according to location, as different parts of the cerebellum are anatomically and functionally connected to different regions of the cerebrum. In particular, the lateral parts of the cerebellar hemisphere are anatomically connected to the opposite side of the association cortices in the cerebrum. Our finding of laterality in terms of the relatively small right cerebellar hemisphere of Neanderthal indicates minimal connection to the left prefrontal regions, which has one of the major roles in language processing, potentially causing disparity of language ability between Neanderthal and Homo sapiens.

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These are exciting times for cerebellar research. If you’d like to learn more about how our cerebellar hemispheres work in conjunction with the left and right cerebral hemispheres to coordinate both our thoughts and our movements, please watch this March 2018 keynote presentation by Jeremy Schmahmann: "Ataxia, Dysmetria of Thought, and the Cerebellar Cognitive Affective Syndrome."