Biology

Scientists Were Wrong About Human Pattern of Brain Asymmetry Being Unique

Human Endocast

The shared brain asymmetry pattern is visualized on a human endocast (cast of the internal bony braincase) in lateral view (left) and from the lower side (right). This pattern includes a more backwards projecting left hemisphere and a more forward projecting right hemisphere with localized larger surface areas (orange) in one hemisphere as compared to corresponding smaller regions (blue) in the other hemisphere. It also includes differential projections of the cerebellar lobes and the temporal poles. Credit: Simon Neubauer, CC BY-NC-ND 4.0

Brain imprints on cranial bones from great apes and humans refute the long-held notion that the human pattern of brain asymmetry is unique.

The left and right side of our brain are specialized for some cognitive abilities. For example, in humans, language is processed predominantly in the left hemisphere, and the right hand is controlled by the motor cortex in the left hemisphere. The functional lateralization is reflected by morphological asymmetry of the brain. Left and right hemispheres differ subtly in brain anatomy, the distribution of nerve cells, their connectivity and neurochemistry. Asymmetries of outer brain shape are even visible on endocasts. Most humans have a combination of a more projecting left occipital lobe (located in the back of the brain) with a more projecting right frontal lobe. Brain asymmetry is commonly interpreted as crucial for human brain function and cognition because it reflects functional lateralization. However, comparative studies among primates are rare and it is not known which aspects of brain asymmetry are really uniquely human. Based on previously available data, scientists assumed that many aspects of brain asymmetry evolved only recently, after the split between the human lineage from the lineage of our closest living relatives, the chimpanzees.

In a new paper, researchers from the Max Planck Institute for Evolutionary Anthropology and the University of Vienna measured the magnitude and pattern of shape asymmetry of endocasts from humans and apes. “Great ape brains are rarely available for study, but we have developed methods to extract brain asymmetry data from skulls, which are easier to access. This made our study possible in the first place,” says lead author Simon Neubauer.

Humans, chimpanzees, gorillas, and orangutans (from left to right) have differently shaped endocasts and brains (see top row). But they share an asymmetry pattern, as visualized in the bottom row. This pattern includes a more backwards projecting left hemisphere and a more forward projecting right hemisphere with localized larger surface areas (orange) in one hemisphere as compared to corresponding smaller regions (blue) in the other hemisphere. Credit: Simon Neubauer, CC BY-NC-ND 4.0

The team found that the magnitude of asymmetry was about the same in humans and most great apes. Only chimpanzees were, on average, less asymmetric than humans, gorillas, and orangutans. They also investigated the pattern of asymmetry and could demonstrate that not only humans, but also chimpanzees, gorillas, and orangutans showed the asymmetry pattern previously described as typically human: the left occipital lobe, the right frontal lobe, as well as the right temporal pole and the right cerebellar lobe projecting more relatively to their contralateral parts. “What surprised us even more,” says Philipp Mitteroecker, a co-author of the study, “was that humans were least consistent in this asymmetry with a lot of individual variation around the most common pattern.” The authors interpret this as a sign of increased functional and developmental modularization of the human brain. For example, the differential projections of the occipital lobe and the cerebellum are less correlated in humans than in great apes. This finding is interesting because the cerebellum in humans underwent dramatic evolutionary changes and it seems that thereby its asymmetry was affected as well.

The finding of a shared asymmetry pattern but greater variability in humans is intriguing for the interpretation of human brain evolution. An endocast of one of our fossil ancestors that shows this asymmetry can no longer be interpreted as evidence for human-specific functional brain lateralization without other (archaeological) data. Philipp Gunz, a co-author of the study, explains: “This shared asymmetry pattern of the brain evolved already before the origin of the human lineage. Humans seem to have built upon this morphological pattern to establish functional brain lateralization related to typical human behaviors.”

Reference: “Evolution of brain lateralization: A shared hominid pattern of endocranial asymmetry is much more variable in humans than in great apes” by Simon Neubauer, Philipp Gunz, Nadia A. Scott, Jean-Jacques Hublin and Philipp Mitteroecker, 14 February 2020, Science Advances.
DOI: 10.1126/sciadv.aax9935

Share
By
Max Planck Institute for Evolutionary Anthropology

Recent Posts

Traffic Pollution Has Been Associated With an Increased Risk of Dementia

The meta-analysis reviewed 17 studies studying traffic-related air pollution. According to a meta-analysis recently published…

December 2, 2022

Startling – Elevated Levels of Arsenic Found in Nevada’s Private Wells

Numerous residential wells need improved drinking water treatment and monitoring, according to the study. Private…

December 2, 2022

Surreal Video of Stressed Cells Helps Biologists Solve a Decades-Old Mystery

Crowded rooms: How Carnegie Mellon University and the University of Pittsburgh researchers solved a cell…

December 2, 2022

Never-Before-Seen Molecule: Webb Reveals a “Hot Saturn” Exoplanet Atmosphere

New Webb Space Telescope observations of WASP-39 b reveal a never-before-seen molecule in the atmosphere…

December 2, 2022

What Predicts Parents’ Desire for More Children?

There are no differences in the desire for more children or the ideal family, according…

December 2, 2022

Anti-Aging Medicines Seek To Eliminate “Zombie” Cells – But Could This Be Dangerous?

Senescent Cells Help To Heal Damaged Tissues According to a recent study from the University…

December 2, 2022