The left and right side of the brain are involved in different tasks.
This functional lateralization and associated brain asymmetry are well
documented in humans, but little is known about brain asymmetry in our closest
living relatives, the great apes. Using endocasts (imprints of the brain on
cranial bones), scientists now challenge the long-held notion that the human
pattern of brain asymmetry is unique. They found the same asymmetry pattern in
chimpanzees, gorillas, and orangutans. However, humans were the most variable
in this pattern. This suggests that lateralized, uniquely human cognitive
abilities, such as language, evolved by adapting a presumably ancestral
asymmetry pattern.
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 hemisphere 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.
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.”
Journal article: https://advances.sciencemag.org/content/6/7/eaax9935
Source: https://myfusimotors.com/2020/02/19/researchers-were-not-right-about-left-brains-study-suggests/
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