Metabolite
study demonstrates human muscle and brain tissue underwent disproportionate
evolutionary change
A new study, published in the open
access journal PLoS One Biology, has used metabolites to track evolutionary
changes in brain and skeletal muscle tissues. Metabolites are metabolic products
or intermediated of low molecular weight (1,500 amu or less), which are
associated with the physiological processes that maintain the functionality of
body tissues. Changes in the concentrations of these metabolites are thought to
be closely related to evolutionary changes in the associated tissues.
Researchers measured the concentrations
of more than 10,000 metabolites in the prefrontal cortex, primary visual
cortex, cerebellar cortex, skeletal muscles and kidneys of humans, chimpanzees,
macaque monkeys and mice using mass spectrometry-based techniques. They found
that in most cases the differences reflected genetic distances between the
species rather than environmental differences.
The striking exception was found in
the human lineage. The concentration profiles of metabolites associated with the
human prefrontal cortex, cerebellar cortex and skeletal tissues showed far
greater changes than could be accounted for by genetic difference: by a factor
of four for the brain tissue, and eight for the muscle tissue. In fact the
muscle tissue is implied to have undergone more evolutionary change in the 6 to
7 million years since the divergence from chimpanzees than it did during the
130 or so million years separating mice from the common ancestor of the apes
and Old World monkeys. No comparable differences were noted for the primary
visual cortex or kidneys. Nor were significant differences to any of these
results found after controlling for differences in diet and levels of physical
activity.
It is well known that humans are
physically quite weak in comparison to chimpanzees, despite weighing in at
around twice the size. Surprisingly, this is largely based on anecdotal
observations mostly predating the 1950s. Accordingly, the researchers set macaque,
chimpanzee and human subjects a ‘pulling task’, which tested both upper and
lower body strength. These tests confirmed the anecdotal observations.
The researchers concluded that the
metabolic changes in human muscle tissue were associated with a drastic
reduction in muscle strength; and that these changes might be linked to the
changes in brain metabolism and enhanced cognitive abilities.
The findings are an extension of
Aiello and Wheeler’s ‘expensive tissue’ hypothesis, which proposed that the
considerable energy requirements of the human brain (around 20 percent of the
total energy budget) could only be met by making savings elsewhere. Aiello and
Wheeler (1995) proposed these savings were made by downsizing other
energetically-expensive organs, principally the gut. Apparently, though, this
was insufficient and further savings were required in the form of a decrease in
the energy expenditure of skeletal muscle.
References:
1. Bozek, K. et al.,
Exceptional Evolutionary Divergence of Human Muscle and Brain Metabolomes
Parallels Human Cognitive and Physical Uniqueness. PLoS One Biology 12
(5), e1001871 (2014).
2. Aiello, L. & Wheeler, P.,
The expensive tissue hypothesis: the brain and the digestive system in human
and primate evolution. Current Anthropology 36, 199-221 (1995).
Link (open access):
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