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Evolution of the primate brain

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10th-Apr-2019       
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Samantha Jones, PhD :
What events led to differences between humans and other primates? What changes throughout evolution underlie such a dramatic shift in how we socialize and process information?
Although somewhat contested, the general consensus is that around 6 million years ago we diverged from the ancestor we share with bonobos and chimpanzees. Over the last 2 million years or so, the fossil record has indicated that our human brains have about tripled in size. That being said, fossils can only tell us so much.
To investigate the anatomical and cognitive changes that accompany this massive size increase, scientists are collecting data from living primate relatives of humans, while also referring to the sequenced genomes of extinct hominins for insight into gene regulation over time.
In doing so, researchers have found that changes at the DNA level may not only have led to human expression of genes that are associated with speech and language, but also increased our susceptibility to disease, which includes mental illness. For instance, geneticists have been working to determine the evolutionary shifts that have led to psychiatric disorders like schizophrenia.
Over the last few years, advances in the field have shed light on what changes likely gave rise to such distinct social and cognitive disparities between humans and non-human primates.
The expanding human brain
Human brains show rapid development after birth-prenatal human brains are much smaller than their adult size. The same can't be said for chimpanzees.
In 2012, a group of researchers in Japan were the first to investigate this development in utero. They worked with two pregnant female chimps (Pan troglodytes), using three-dimensional ultrasound imaging to track brain development in their fetuses, from 14 to 34 weeks gestation. These findings were then compared to ultrasound images of human fetuses, from 16 to 32 weeks gestation.
Brain volume consistently increased in both chimps and humans until around 22 weeks gestation. After that, chimp brain growth slowed while it continued to increase in humans. By 32 weeks gestation, the rate of chimpanzee brain growth was about 20% of that observed in humans--4.1 cm3/week versus 26.1 cm3/week, respectively.
The researchers found that, at 32 weeks gestation, the brain comprised about 40% of the chimp fetus's total body weight. In human fetuses the same age, brain volume comprised only 23% of their total weight.
This research was the first to show that the differences we see in chimp versus human brain development begins in the womb and also pinpointed when this disparity arises. So human brains grow for longer than chimp brains; what's the big deal? Turns out, these differences in the rate and time frame of brain growth are just the tip of the iceberg.
This is the first of a three-part series about what makes our human brains, and thereby the human experience, so unique.
Next week, I'll dive into the structures and cells that accompany these changes, starting with the prefrontal cortex-the structure that is so important for complex cognitive behaviors, allowing us to make productive decisions ("I guess I shouldn't kill that person who cut me in line") and respond to social cues ("That person looks unhappy, maybe I should ask if they're okay"). In the third installment, I'll get into how genetics is changing what we know about human evolution and human propensity for disease - i.e., why do we have all of these diseases that chimps don't?
(Samantha Jones, Ph.D., is a science writer based in San Diego. She received her Ph.D. in Biomedical Sciences from the University of California San Diego in 2018).

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