Newborn babies' brains aren't "underdeveloped" compared with those of other primates at birth, a new study suggests.
In the past, scientists typically compared brain development between species by measuring how much each species' newborn brain size differs from its adult brain size. Compared with other primates, human babies' brains are significantly smaller than adults' brains. Meanwhile, newborn and adult primates have a smaller gap, which has led to the popular misconception that human newborns are "underdeveloped" in comparison.
In the new study, published Monday (Dec. 4) in the journal Nature Ecology & Evolution, scientists instead considered how the absolute size of the brain at birth, compared with adulthood, has varied across mammalian evolution. They found that of all mammals with a placenta, humans showed the strongest evolutionary drive toward having a proportionally small brain size at birth. However, this is not because newborn humans' brains are smaller than expected but rather that our adult brains are dramatically larger.
In other words, we're not developmentally behind primates at birth — we just have significantly more growing to do.
"Our study shows that human brains are not substantially less developed than the brains of other primates at birth, and that they simply appear so because we normally compare brain size in newborns with adult brain size, which is much bigger in humans," lead study author Aida Gomez-Robles, an associate professor of anthropology at University College London, told Live Science in an email.
Related: In rare cases, COVID-19 infection in pregnancy can cause brain damage to fetuses
To better understand how babies' small brains stack up against other newborn animals, Gomez-Robles and colleagues analyzed brain development in modern humans, our extinct relatives the Neanderthals, and various primates, including chimpanzees, bonobos, gorillas and orangutans. In all, the study included 44 primate species, as well as dozens of additional mammals, from rodents to hooved animals and large carnivores.
They found that humans had the smallest relative brain size at birth compared with adulthood of all primates — newborn brains are less than 25% the size of adults.
Researchers believe human babies have relatively small brains because a smaller brain means childbirth is more likely to be successful.
"Classic models assume that human babies are born before they are as developed as other primates because otherwise their brains (and heads) would be too big to go through the birth canal," Gomez-Robles said. This developmental pattern ties into the fact that humans are bipedal, meaning they move around while upright on two feet, which requires a narrow pelvis compared with primates, she said.
However, human baby brains were not significantly underdeveloped compared to age-matched primates, when you consider key steps in early brain development.
When the authors focused specifically on human evolution, they found that only a few of these steps were shifted to occur after birth, instead of in the womb. These processes mainly include the insulation of nerves within brain structures, such as the hippocampus. As this makes nerves more efficient at communicating with each other, it may subsequently play a big role in driving human brain plasticity after birth, the authors wrote in the paper.
The researchers also looked at the time that human babies spend in the womb, and they found that this was no shorter than would be expected for other primates. This suggests that having relatively smaller brains as newborns is not because humans spend comparatively less time developing in the womb.
Most of the authors' results are based on estimates from mathematical models, as they were studying brain development across evolution. For example, when studying our human ancestors, they relied on approximate patterns of brain development inferred from fossilized remains.
"These estimates are valuable because they help us understand general patterns of evolution of brain development, but they are not empirical data, so they are not expected to reflect perfectly the actual ancestral values" in terms of their relative brain size over their life span, for example, Gomez-Robles said.
In future studies, the authors would like to compare these estimates with actual measurements of brain development in newborns across different species. Depending on time and resource constraints, this would be possible to a certain extent in present-day mammalian species but not in extinct species, Gomez-Robles said.
For that, they'll have to go back to the drawing board.
"The major challenge is inferring patterns of brain development in fossil hominins," our extinct ancestors, she said.
Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!
