Neuroscientists have long debated whether our prefrontal cortex takes longer to develop than in other primates — and whether this extended period gives us exceptional cognitive abilities — but new research suggests that human brain circuitry matures much like that of chimpanzees and macaques.
"It's true that our development and especially our life span is extended compared with other primate species," Christine Charvet, an assistant professor and a neurobiologist at Delaware State University and the author of the study, told The Academic Times. "But the question is, what is unusually extended after we control for the life span?"
The findings, presented Tuesday at a conference hosted by Experimental Biology, are explored in a Feb. 10 study published in Proceedings of the Royal Society B and in a forthcoming research paper authored by Charvet and several colleagues. The investigators found no evidence that the prefrontal cortex maturation period is longer in humans than in chimpanzees or macaques. "We often talk about how the prefrontal cortex development should be extended relative to other primates," Charvet said. "We're just not finding it."
Separate studies have recently found that early humans had great-ape-like brains and that other primates have logical-reasoning skills that exceed those of human toddlers. Charvet believes her research, which has revealed a number of unexpected parallels in the frontal cortex neural circuitry of primates and humans, is a starting point for scientists to better understand what makes the human brain distinct from the brains of similar mammals. Her yet-to-be-published study advances a new approach for visualizing the brain connectome in humans and nonhuman primates, allowing for unprecedented comparisons between species.
"We often compare the biology of humans with chimpanzees, with the goal of identifying what is it about our brains or biology that is different from our closest relatives," she said. "But we actually don't know how to compare chimpanzees to humans: We don't know what the corresponding ages are; we don't know how old chimpanzees are in human days. If we had a way to compare the biology of these two species, we would be better equipped to identify what is it about our biology that stands out."
The researchers found equivalent ages across different species by incorporating behavioral markers with measures of gene expression and brain structure. "If we were to focus on one metric — like RNA-Seq, neuroimaging or behavior alone — we wouldn't be able to find corresponding ages across the entire life span," Charvet said. "Now that we have a way to compare the biology of two species, we can better address these questions that have been debated."
Calculating equivalent ages between species isn't a new concept; at its most basic level, pet owners often discuss ages in "dog years" or "cat years." Charvet helped develop the website Translating Time, which uses a database of "equivalent post conception (PC) dates across mammalian species," to estimate, for example, a mouse's age in human days. "In the past, we relied on the timing of abrupt transformations that occur during development as a basis to find those ages," she said. "During prenatal development, things change rapidly: When do the eyes open? When do axons myelinate? We captured that information to find corresponding ages."
Focusing on abrupt transformations is a good approach for calculating equivalent ages during fetal stages of development, when changes happen relatively quickly, but it's not as accurate for postnatal development and aging, Charvet said. To remedy this issue, she added time points from more gradual changes that occur throughout the life span. "I considered metrics that change very gradually from neuroimaging or gene expression, and I aligned those trajectories — they were very similar between humans and chimpanzees — and extracted time points from those alignments," she said.
The project has been a long-term effort for Charvet, who studied neuroimaging at Boston Children's Hospital and then genetics at Cornell University, with the goal of clearing up this very debate about precortex development in humans versus nonhuman primates. But she was limited by the recurring issue of small sample sizes. "When I was a postdoc in neuroimaging, we were working with one to two baby chimp brains," she said. "We didn't have the samples to answer the questions rigorously."
Through the rare combination of neuroimaging and genetic measurements, however, it's possible to expand sample sizes and "more rigorously test questions and answer them," she said. "I think it's a much stronger approach than using one metric on its own, and it has much more potential to solve the questions we have in neuroscience."
Charvet says she intends to keep finding new ways of approaching the question of what makes us different from our closest relatives: "We're hoping to expand on this, look at more great apes and, hopefully, we'll find what is dissimilar.".
The study, "Cutting across structural and transcriptomic scales translates time across the lifespan in humans and chimpanzees," published Feb. 10 in Proceedings of the Royal Society B, was authored by Christine J. Charvet, Delaware State University.
The study, "Tracing cortical circuits in humans and non-human primates from high resolution connectomic, transcriptomic, and temporal dimensions," yet to be submitted for publication, was authored by Christine J. Charvet, Kwadwo Ofori, Jianli Sun and Melinda S. Modrell, Delaware State University; Christine Baucum, Bath Spa University; Khan Hekmatyar, University of Delaware; and Brian L. Edlow and Andre J. van der Kouwe, Massachusetts General Hospital.