The human brain is unique: Our remarkable cognitive capacity has allowed us to invent the wheel, build the pyramids and land on the moon. In fact, scientists sometimes refer to the human brain as the “crowning achievement of evolution.”
But what, exactly, makes our brains so special? Some leading arguments have been that our brains have more neurons and expend more energy than would be expected for our size, and that our cerebral cortex, which is responsible for higher cognition, is disproportionately large—accounting for over 80 percent of our total brain mass.
Suzana Herculano-Houzel, a neuroscientist at the Institute of Biomedical Science in Rio de Janeiro, debunked these well-established beliefs in recent years when she discovered a novel way of counting neurons—dissolving brains into a homogenous mixture, or “brain soup.” Using this technique she found the number of neurons relative to brain size to be consistent with other primates, and that the cerebral cortex, the region responsible for higher cognition, only holds around 20 percent of all our brain’s neurons, a similar proportion found in other mammals. In light of these findings, she argues that the human brain is actually just a linearly scaled-up primate brain that grew in size as we started to consume more calories, thanks to the advent of cooked food.
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Other researchers have found that traits once believed to belong solely to humans also exist in other members of the animal kingdom. Monkeys have a sense of fairness. Chimps engage in war. Rats show altruism and exhibit empathy. In a study published last week in Nature Communications, neuroscientist Christopher Petkov and his group at Newcastle University found that macaques and humans share brain areas responsible for processing the basic structures of language. [Scientific American is part of Nature Publishing Group.]
Although some of the previously proposed reasons our brains are special may have been debunked, there are still many ways in which we are different. They lie in our genes and our ability to adapt to our surroundings. Two other recently published studies add new insight to the debate.
Unique genetic signatures
At the genetic level, humans are similar to other animals. We share more than 90 percent of our DNA with our closest relatives, including chimpanzees, bonobos and gorillas. Mice and humans also share many of the same genes—which is why scientists use them as a model to study many human diseases. Studies in recent years, however, have revealed that the way in which genes, the segments of DNA that code for specific proteins, are expressed can be quite different among humans and other animals.
One reason scientists can now unravel these more nuanced differences between the human brain and those of other species is the rise of more robust data collection techniques. For example, scientists at the Allen Institute for Brain Science have developed detailed atlases of the expression patterns of thousands of genes in various species, including those of adult mice and human brains. In a study published last week in Nature Neuroscience researchers used these enormous data sets to look for the patterns of gene expressionthat are shared within the human population. They were able to identify 32 unique signatures within 20,000 genes that appear to be shared across 132 brain regions in six individuals (see a map here). This unique genetic code may help explain what gives rise to our distinctly human traits.
When the researchers compared humans with mice, they found that whereas the genes associated with neurons were well preserved among species, those associated with glial cells—nonneuronal cells with a wide variety of functions—were not. They also found the gene patterns associated with glia overlap with those implicated with disorders of the brain, such as Alzheimer’s disease. This adds to the recent developments revealing that glial cells, which for a long time were thought to simply be the brain’s support cells, are actually a major player in both development and disease. “It affirms the importance of these glial patterns in brain disease,” says Michael Hawrylycz, a computational biologist at the Allen Institute and first author of the study.
This finding may have another important implication—the capacity for plasticity; researchers have found the glia play an important role in shaping the brain. “One interesting thing in the context of [the uniqueness of the human brain] is that you could imagine that one way to enhance the system would be to make it more plastic—I'm hypothesizing here, but [glia] could potentially be one route to do that,” says Allen neuroscientist Ed Lein, senior author of the paper. “[But] we still need to do the analysis to see whether this is specific to humans or is common among primates.”
From monkey to human
Plasticity may be what underlies the specific differences in our brain that lead to our unique cognitive abilities. A study published last week in Proceedings of the National Academy of Sciences revealed that human brains may be less genetically inheritable, and therefore more plastic, than those of chimpanzees, our closest ancestors.
Aida Gómez-Robles, an anthropologist at The George Washington University, and her colleagues compared the effect of genes on brain size and organization in 218 human and 206 chimpanzee brains. They found that although brain size was highly heritable in both species, the organization of the cerebral cortex—especially in areas involved in higher-order cognition functions—was much less genetically controlled in humans than in chimps. One potential explanation for this difference, according to the researchers, is that because our brains are less developed than those of our primate cousins at birth, it creates a longer period during which we can be molded by our surroundings.
More research, however, is required to pinpoint exactly where those differences lie. “There are still many things we don’t know about what humans share with great apes and with other mammals,” Gómez-Robles says. Understanding where we are unique and where we are not will not only shed light on how we became the dominant species on the planet but can also help us better understand ourselves. Knowing where the parallels between humans and other species lie can also help scientists develop more effective therapies and treatments for disorders and diseases.