Down From the Trees
As we made our way down from the trees
we were careful. We stayed close to one another.
The grasslands stretched in all directions, and we
felt them pulling us, ineffably, elsewhere.
As we made our way down from the trees
we were careful to pluck with each hand a fig
from the sturdy branches. We didn't know
how the universe might unfold, but we knew
it would bloom from these orbs.
— Matthew Kosinski
Inspired by Wild Figs, this month’s Ignite story explores the evolution of the human brain. What our ancestors ate — including the nutrient-rich fig — may have played a decisive role in the development of human intelligence.
Humans and chimpanzees, our closest living primate relatives, share 98.8 percent of their DNA.1 That similarity extends, unsurprisingly, to our brains. At a fundamental level, the resemblance between a human brain and a chimp brain is striking. To put it in perspective: There is more variation between different structures within the human brain than there is between parallel structures across human and chimp brains. For example, take the cerebellum, a part of the brain involved in coordinating bodily movements, among other things. The molecular structure of a human cerebellum is closer to the molecular structure of a chimp’s cerebellum than it is to the molecular structure of a human hippocampus, a separate part of the human brain that processes episodic memories.2
Comparison of human and chimpanzee brains. Diagram by paleontologist Paul Gervais, 1854; Source: Wikipedia
Yet that 1.2 percent DNA gap — as miniscule as it is — seems to have made all the difference. Sure, chimpanzees engage in all kinds of behaviors that echo our own. They’re highly social creatures who thrive in complex communities.3 They know how to use tools, and they even pass that knowledge on to other generations, indicating a form of cultural transmission.4 They may even have a kind of consciousness similar to our own.5 But humans seem to be able to achieve all these things at an even higher level and to a greater extent. Our communities are more complex, our memory and communication far superior, our cultural transmission and production more robust, and our consciousness — according to the evidence available to us — even more refined.
What makes our brains so special? What tiny tweak in our DNA is responsible for the gulf between us and our most genetically similar relatives?
“What tiny tweak in our DNA is responsible for the gulf between us and our most genetically similar relatives?”
Bonobos, another of our closest primate relatives, are said to have a level of tool use on par with Stone Age humans. Here, a bonobo “fishes” for termites using a stick. Source: Wikipedia
We’re Not So Different After All
The answer, at least for now, is that no one really knows. It’s clear that something happened in the course of the human brain’s evolution to take us off on our own, individual journey. What exactly that was — and how exactly our brains differ from those of other animals — remains a bit of a mystery.
For a long time, one popular theory held that human brains were simply more specialized than those of other animals. It was thought that humans had evolved a uniquely complex neural architecture, with highly differentiated areas of the brain refined to focus on specific tasks. This specialization of the brain’s labor was thought to be unique to humans. Other animals, it was assumed, had more uniform brain structures.
It turns out that’s not quite correct: Researchers at the University of Sydney uncovered that mouse brains are very structurally similar to human brains. Like our own, a mouse’s brain structure varies in form and function, with different areas dedicated to different tasks. While the researchers did find that human brains vary a little more, the idea that specialization is unique to human or even higher primate brains is erroneous.6
3-D map from the Allen Institute for Brain Science showing neuronal connections between different mouse brain areas; Sydney.edu
Another theory suggests plasticity, the ability of the brain to change its own structure over time, is the key. All brains — not just human brains — have a certain amount of plasticity. As we learn new things or try new activities, our neurons — the cells responsible for transmitting information throughout the brain — forge new connections or strengthen existing connections between one another.7 For example, when you learn a new language, research suggests your brain density increases because more connections are forged between neurons.8
According to this hypothesis, human brains are more plastic than others, allowing us to learn more from our surroundings than other animals. The research of anthropologist Aida Gómez-Robles backs this up: She found that the organization of the cerebral cortex is less genetically controlled in humans than in chimpanzees. The cerebral cortex is responsible for a whole host of functions, including higher-order operations like understanding language and processing new information.9 A chimpanzee’s cerebral cortex stays relatively stable from birth, but human cortexes can change quite a bit throughout our lives.10 Still, the specific mechanism for this isn’t quite understood, so it’s far from a conclusive answer.
Drawing of pigeon neurons by neuroscientist Santiago Ramón y Cajal, 1899. Source: Wikipedia
Perhaps the Kitchen Was Our Classroom
The simplest explanation for why we’re different from other primates might be that our brains are just bigger. In general, primates tend to have more neurons in our cerebral cortexes than other species.11 Even for primates, humans’ brains are big — about three times as big as a chimp’s12 and about seven times bigger than the brains of similarly sized, non-primate animals.13
The neuroscientist Suzana Herculano-Houzel, who pioneered one of the most accurate techniques for counting neurons, found that humans have about as many neurons as you would expect for their brain size. So it’s not that we have more neurons per square inch, but that our bigger brains have more room for more neurons.14
But how did our brains get so big? The brain uses more energy than any other organ, about 20 percent of our total caloric intake.15 Evolving and sustaining such an energy-hungry organ is no small task.
“But how did our brains get so big? The brain uses more energy than any other organ, about 20 percent of our total caloric intake.”
A photomicrograph of tissue from the visual cortex of the human brain. Source: Wikipedia
Herculano-Houzel believes cooking may have been the key. When humans figured out how to cook, they essentially learned how to “cheat nature,” she says. Cooking makes it easier to digest food, which means we have to spend less energy breaking down the things we eat, and we get to reap more energy in return. That allowed humans to extract many more calories than other animals could from the same amount of food — and much of that extra energy went to beefing up our brains.16
“Even before they discovered cooking, humans may have relied on particularly nutrient-dense foods to sustain themselves, which would have given their brains a similar caloric kickstart.”
Even before they discovered cooking, humans may have relied on particularly nutrient-dense foods to sustain themselves, which would have given their brains a similar caloric kickstart. For example, ecologist Mike Shanahan argues that figs may have been a key food for our early human ancestors. These calorie-packed fruits were easy to find and consume, and the energy they contained may have pumped our brains up.17
The Virtuous Cycle of Evolution
While cooking may have played a key role, it would be inaccurate to boil the human brain down to a culinary byproduct. Evolution tends to be a much more complicated process, involving disparate but overlapping feedback loops and lots of happy accidents. So what other factors went into our massive noggins?
Drawing of the human brain from Andreas Vesalius’s De humani corporis fabrica libri septem, 1543.; Source: Wikpedia
Climate change may have actually had a hand in it. The earth’s climate has fluctuated over time, and scientists have noticed that, around 800,000-200,000 years ago, we saw a period of intense climate flux. This coincides with a period of dramatic increase in our ancestors’ brain sizes. The thinking goes that bigger brains would have been an advantage that allowed our ancestors to creatively adapt to and survive in their changing environments.18
The human penchant for wanderlust may have also helped: Around 2 million years ago, our early ancestors began to venture out of modern-day Africa into what is now Asia and, later, Europe. Like a changing climate, a changing physical environment would present all kinds of novel problems, and bigger brains would again be an advantage in finding ways to thrive.19
We humans ourselves probably played a role, too. According to the “cultural brain hypothesis,” humans evolved large brains and great intelligence in order to keep up with our complex social groups. We’ve always been a social species, and we may have developed our intelligence in part to maintain those relationships and function successfully in these environments.20
Cetaceans display remarkably human-like behavior, including working in groups to create “bubble nets” in which they trap their prey; Source: Wikipedia
Part of what makes the cultural brain hypothesis compelling is that it may help explain how other animals developed similar levels of intelligence along very different lineages. For example, cetaceans — the order of marine mammals that includes whales, dolphins, and porpoises — are incredibly intelligent creatures. They have been known to engage in activities like playing with other species, using tools, teaching one another new behaviors, and even gossiping.20
“[Marine mammals] have been known to engage in activities like playing with other species, using tools, teaching one another new behaviors, and even gossiping.”
Yet their brains vary a great deal from our own — which makes sense, given that they evolved in entirely different environments.22 For example, human self-recognition relies on a highly developed frontal cortex, but such development is missing in dolphins. However, dolphins are still capable of self-recognition and may even be able to recognize themselves in mirrors at younger ages than chimpanzees and humans. Scientists believe a dolphin’s comparatively hyper-developed parietal and temporal lobes may play a role here. Dolphins also have an entirely unique brain structure — the paralimbic lobe — which is absent in humans.23 It is thought that this lobe helps them manage social and emotional information.24
Cetaceans and humans both tend to be highly communal species that do most of their learning socially, rather than individually.25 Two different species in dramatically different environments but subject to similar pressures end up evolving in a similar direction.
Dolphin fresco found on the Greek island of Crete, 1600; Source: Wikipedia
The human brain, then, is not a wholly distinct achievement. We didn’t rise heroically from the primordial soup. We, like every other species, are the result of our earthen environment. It wasn’t just one thing, such as the discovery of delicious figs, that made us who we are: It was a mixture of pressures and adaptations, optimizing unwittingly toward a better brain . As science writer David Robson puts it, “cultural and genetic evolution can feed off each other. The overall picture [of the human brain’s evolution] is one of a virtuous cycle involving our diet, culture, technology, social relationships, and genes.”26
We humans have a habit of assuming our brains are the best in the neurological business, but there is no objective interspecies standard for intelligence. If you could somehow throw a human into an ant colony, they'd probably be the least "intelligent" ant in town — because our brains evolved in response to an entirely different set of circumstances. Each species encounters its own particular mixture of environmental pressures, leading to the development of brains that help it excel in whatever its ecological context might be. Some species even run into similar pressures in different environments, leading to different brains with parallel functions.
Stepping back, that shouldn’t come as a surprise. We are, after all, products of the same mother: nature.