How Are Mammals More Advanced Than Other Animals
Southwarduzana Herculano-Houzel spent most of 2003 perfecting a macabre recipe—a formula for brain soup. Sometimes she froze the jiggly tissue in liquid nitrogen, and and so she liquefied it in a blender. Other times she soaked it in formaldehyde and then mashed information technology in detergent, yielding a polish, pink slurry.
Herculano-Houzel had completed her Ph.D. in neuroscience several years before, and in 2002, she had begun working equally an assistant professor at the Federal University of Rio de Janeiro in Brazil. She had no real funding, no laboratory of her own—just a few feet of counter space borrowed from a colleague.
"I was interested in questions that could exist answered with very niggling coin [and] very piddling technology," she recalls. Even and then, she had a bold idea. With some attempt—and luck—she hoped to accomplish something with her kitchen-blender project that had bedeviled scientists for over a century: to count the number of cells in the encephalon—not but the human being brain, but also the brains of marmosets, macaque monkeys, shrews, giraffes, elephants, and dozens of other mammals.
Her method might have seemed carelessly destructive at first. How could annihilating such a fragile and complex organ provide any useful insights? Just 15 years on, the work of Herculano-Houzel and her team has overturned some long-held ideas about the evolution of the human mind. It is helping to reveal the fundamental blueprint principles of brains and the biological basis of intelligence: why some large brains lead to enhanced intelligence while others provide no benefit at all. Her work has unveiled a subtle tweak in brain organization that happened more than threescore million years agone, non long after primates branched off from their rodent-like cousins. It might accept been a tiny change—just without it, humans never could have evolved.
The questions that Herculano-Houzel sought to answer go back more than than 100 years, to a time when scientists were but starting to study the relationship betwixt brain size and intelligence.
In August 1891, laborers working for the Dutch anatomist Eugène Dubois began excavating trenches along a steep riverbank on the Indonesian island of Java. Dubois hoped to find early hominin remains.
Over the course of 15 months, layers of sandstone and hardened volcanic gravel yielded the petrified bones of elephants and rhinos, and, about importantly, the skullcap, left femur, and two molars of a human being-like creature thought to have died nearly a 1000000 years before. That specimen, namedPithecanthropus erectus, and subsequently Coffee man, would somewhen come up to be known equally the beginning example ofHuman erectus.
Dubois made it his mission to infer the intelligence of this early hominin. But he had simply 3 fragments of seemingly relevant information: its estimated encephalon size, stature, and body weight. Would this be enough?
Zoologists had long noticed that when they compared different species of animals, those with bigger bodies had larger brains. Information technology seemed as if the ratio of brain weight to body weight was governed past a mathematical police. As a start, Dubois prepare out to identify that police. He gathered the brain and body weights of several dozen animal species (every bit measured by other scientists), and using these, he calculated the mathematical rate at which brain size expands relative to body size. This exercise seemed to reveal that across all vertebrates, the brain really does expand at a like rate relative to body size.
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Dubois reasoned that as torso size increases, the brain must aggrandize for reasons of neural housekeeping: Bigger animals should require more neurons just to keep up with the mounting chores of running a larger torso. This increase in brain size would add nothing to intelligence, he believed. After all, a moo-cow has a encephalon at least 200 times larger than a rat, only it doesn't seem any smarter. Simply deviations from that mathematical line, Dubois thought, would reverberate an beast's intelligence. Species with bigger-than-predicted brains would exist smarter than average, while those with smaller-than-predicted brains would be dumber. Dubois' calculations suggested that his Coffee man was indeed a smart cookie, with a relative brain size—and intelligence—that fell somewhere between modern humans and chimpanzees.
Dubois' formula was later revised by other scientists, only his general arroyo, which came to be known as "allometric scaling," persisted. More than mod estimates have suggested that the mammalian brain mass increases by an exponent of two-thirds compared to torso mass. So a dachshund, weighing roughly 27 times more than a squirrel, should have a brain about 9 times bigger—and in fact, it does. This concept of allometric scaling came to permeate the word of how brains relate to intelligence for the side by side hundred years.
Southwardeeing this uniform relationship betwixt trunk and brain mass, scientists developed a new measure out called encephalization quotient (EQ). EQ is the ratio of a species' actual brain mass to its predicted brain mass. It became a widely used shorthand for intelligence. As expected, humans led the pack with an EQ of vii.4 to 7.8, followed by other loftier achievers such equally dolphins (nearly 5), chimpanzees (2.2 to ii.5), and squirrel monkeys (roughly 2.3). Dogs and cats barbarous in the eye of the pack, with EQs of around 1.0 to ane.ii, while rats, rabbits, and oxen brought up the rear, with values of 0.4 to 0.5. This way of thinking about brains and intelligence has been "very, very dominant" for decades, says Evan MacLean, an evolutionary anthropologist at the Academy of Arizona in Tucson. "It's sort of a fundamental insight."
This paradigm still held sway when Herculano-Houzel was going through graduate schoolhouse in the 1990s. "The intuition behind it made perfect sense," she says. When she began trying to count neurons in the early on 2000s, she imagined herself simply adding a layer of dash to the conversation. She didn't necessarily look to undermine it.
By the early 2000s, scientists had already been counting neurons for decades. Information technology was slow, painstaking work, unremarkably washed past cut brain tissue into ultra-thin prosciutto-like slices and viewing these under a microscope. Researchers typically counted hundreds of cells per slice. Tallying plenty neurons to estimate the boilerplate number of cells for a single species was time-consuming, and the results were often uncertain. Each nerve cell is branched like a twisty oak tree; its limbs and twigs crisscross with those of other cells, making information technology hard to know where 1 cell ends and another begins.
This is the problem that Herculano-Houzel prepare out to solve. By early 2003, she realized that the all-time way to count nerve cells in brain tissue might exist to eliminate the complexity altogether. It occurred to her that each nerve prison cell, no matter how branched and contorted, should contain only one nucleus—the little sphere that holds the cell's DNA. All she had to exercise was find a mode to deliquesce the brain tissue while keeping the nuclei intact. Then she could count the nuclei to figure out how many cells there were; it would exist equally uncomplicated as counting checkers on a checkerboard.
After xviii months, she settled on a procedure that involved hardening the brain tissue with formaldehyde and and so mashing it gently with detergent—repeatedly pushing a plunger into the drinking glass tube, turning it as she went, until she had a uniform slurry. She diluted the liquid, squeezed a drop of it onto a glass slide, and peered at it through a microscope. A constellation of blue dots lay scattered across her field of view: the cell nuclei, lit up with a Dna-bounden dye. By staining the nuclei with a second dye, which binds to specialized nerve proteins, she could count how many of them came from nerve cells—the cells that really procedure data in brains—rather than other types of cells found in brain tissue.
Herculano-Houzel counted a few hundred nerve cells over the course of xv minutes; by multiplying this number up to the entire volume of liquid, she was able to calculate a totally new piece of information: An entire rat brain contains about 200 million nerve cells.
She looked at brains from 5 other rodents, from the 40-gram mouse to the 48-kilogram capybara (the largest rodent in the earth, native to Herculano-Houzel's home land of Brazil). Her results revealed that as brains get larger and heavier from one species of rodent to another, the number of neurons grows more slowly than the mass of the brain itself: A capybara's brain is 190 times larger than a mouse'south, but it has only 22 times as many neurons.
Then in 2006, Herculano-Houzel got her hands on the brains of half-dozen primate species during a visit with Jon Kaas, a brain scientist at Vanderbilt Academy in Nashville, Tennessee. And this is where things got fifty-fifty more than interesting.
What Herculano-Houzel found in these primates was totally dissimilar from rodents. "The primate brains had many more neurons than nosotros expected," she says. "It was right there, staring us in the confront."
Herculano-Houzel saw a clear mathematical trend among these six species that are alive today: As the primate brain expands from one species to another, the number of neurons rises apace enough to keep step with the growing brain size. This means that the neurons aren't ballooning in size and taking up more infinite, as they exercise in rodents. Instead, they stay compact. An owl monkey, with a encephalon twice as large every bit a marmoset, actually has twice every bit many neurons—whereas doubling the size of a rodent brain oft yields only 20 to xxx pct more than neurons. And a macaque monkey, with a encephalon 11 times larger than a marmoset, has 10 times as many nervus cells.
The assumption that everyone had been making, that dissimilar mammalian species' brains scaled upwards the same way, "was very obviously wrong," says Herculano-Houzel. Primate brains were very different from those of rodents.
Herculano-Houzel published these outset nonhuman primate results with Kaas and two other co-authors in 2007. And in 2009, she confirmed that this pattern holds truthful from small-brained primates all the mode up to humans: At roughly 1,500 grams, the human brain weighs 190 times every bit much as a marmoset brain and holds 134 times every bit many nerve cells—nigh 86 billion in total. Her subsequent studies, published between 2009 and 2017, advise that other major mammal groups, such as insectivores and cloven-hoofed artiodactyls (like pigs, antelopes, and giraffes), follow the rodent-like scaling design, with neuron numbers increasing much more than slowly than brain mass. "There's a huge departure between primates and non-primates," says Herculano-Houzel, who moved to Vanderbilt University in 2016.
Her results didn't reveal the exact procedure of evolution that led to the modern homo brain. After all, she could only count brain cells in species that currently exist—and because they're alive today, they aren't man ancestors. But by studying a diversity of brains, from small-scale to big, Herculano-Houzel learned well-nigh the design principles of brains. She came to empathise that primate and rodent brains faced very different constraints in the way that they could evolve.
People in the anthropological community accept responded positively to her work—though with a touch of caution. Robert Barton, an anthropologist who studies brain evolution and behavior at Durham University in the U.Thousand., is convinced that neurons are packed more densely in the brains of primates than they are in those of other mammals. But he's non however convinced that the mathematical trend line—the rate at which brains add new neurons as they become bigger from species to species—is any greater in primates compared to other mammals. "I'd like to meet more data before I completely believe it," he says. He points out that Herculano-Houzel has and then far studied the brains of about a dozen, out of several hundred known, primate species.
But Herculano-Houzel's results have already dealt a serious blow to conventional wisdom. Scientists who calculated EQs had assumed that they were making apples-to-apples comparisons—that the relationship between brain size and number of neurons was uniform across all mammals. Herculano-Houzel showed that this wasn't then.
"Information technology'due south a brilliant insight," says MacLean, who himself has spent years studying the intellectual capacities of animals. "Information technology's pushed the field forward enormously."
MacLean's own work has also undermined the universality of EQ. His study, published with a large consortium of co-authors in 2014, compared the brains and cerebral abilities of 36 animate being species—including 23 primates and a sprinkling of other mammals, and seven birds. MacLean assessed them on their capacity for impulse control (measured, for example, by an animal'south ability to calmly reach around a transparent barrier to obtain some food, rather than bully against it in an impulsive grab). Impulse command is an important component of intelligence, which, unlike algebra skills, tin exist measured across diverse species.
MacLean plant that EQ did a poor job of predicting this quality. Chimpanzees and gorillas accept mediocre EQs of 1.5 to 2.5, only, says MacLean, "they did super well [in impulse control]. They were at the height." Squirrel monkeys, meanwhile, scored far worse than chimps and gorillas on self-control, even though this species sports an EQ of ii.3.
Despite a relatively small sampling of animals and a lot of scatter in the information, MacLean found that the best predictor for self-control was absolute brain volume, uncorrected for trunk size: Chimps and gorillas may accept EQs no better than squirrel monkeys, but their brains, in absolute terms, are 15 to 20 times bigger. (Their EQs may be thrown off because they have unusually large bodies, not small brains.) For primates, a bigger encephalon was a better brain, regardless of the animal'due south size. (This was also the case for birds.)
In 2017, Herculano-Houzel published a study in which she looked at the same measurements of impulse control that MacLean had used, but she compared them to a new variable: the number of neurons that each species has in its cerebral cortex—the upper layer of brain tissue, frequently folded, that performs advanced cognitive functions, such as recognizing objects. Herculano-Houzel found that the number of cortical neurons predicted self-control about also equally absolute encephalon size had in MacLean'southward report—and it also smoothed out a major glitch in his results: Birds may have tiny brains, only Herculano-Houzel constitute that those brains are densely packed. The Eurasian jay has a encephalon smaller than a walnut, but information technology has most 530 million neurons in its pallium (the brain structure in birds that is roughly equivalent to the mammalian cortex). Her numbers provided a compelling explanation for why these birds scored better on impulse control than did some primates with brains five times larger.
"The simplest, about important cistron that should limit cerebral capacity," concludes Herculano-Houzel, "is the number of neurons that an creature has in the cortex."
If the secret to intelligence is simply having more than neurons, then one might ask why rodents and other mammals didn't just evolve bigger brains to arrange their larger neurons. The reason is that ballooning neuron size presents a staggering problem. It somewhen becomes unsustainable. Just consider a hypothetical rodent with the aforementioned number of neurons as a man—almost 86 billion. That animal would demand to drag around a brain weighing 35 kilograms. That's well-nigh 25 times bigger than a human brain—about equally heavy as nine gallons of water. "Information technology'south biologically implausible," says MacLean. It "would be insane—y'all couldn't walk."
This problem of ballooning neuron size was probably one of the major factors that limited encephalon expansion in most species. The burning question is how primates managed to avoid this trouble.
The usual curse of an ever-expanding neuron size may stem from the basic fact that brains office every bit networks in which individual neurons send signals to one another. As brains get bigger, each nerve prison cell must stay connected with more and more than other neurons. And in bigger brains, those other neurons are located farther and farther abroad.
"Those are problems that have to be solved when you lot enlarge brains," says Kaas, who oftentimes collaborates with Herculano-Houzel. He hypothesized that rodents and near other mammals addressed these problems in a simple manner: by growing communication wires, called axons, that are longer, causing each neuron to have up more infinite.
Inorthward 2013, Herculano-Houzel found evidence for this theory by looking at white affair in the brains of v rodent and nine primate species. White thing contains much of the encephalon'southward wiring—the fat-coated axons that cortical neurons utilise to make long-altitude connections. Her piece of work showed that the book of white matter grows much more quickly in rodent species with larger brains than information technology does in primates. A large rodent called an agouti has eight times as many cortical nerve cells as a mouse, while its white matter takes up an astonishing 77 times as much space. But a capuchin monkey, with viii times as many cortical neurons every bit a small primate called a galago, has only 11 times as much white matter.
Due southo equally rodent brains get bigger, more and more brain volume has to be devoted to the wires that but transmit information. Those wires don't merely get longer, they as well become thicker—which allows signals to travel at a higher speed, to make upward for the longer distances they have to cover. As a effect, less and less space is available for the nerve cells that do the of import work of really processing information.
The downfall of rodents, in other words, is that their brains don't adapt well to the problems of existence big. They don't recoup efficiently for the communication bottlenecks that emerge as brains increase in size. This constraint has severely limited their capacity for intelligence.
Primates, on the other hand, practice adapt to these challenges. Every bit primate brains become larger from species to species, their blueprints practise gradually alter—assuasive them to circumvent the problem of long-distance communication.
1000aas thinks that primates managed to keep most of their neurons the same size by shifting the burden of long-altitude advice onto a modest subset of nerve cells. He points to microscopic studies showing that perhaps i percent of neurons do expand in big-brained primates: These are the neurons that gather information from huge numbers of nearby cells and transport it to other neurons that are far away. Some of the axons that make these long-distance connections also get thicker; this allows time-sensitive information, such every bit a visual image of a rapidly moving predator, or prey, to reach its destination without delay. Merely less-urgent information—that is, most of information technology—is sent through slower, skinnier axons. So in primates, the average thickness of axons doesn't increment, and less white matter is needed.
This pattern of keeping most connections local, and having only a few cells transmit information long-distance, had huge consequences for primate development. It didn't merely allow primate brains to squeeze in more neurons. Kaas thinks that it also had a more profound outcome: Information technology really changed how the brain does its piece of work. Since most cells communicated just with nearby partners, these groups of neurons became cloistral into local neighborhoods. Neurons in each neighborhood worked on a specific job—and only the end outcome of that work was transmitted to other areas far away. In other words, the primate encephalon became more than compartmentalized. And as these local areas increased in number, this organizational change allowed primates to evolve more and more cognitive abilities.
All mammal brains are divided into compartments, called "cortical areas," that each contain a few million neurons. And each cortical area handles a specialized task: The visual organization, for example, includes different areas for spotting the simple edges of shapes and for recognizing objects. Rodent brains don't seem to become more compartmentalized as they get larger, says Kaas. Every rodent from the bite-sized mouse to the Doberman-sized capybara has virtually the aforementioned number of cortical areas—roughly 40. But primate brains are dissimilar. Small primates, such as galagos, take around 100 areas; marmosets accept well-nigh 170, macaques about 270—and humans around 360.
In primates, some of these new areas took on novel social tasks, such as recognizing faces and the emotions of others, and learning written or spoken language—the very skills that helped to drive the evolution of hominin culture, and, arguably, human intelligence. "Primates with large brains have really superior processing," says Kaas. "Only rodents with larger brains may be processing things almost the aforementioned every bit rodents with smaller brains. They haven't gained much."
Anthropologists accept spent decades studying the of import changes in brain structure that happened subsequently the appearance ofH. erectus (1.9 meg years agone) or the separate between hominins and nifty apes (viii meg years ago). But Herculano-Houzel has now added a new piece to this pic by identifying another key moment in the evolution of human intelligence. In a sense, she has unearthed a new origin story for humanity—one that is no less important than the others we already knew.
This story unfolded a fiddling over 60 million years ago, not long after early primates had split up off, in quick succession, from three other major groups of mammals that include modernistic-day rodents, tree shrews, and colugos (a.k.a. "flying lemurs").
These early primates were smaller than rats. They crept quietly along tree branches at night, grasping twigs with their prehensile fingers and toes as they hunted insects. They didn't look like much at all, says Herculano-Houzel.
But a subtle tweak had already occurred deep in their little brains—a change in the genes that guide how neurons connect to i some other during fetal development. This change probably fabricated little difference at first. Simply over the long run, it would profoundly separate primates from the rodents and other groups that they had parted ways with. This tiny change would go on nervus cells pocket-size, even as brains gradually got bigger and bigger. It would curve the arc of development for tens of millions of years to come. Without information technology, humans never would have walked the earth.
Douglas Fox is a freelance journalist who writes well-nigh the earth, the Antarctic, and polar sciences—with an occasional foray into neuroscience. His stories have appeared in Scientific American, National Geographic, and other publications. Trick is a contributing author to The Scientific discipline Writers' Handbook: Everything You Need to Know to Pitch, Publish, and Prosper in the Digital Historic period.
A version of this commodity was originally published on Sapiens' website as "How Human Smarts Evolved" and has been republished hither with permission.
Source: https://geneticliteracyproject.org/2018/09/24/why-are-humans-so-much-smarter-than-other-primates/
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