Did the Smarter Apes Stay in the Trees?
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This site is being redeveloped. For all the latest ABC Science content click here. Site Navigation Video Audio Photos. Share Print News in Science Chimps 'talk' about favourite fruits, best trees Tuesday, 20 January Jennifer Viegas Discovery News New research shows that chimpanzees use vocalisations in a sophisticated manner. Email the editor Share this article Email a friend. All three groups slept for an average of 6. That is even less than the average for an industrial society. The people in the study woke before sunrise and went to bed several hours after sunset, suggesting that the natural light and dark cycles were not determining when they went to sleep.
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What's more, getting less sleep did not appear to reduce their cognitive abilities, and they were all generally healthy, says lead author Jerome Siegel of the University of California, Los Angeles in the US. This suggests they were sleeping enough. Siegel's study does not tell us anything about ancestral species like H. But it does suggest that the earliest modern humans, who lived in similar environments to the people in the study, also needed surprisingly little sleep. View image of This looks uncomfortable, but to each their own Credit: It is tricky to be sure about this, because some people need more sleep than others.
There could even be population-level patterns: Samson and Nunn's study focused on average amounts of sleep, taken across entire populations. Sleep is particularly important when we are very young, especially REM sleep.
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Infants spend far more time in REM sleep than children or adults. If you compare a child's REM sleep to that of an adult, the difference is much greater than it is between humans and chimpanzees, he says. This might mean that getting enough quality sleep early on in life was more important in helping our ancestors develop into ever big-brained hominins. View image of Babies need far more sleep than adults Credit: The biggest problem with Samson and Nunn's idea is that we do not yet understand all the functions of sleep, so we cannot say for sure what benefits we might get from extra REM sleep.
In particular, we periodically switch from REM sleep to other kinds of sleep. The picture gets even muddier when we start to compare our own REM sleep to that of other animals, beyond our close relatives the primates. View image of Dolphins never completely switch off Credit: He argues that we should consider all mammals, not just primates.
When we do so, humans do not stand out as needing a particularly large proportion of REM sleep. The link between REM sleep and intelligence also fades away. For example, dolphins are intelligent and have large brains, yet they do not need any REM sleep. Meanwhile opossums, which are not noted for their deep thinking, need over six hours. He points out that species' distinct lifestyles can result in differing needs for sleep. Famously, dolphins only sleep on one side of their brain at a time.
The reason for this is simple: Looked at in that light, it makes sense that dolphins do not sleep deeply. View image of Our closest living relatives sleep for longer than we do.
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Regardless of why it happened, the fact remains that human sleep is strange compared to our closest living relatives. This suggests that we have evolved to need less of it. Obviously, we cannot ever study how long our hominin ancestors slept for, because they have gone extinct. This exercise seemed to reveal that across all vertebrates, the brain really does expand at a similar rate relative to body size. D ubois reasoned that as body size increases, the brain must expand for reasons of neural housekeeping: Bigger animals should require more neurons just to keep up with the mounting chores of running a larger body.
This increase in brain size would add nothing to intelligence, he believed. Species with bigger-than-predicted brains would be smarter than average, while those with smaller-than-predicted brains would be dumber. More modern estimates have suggested that the mammalian brain mass increases by an exponent of two-thirds compared to body 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 discussion of how brains relate to intelligence for the next hundred years.
S eeing this uniform relationship between body and brain mass, scientists developed a new measure called encephalization quotient EQ. It became a widely used shorthand for intelligence. As expected, humans led the pack with an EQ of 7. Dogs and cats fell in the middle of the pack, with EQs of around 1. T his paradigm still held sway when Herculano-Houzel was going through graduate school in the s.
When she began trying to count neurons in the early s, she imagined herself simply adding a layer of nuance to the conversation. B y the early s, scientists had already been counting neurons for decades. It was slow, painstaking work, usually done by cutting brain tissue into ultra-thin prosciutto-like slices and viewing these under a microscope.
Researchers typically counted hundreds of cells per slice. Tallying enough neurons to estimate the average 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 it hard to know where one cell ends and another begins.
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T his is the problem that Herculano-Houzel set out to solve. By early , she realized that the best way to count nerve cells in brain tissue might be to eliminate the complexity altogether. All she had to do was find a way to dissolve the brain tissue while keeping the nuclei intact. Then she could count the nuclei to figure out how many cells there were; it would be as simple as counting checkers on a checkerboard. A fter 18 months, she settled on a procedure that involved hardening the brain tissue with formaldehyde and then mashing it gently with detergent—repeatedly pushing a plunger into the 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.
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A constellation of blue dots lay scattered across her field of view: 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 actually process information in brains—rather than other types of cells found in brain tissue. Neuroscientist Suzana Herculano-Houzel holds up a tube that contains a liquid suspension of all the cell nuclei that once made up a mouse brain. An entire rat brain contains about million nerve cells. 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: T hen in , Herculano-Houzel got her hands on the brains of six primate species during a visit with Jon Kaas, a brain scientist at Vanderbilt University in Nashville, Tennessee.
And this is where things got even more interesting. W hat Herculano-Houzel found in these primates was totally different from rodents.
As Maurice promised to Caesar, Cornelius will know about his late father's life so that the young prince doesn't forget him, what he did to ensure the tribe's safety, and strive to be a true leader. Female bonobos, like human females, develop strong bonds, whereas female chimps generally don't. Already a subscriber or registered access user? Low-ranking chimpanzees will always go for the food that's hidden from a dominant chimpanzee's view, because they know the dominant has not seen it. A constellation of blue dots lay scattered across her field of view:
H erculano-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 quickly enough to keep pace with the growing brain size. Instead, they stay compact. An owl monkey, with a brain twice as large as a marmoset, actually has twice as many neurons—whereas doubling the size of a rodent brain often yields only 20 to 30 percent more neurons.
And a macaque monkey, with a brain 11 times larger than a marmoset, has 10 times as many nerve cells. Primate brains were very different from those of rodents. At roughly 1, grams, the human brain weighs times as much as a marmoset brain and holds times as many nerve cells—about 86 billion in total. Her subsequent studies, published between and , suggest that other major mammal groups, such as insectivores and cloven-hoofed artiodactyls like pigs, antelopes, and giraffes , follow the rodent-like scaling pattern, with neuron numbers increasing much more slowly than brain mass.
But by studying a diversity of brains, from small to big, Herculano-Houzel learned about the design principles of brains. She came to understand that primate and rodent brains faced very different constraints in the way that they could evolve. P eople in the anthropological community have responded positively to her work—though with a touch of caution.
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Robert Barton, an anthropologist who studies brain evolution and behavior at Durham University in the U. He points out that Herculano-Houzel has so far studied the brains of about a dozen, out of several hundred known, primate species. As brain size expanded over the course of primate evolution, the number of neurons in the primate brain increased quickly, leading to big improvements in cognition.