Interactive by Emily Fuhrman and artwork by MRK for Quanta Magazine, with art direction by Olena Shmahalo/Quanta Magazine

Chapter 4: Big Brains

How Humans Evolved Supersize Brains

Scientists have begun to identify the symphony of biological triggers that powered the extraordinary expansion of the human brain.


There it was, sitting on the mantelpiece, staring at her with hollow eyes and a naked grin. She could not stop staring back. It looked distinctly like the fossilized skull of an extinct baboon. That was the sort of thing Josephine Salmons was likely to know. At the time — 1924 — she was one of the only female students of anatomy attending the University of the Witwatersrand in South Africa. On this particular day she was visiting her friend Pat Izod, whose father managed a quarry company that had been excavating limestone near the town of Taung. Workers had unearthed numerous fossils during the excavation, and the Izods had kept this one as a memento. Salmons brought news of the skull to her professor, Raymond Dart, an anthropologist with a particular interest in the brain. He was incredulous. Very few primate fossils had been uncovered this far south in Africa. If the Taung site really housed such fossils, it would be an invaluable treasure trove. The next morning Salmons brought Dart the skull, and he could see that she was right: The skull was undeniably simian.


Dart promptly arranged to have other primate fossils from the Taung quarry sent to him. Later that year, as he was preparing to attend a close friend’s wedding, he received a large crate. One of the specimens it contained was so mesmerizing that he nearly missed the ceremony. It came in two pieces: a natural endocast — the fossilized mold of the inner cranium, preserving the brain’s topography — and its matching skeletal face, with eye sockets, nose, jaw and teeth all intact. Dart noticed right away that this was the fossil of an extinct ape, not a monkey. The teeth suggested that the individual had died at age 6 or so. The point where the spinal cord had joined the skull was too far forward for a knuckle walker, indicating bipedalism. And the endocast, which was a little too large for a nonhuman ape of that age, had surface features characteristic of a human brain. After further study, Dart reached a bold conclusion: this was the fossil of a previously unknown ancestor of modern humans — Australopithecus africanus, the “Man-Ape of South Africa.”

New York Public Library / Science Source

Raymond Dart with the Taung child.

At first, the greater scientific community lambasted Dart’s proposal. If the Taung child, as the fossil was nicknamed, truly belonged to a hominin, surely it would have a far larger brain. Its cranium was a bit bigger than that of a chimpanzee, but not by much. Besides, it was generally believed that humans had evolved in Asia, not Africa. The “absurdly tiny” illustration accompanying Dart’s 1925 Nature paper, and his initial possessiveness of the specimen, did not help matters. Eventually, though, as prominent experts got to see the Taung child for themselves, and similar fossil discoveries came to light, attitudes began to change. By the 1950s, anthropologists had accepted that Taung was indeed a hominin and that an exceptionally large brain had not always been a distinguishing characteristic of humans. Dean Falk, a professor of anthropology at Florida State University and an expert on brain evolution, has called the Taung child “one of the most (if not the most) important hominin discoveries of the 20th century.”

In subsequent decades, by uncovering and comparing other fossil skulls and endocasts, paleontologists documented one of the most dramatic transitions in human evolution. We might call it the Brain Boom. Humans, chimps and bonobos split from their last common ancestor between 6 and 8 million years ago. For the next few million years, the brains of early hominins did not grow much larger than those of our ape ancestors and cousins. Starting around 3 million years ago, however, the hominin brain began a massive expansion. By the time our species, Homo sapiens, emerged about 200,000 years ago, the human brain had swelled from about 350 grams to more than 1,300 grams. In that 3-million-year sprint, the human brain almost quadrupled the size its predecessors had attained over the previous 60 million years of primate evolution.

Fossils established the Brain Boom as fact. But they tell us next to nothing about how and why the human brain grew so large so quickly. There are plenty of theories, of course, especially regarding why: increasingly complex social networks, a culture built around tool use and collaboration, the challenge of adapting to a mercurial and often harsh climate — any or all of these evolutionary pressures could have selected for bigger brains.

Although these possibilities are fascinating, they are extremely difficult to test. In the last eight years, however, scientists have started to answer the “how” of human brain expansion — that is, the question of how the supersizing happened on a cellular level and how human physiology reconfigured itself to accommodate a dramatically enlarged and energy-guzzling brain. “It was all speculation up until now, but we finally have the tools to really get some traction,” said Gregory Wray, an evolutionary biologist at Duke University. “What kinds of mutations occurred, and what did they do? We’re starting to get answers and a deeper appreciation for just how complicated this process was.”

What Makes the Human Brain Special

One scientist, in particular, has transformed the way researchers size up brains. Rather than fixating on mass or volume as a proxy for brainpower, she has focused on counting a brain’s constituent parts.

In her laboratory at the Institute of Biomedical Sciences at the Federal University of Rio de Janeiro, Suzana Herculano-Houzel routinely dissolves brains into a soup of nuclei — cells’ genetic control rooms. Each neuron has one nucleus. By tagging the nuclei with fluorescent molecules and measuring the glow, she can get a precise tally of individual brain cells. Using this method on a wide variety of mammalian brains, she has shown that, contrary to long-standing assumptions, larger mammalian brains do not always have more neurons, and the ones they do have are not always distributed in the same way.

Olena Shmahalo/Quanta Magazine;
source: and Herculano-Houzel et al.

When it comes to brains, size isn’t everything. The human brain is much smaller than that of an elephant or whale. But there are far more neurons in a human’s cerebral cortex than in the cortex of any other animal.
Data taken from the following studies: Cellular scaling rules for primate brains; Cellular scaling rules for rodent brainsGorilla and Orangutan Brains Conform to the Primate Cellular Scaling Rules: Implications for Human Evolution; The elephant brain in numbers.

The human brain has 86 billion neurons in all: 69 billion in the cerebellum, a dense lump at the back of the brain that helps orchestrate basic bodily functions and movement; 16 billion in the cerebral cortex, the brain’s thick corona and the seat of our most sophisticated mental talents, such as self-awareness, language, problem solving and abstract thought; and 1 billion in the brain stem and its extensions into the core of the brain. In contrast, the elephant brain, which is three times the size of our own, has 251 billion neurons in its cerebellum, which helps manage a giant, versatile trunk, and only 5.6 billion in its cortex. Considering brain mass or volume alone masks these important distinctions.

Based on her studies, Herculano-Houzel has concluded that primates evolved a way to pack far more neurons into the cerebral cortex than other mammals did. The great apes are tiny compared to elephants and whales, yet their cortices are far denser: Orangutans and gorillas have 9 billion cortical neurons, and chimps have 6 billion. Of all the great apes, we have the largest brains, so we come out on top with our 16 billion neurons in the cortex. In fact, humans appear to have the most cortical neurons of any species on Earth. “That’s the clearest difference between human and nonhuman brains,” Herculano-Houzel says. It’s all about the architecture, not just size.

The human brain is also unique in its unsurpassed gluttony. Although it makes up only 2 percent of body weight, the human brain consumes a whopping 20 percent of the body’s total energy at rest. In contrast, the chimpanzee brain needs only half that. Researchers have long wondered how the human body adapted to sustain such a uniquely ravenous organ. In 1995, the anthropologist Leslie Aiello and the evolutionary biologist Peter Wheeler proposed the “expensive tissue hypothesis” as a possible answer. The underlying logic is straightforward: Human brain evolution likely required a metabolic trade-off. In order for the brain to grow, other organs, namely the gut, had to shrink, and energy that would typically have gone to the latter was redirected to the former. For evidence, they pointed to data showing that primates with larger brains have smaller intestines.

A few years later, the anthropologist Richard Wrangham built on this idea, arguing that the invention of cooking was crucial to human brain evolution. Soft, cooked foods are much easier to digest than tough raw ones, yielding more calories for less gastrointestinal work. Perhaps, then, learning to cook permitted a bloating of the human brain at the expense of the gut. Other researchers have proposed that similar trade-offs might have occurred between brain and muscle, given how much stronger chimps are than humans.

Collectively, these hypotheses and observations of modern anatomy are compelling. But they are based on the echoes of biological changes that are thought to have occurred millions of years ago. To be certain of what happened, to pinpoint the physiological adaptations that made the brain’s evolutionary growth spurt possible, we will have to dive deeper than flesh, into our very genome.

Olena Shmahalo/Quanta Magazine;
source: Herculano-Houzel et. al. (2014)

How does the number of neurons in the cerebral cortex vary with the size of that part of the brain? Different scaling rules apply. In rodents, a 10-fold increase in the number of cortical neurons leads to a 50-fold increase in the size of the cortex. In primates, by contrast, the same neural increase leads to only a 10-fold increase in cortex size — a far more economical relationship.

How Genes Build the Brain

About eight years ago, Wray and his colleagues began to investigate a family of genes that influence the movement of glucose into cells to be used as energy. One member of the gene family is especially active in brain tissue, whereas another is most active in muscle. If the size of the human brain required a metabolic trade-off between brain tissue and muscle, then these genes should behave differently in humans and chimpanzees.

Wray and his team collected brain, muscle and liver samples from deceased humans and chimpanzees and attempted to measure gene activity in each sample. When a cell “expresses” a gene, it translates the DNA first into a signature messenger RNA (mRNA) sequence and subsequently into a chain of amino acids that forms a protein. Varying levels of distinct mRNAs can therefore provide a snapshot of gene activity in a particular type of tissue.

Wray’s team extracted mRNA from the tissues and amplified it many times over in the lab in order to measure the relative abundance of different mRNAs. They found that the brain-centric glucose-transporting gene was 3.2 times more active in human brain tissue than in the chimp brain, whereas the muscle-centric gene was 1.6 times more active in chimp muscle than in human muscle. Yet the two genes behaved similarly in the liver of both species.

Given that the human and chimp gene sequences are nearly identical, something else must explain their variable behavior. Wray and his colleagues found some intriguing differences between the genes’ corresponding regulatory sequences — stretches of DNA that stimulate or stifle gene activity. In humans, but not in chimps, the regulatory sequences for the muscle and brain-focused glucose-transporting genes had accumulated more mutations than would be expected by chance alone, indicating that these regions had undergone accelerated evolution. In other words, there was a strong evolutionary pressure to modify the human regulatory regions in a way that sapped energy from muscle and channeled it to the brain. Genes had corroborated the expensive tissue hypothesis in a way fossils never could.

Last year, the computational biologist Kasia Bozek, who now works at the Okinawa Institute for Science and Technology in Japan, published a similar study that examined metabolism from a different angle. In addition to looking at gene expression, Bozek and her colleagues analyzed levels of metabolites, a diverse group of small molecules that includes sugars, nucleic acids and neurotransmitters. Many metabolites are either necessary for metabolism or produced by it. Different organs have distinct metabolite profiles, depending on what they do and how much energy they require. In general, metabolite levels in the organs of closely related species are more in sync than levels between distantly related species. Bozek found that the metabolite profiles of human and chimp kidneys, for example, were pretty similar. But the variation between chimp and human brain metabolite levels was four times higher than would be expected based on a typical rate of evolution; muscle metabolites differed from the expected levels by a factor of seven. “A single gene can probably regulate a lot of metabolites,” Bozek said. “So even if the difference is not huge at the gene level, you could get a big difference in the metabolite levels.”

“It wasn’t just a couple mutations and — bam! — you get a bigger brain.”

Bozek and her colleagues then pitted 42 humans, including college basketball players and professional rock climbers, against chimpanzees and macaques in a test of strength. All of the primates had to pull a sliding shelf saddled with weights toward themselves. Accounting for body size and weight, the chimps and macaques were twice as strong as the humans. It’s not entirely clear why, but it is possible that our primate cousins get more power out of their muscles than we get out of ours because they feed their muscles more energy. “Compared to other primates, we lost muscle power in favor of sparing energy for our brains,” Bozek said. “It doesn’t mean that our muscles are inherently weaker. We might just have a different metabolism.”

Meanwhile, Wray had turned to his Duke colleague Debra Silver, an expert in embryonic brain development, to embark on a pioneering experiment. Not only were they going to identify relevant genetic mutations from our brain’s evolutionary past, they were also going to weave those mutations into the genomes of lab mice and observe the consequences. “This is something no one had attempted before,” Silver said.

The researchers began by scanning a database of human accelerated regions (HARs); these regulatory DNA sequences are common to all vertebrates but have rapidly mutated in humans. They decided to focus on HARE5, which seemed to control genes that orchestrate brain development. The human version of HARE5 differs from its chimp correlate by 16 DNA letters. Silver and Wray introduced the chimpanzee copy of HARE5 into one group of mice and the human edition into a separate group. They then observed how the embryonic mice brains grew.

After nine days of development, mice embryos begin to form a cortex, the outer wrinkly layer of the brain associated with the most sophisticated mental talents. On day 10, the human version of HARE5 was much more active in the budding mice brains than the chimp copy, ultimately producing a brain that was 12 percent larger. Further tests revealed that HARE5 shortened the time required for certain embryonic brain cells to divide and multiply from 12 hours to nine. Mice with the human HARE5 were creating new neurons more rapidly.

“This sort of study would have been impossible to do 10 years ago when we didn’t have the full genome sequences,” Silver said. “It’s really exciting.” But she also stressed that it will take a great deal more research to fully answer how the human brain blew up. “It’s a mistake to think we can explain brain size with just one or two mutations. I think that is dead wrong. We have probably acquired many little changes that are in some ways coopting the developmental rules.”

Wray concurs: “It wasn’t just a couple mutations and — bam! — you get a bigger brain. As we learn more about the changes between human and chimp brains, we realize there will be lots and lots of genes involved, each contributing a piece to that. The door is now open to get in there and really start understanding. The brain is modified in so many subtle and nonobvious ways.”

Brain and Body

Although the mechanics of the human brain’s expansion have long been mysterious, its importance has rarely been questioned. Again and again, researchers have cited the evolutionary surge in human brain size as the key reason for our exceptionally high degree of intelligence compared to other animals. As recent research on whale and elephant brains makes clear, size is not everything, but it certainly counts for something. The reason we have so many more cortical neurons than our great-ape cousins is not that we have denser brains, but rather that we evolved ways to support brains that are large enough to accommodate all those extra cells.

There’s a danger, though, in becoming too enamored with our own big heads. Yes, a large brain packed with neurons is essential to what we consider high intelligence. But it’s not sufficient. Consider, for a moment, what the world would be like if dolphins had hands. Dolphins are impressively brainy. They have demonstrated self-awareness, cooperation, planning and the rudiments of language and grammar. Compared to apes, though, they are severely limited in their ability to manipulate the world’s raw materials. Dolphins will never enter the Stone Age; flippers cannot finesse.

Similarly, we know that chimps and bonobos can understand human language and even form simple sentences with touch-screen keyboards, but their vocal tracts are inadequate for producing the distinct series of sounds required for speech. Conversely, some birds have the right vocal anatomy to flawlessly mimic human speech, but their brains are not large enough or wired in the right way to master complex language.

No matter how large the human brain grew, or how much energy we lavished upon it, it would have been useless without the right body. Three particularly crucial adaptations worked in tandem with our burgeoning brain to dramatically increase our overall intelligence: bipedalism, which freed up our hands for tool making, fire building and hunting; manual dexterity surpassing that of any other animal; and a vocal tract that allowed us to speak and sing. Human intelligence, then, cannot be traced to a single organ, no matter how large; it emerged from a serendipitous confluence of adaptations throughout the body. Despite our ongoing obsession with the size of our noggins, the fact is that our intelligence has always been so much bigger than our brain.

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  • Although I agree with most of the article, why couldn't intelligent life use some sort of visual communication?

  • A root cause was that we where once predators hunters of larger pray.
    We had to outsmart it, work together, and weak as we are we needed sticks and stones.
    We had to imagine the world and outsmart it.

    And by eating more flesh then a bonobo does, we got high protain meals.
    Food needed to grow larger brains.

    In the end our physical weakness made us strong minded, just evolution waiting to happen.

  • "Human intelligence…emerged from a serendipitous confluence of adaptations throughout the body."

    Are you certain it was serendipity?

  • Pilot whales (a misnomer, they're actually dolphins) have more cortical neurons than humans

    My theory: they're actually smarter than humans, but without opposable thumbs or the ability to make fire, how are they going to develop technology?

  • An additional element is the reduction and weakening of the jaw muscles that resulted from the softening of foods through cooking and other processing. That allowed for the expansion of the skull to accommodate a larger brain.

  • @Tom O: Thanks for pointing out that claim about pilot whales. The Wikipedia data is based on this recent paper. In the course of reporting this story, independent researchers called the conclusions of that study into question. The authors used stereology to create estimates of the number of neurons. The process involves slicing the brain into thin sections, counting cells in some fraction of the slices, then extrapolating up. Independent researchers felt that the authors of this study began with a too-small sample of whale brain tissue, and extrapolated too far. Their results for neuron density also do not match other published neuronal densities for whale brains. Until this result is replicated and confirmed by other groups—preferably using a complementary cell-counting technique—we determined that it would be irresponsible to publicize the findings.

  • Re: Andrew Patterson

    There is a theory that the areas of the brain involved in speech originally evolved for tool making. Brain scans of people making stone tools seem to activate areas of the brain involved in speech.

    If this is the case, then the first language may have been sign languages.. For tool making to persist across generations it would have to be taught. The way it would be taught would be by another watching someone else who was already skilled in making tools. This would require watching the hands and imitating their motions. It would be a very small step from that to using the hands to signal other things and over time we would develop a sign language capability. Sign language uses the same part of the brain as spoken language. If we add to that some rudimentary control of sounds – whistles, clicks, etc – that might combine with gestures and hand motions, particularly for drawing attention when another’s gaze was averted or out of sight, we might have a selection pressure for the anatomical modifications that allowed full speech.

  • Great article about the physical brain. Human behavior in modifying and utilizing the environment contributed to the intelligence gap as we developed ways to share and teach. IMO the greatest distinction of humans, compared to other animals, is the extent to which we save and share our knowledge. Neurons in the cortex are like the hardware which is super important but the software matters too.

  • Enjoyable article on the evolution of the human brain. I do have an issue with the opening statement, however.

    "Life’s extraordinary menagerie sprang up from the simplest of roots — chance chemical connections in the chaotic broth of early Earth."

    Only if you accept a multiverse scenario. Our universe neither contains enough chemical building blocks nor is old enough to allow random combinations of molecules and immense period of time to overcome the extraordinarily prohibitive odds against the complexity we see in nature today.

    This is common knowledge accepted by most mathematicians and physicists. And, to accept the multiverse scenario, you must also accept one of the many flavors of String Theory, which has floundered aimlessly for the last three decades or so with no real progress.

  • No one apparently thinks that 'losing the tail' was important for our advancement. Probably that freed up or added dexterity to our hands, and perhaps walking with an efficient stride. Although I miss having a prehensile tail, I think that amount of added brain power is worth it.

  • I think that the reference to chance chemical reactions leading to life is a little off the mark. The chances for replicating existing organic molecules or synthesizing new ones is rapidly increased because of catalysts. Some molecules naturally catalyze others, which in turn catalyze still more molecules. Chains of catalytic molecules eventually form loops when one molecule in the chain catalyzes a molecule appearing earlier in the chain. This then forms a positive feedback loop for creating more of these loops. This was chemical replication which eventually increased in complexity to biological replication (which is still basically chemical replication). It is surprising that this theory of auto catalysis is hardly recognized. Without catalysts, organic molecules would quickly degrade and not sustain organized structures such as cells.

    Addressing the biological reasons for a more sophisticated human brain also is a little off the mark. The reason for our sophisticated brains goes beyond biology. Our sophisticated brains are the implementors for creating culture and technology. Both of these non-biological developments formed a positive feedback loop for driving increasingly sophisticated brains. Culture and technology enabled the acquisition of more calories to supply larger brains, and the larger brains created more sophisticated culture and technology.

    Great article focusing on the how of our brains though.

  • Another important factor that (I believe) has impacted our ability to surpass other animals in brain power is *the length of time* we spend in infancy and adolescence. We come into the world with brains that are not fully developed and then spend 20+ years developing them. This is unique among animals.

    (I can't remember where I learned this, but I'm not the first to raise the point.)

    Great article, thanks!

  • Nice piece, but I can't help the feeling that the science behind it is a bit soft.
    Yet, I am intrigued by Oscar O.'s comment that "Our universe neither contains enough chemical building blocks nor is old enough to allow random combinations of molecules and immense period of time to overcome the extraordinarily prohibitive odds against the complexity we see in nature today. "
    And what about accepting the Multiverse scenario, and string theory as a precondition to the belief in "chance chemical connections in the chaotic broth of early Earth."?
    May be I am wrong, but does Oscar mean to point out a basic problem with the study of evolution?
    The question is for Oscar….

  • To add to David McMahon says: [November 11, 2015 at 10:35 am. "Human intelligence…emerged from a serendipitous confluence of adaptations throughout the body." Are you certain it was serendipity?]

    Indeed if one says that serendipity is "chance" as in random. Most evolutionary biologists would say that random chance is not the mechanisms of a evolution; rather the selective pressures that influence the relative rates of specie adaptations, vis a vis, traits that endure and are passed on to successive generations that improve or increase survivability (on a specie scale). I think the author may have been pointing to a colloquial sense of serendipity (that is to say, that it was a favorable outcome for such confluence of adaptations to simultaneously or concurrently evolve to the complicated splendor of 'human').

    That is my interpretation of that sentence.

  • I would like to know how they separated the nuclei of the neurons from the nuclei of the glia. I realize they stained the nuclei of the neurons with a fluorescent antibody, but that would stain all the nuclei in a dissolved brain.

  • The article is informative, but misses a very important point. Eukaryotic organisms evolve to achieve specific survival goals, as environments change.

    What exactly or approximately were the mitigating environmental factors for the development of our high energy consuming brains? Or, if that information is not known, what are some viable candidates?

  • Here is a question not posed in this excellent article. Why did big-brained hominins emerge in the last 3 million years? We argued that Pleistocene glacial episodes were likely sufficient to serve as prime releasers for emergence of Homo habilis and Homo erectus. Given the energy-intensity demand of large brains, its temperature differential with the ambient temperature created by climatic cooling opened up evolutionary space for the emergence of these hominins in Africa, either in higher elevation East Africa or the already temperate climate of South Africa. Reference: Schwartzman D, Middendorf M., and M. Armour-Chelu, 2009. Was climate the prime releaser for encephalization? Climatic Change 95, Issue 3: 439-447.

  • As my colleague Schwartzman noted, climatic cooling appears to have been a prime releaser by providing the environmental context, e.g. the necessary heat sink that the enlarged brain required. However, the evolution of a larger brain capacity was a complicated event, requiring a suite of supporting changes. Some, like the loss of body hair and the adaptation of an upright posture, were also linked to thermoregulation. Other changes, like dietary-driven shifts in skull architecture that fostered reduction in jaw size and musculature and shifts in fetal development coupled with modifications in the pelvic girdle, while not driven by thermoregulation were clearly necessary or supportive.

  • To Oscar and S Hussein, it is misleading when reading the word "chance" and "accident" to imagine a continuous series of events each of equal likelihood and impact, influencing the future state of self replicating molecules.

    The most important observation is that chance events can be of cumulative consequence. Hence it is not as if n events must occur in time dt and then if they don't then you have to go back to t=0

    This means that some chemical combinations (and I assume you agree Oscar that life is chemical), may only have to occur once in a preexisting system in order to form some kind of self replicating molecule.

    If you haven't already read about it, have a look at the huge mistake Hoyle made when blundering into biology with mathematics but not the idea of changes building on past events. His classic facepalm of the "Boeing 747 argument".

    Also in an organised system, and actually, even a puddle resting on clay or chemically laden hydrothermal vent is organised, particle interactions are dependent on known chemical principles, such as proximity, temperature, presence of catalytic species and energetic state

  • Yes, I for one am glad that my brain, with all those extra neurones, is so much bigger than that of other primates. Seems to make sense. Folks with big muscles are stronger than folks with small muscles. So surely folks with big brains are smarter than folks with small brains? An article in Science in 1980 turned this, if you will excuse, on its head. There are among us apparently normal individuals with only 5% the normal quantity of brain tissue – one has a maths Ph.D. The scan of his normal size skull reveals a large fluid-filled space. This met with much scepticism – to say the least. In early 2007 anthropologist John Hawks found it "quite obviously incredible." However, a few months later independent confirmatory evidence was presented in the Lancet by French neurologists. In 2012 Brazilian neurosurgeons reported another case. A new paper in the December 2015 issue of Biological Theory (Volume 10, Issue 4, 336-342) provides further discussion of this.

  • Well written article. However I would have loved to hear the exact details of how evolutionary pressures were able to direct the necessary specific mutations for human brains to develop if the changes are beyond the reach of chance. If random mutations are an ineffective mechanism, how exactly do evolutionary pressures ensure that the necessary specific mutations occur? Secondly what are the author's thoughts on the waiting time problem – it seems it quantifies what you intuitively allude to -"Human intelligence, then, cannot be traced to a single organ, no matter how large; it emerged from a serendipitous confluence of adaptations throughout the body". Durett and Schmidt are one among several researchers who have modeled how long it will take to for a new gene requiring 2 specific changes from an existing gene to arrive within any individual in the human population. They calculate 216 million years, others have calculated 84 million years. Is natural selection acting on random mutations still capable then of generating the changes you have illuminated so wonderfully?

  • Interestingly psychoactive drugs work by depressing breathing and reduce oxygen to the brain rather than depressing metabolism. Highly developed neural networks require proportionally more oxygen than sugars to be efficient….

  • How Humans Evolved Supersize Brains | Quanta Magazine
    This is one of most important finds about brain size versus where energy goes from food. In this study if food supplies energy more to the brain versus muscles, the brain need not grow in size too much. In some evolutionary cycle perhaps more energy is spent on muscle density and growth. How lmportant and frequent eating of soft easily digested foods is to boosting intellectual output. When I studied for exams, I used to lose weight like crazy, but when I walked until I dropped every day or starved myself on special diets, I hardly lost any weight. That happened, I realize from this article, because my brain was built to consume far more energy than my muscles. It could be small, but it needed to be supplied from food more often or rest more to build or conserve energy. Comment by Agnes Temesvari known in native Hungary as Tothkása, Ágnes Katalin Mária

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