Human brains got bigger...

               This...

              became this

Photo adapted from:  Molecular Insights into Human Brain Evolution :Jane Bradbury

             Ape Brain 400gm

                     Human Brain 1350 gm

Actually the chimpanzee (left-hand brain) didn't develop into the human one (right) at all. We and chimpanzees merely had a common ancestor whose brain may have been like the chimp's. But chimpanzees have had as long a period of evolution since that common ancestor as we have and no one has found any early fossils of intervening chimp ancestors. There are really no convincing arguments against the possibilities that chimp brains have either shrunk or grown.

and Man progressed...

... Somehow

Human brain sizes are top of the primate (& every other) scale.

Brain sizes in animal species should follow a natural trend line as body sizes get larger:

I have adapted Jerison's well known chart to show just the gist of it. 
L to R  axis - body size.
EQ is up and down.
Upper blue line - expected trend for brain sizes as bodies get larger ('Higher' vertebrates only)
Lower blue line - everyone else (like Latimeria, the coelacanth, a still new 'rediscovery' in 1973). 
Above the lines, brain sizes bigger than expected, below smaller.

Adapted - 'Encephalisation Quotient'  (Jerison 1973)  Why Are Primate Brains So Big?

Viewing the chart in this unorthodox way reveals some unexpected relationships:

v  Vampire Bat, Crow and Wolf alike have a little bit more brain (EQ) than expected .
v So do Gorilla and Elephant, but not much.
v Mole, Rat and Lion have a little less.
v Baboon and Chimp are about on the same EQ level, but not much above Crow or Wolf.
v Goldfish and Blue Whale are almost level, but Goldfish has a smidgen less brain for its size. 
v If Goldfish is a 'Lower vertebrate' it should be smart, as much above its own trend as Australopithecus.
v Lion, Possum, Rat and Mole share about the same EQ.
v Only the Hummingbird has a 'normal' brain size, but perhaps both it and Blue Whale are 'off scale'.
v Australopithecus has as much more EQ as an Ostrich has less. 
v Ostrich shows the rule that larger herbivores grow equivalently smaller brains.
v Between Australopithecus and Modern Man, Man finally overtook Porpoise in EQ, but not by much more than the Gorilla over the Lion.
v Modern Man, Australopithecus, Chimp and Gorilla are much more closely related to each other than to anyone else on the chart, yet show the widest differences in EQ.

For those who are really interested in Encephalisation Quotients, there are a couple of explanations at:

Encephalisation Quotients. Jim Moore shows how Jerison manipulated his lines to fit his theory.

Some don't think human brain size is extraordinary at all 

Human brains are extraordinary: "Brains have become increasingly commonplace in the five hundred million years or so since they first appeared in vertebrates (e.g., in the early fish Haikouichthys—see Shu et al, 2003), though vanishingly few exhibit an EQ greater than one with respect to today's ecosystem" (Macphail 1982, p243). 

Why Are Primate Brains So Big? Robin Prior 

Do human brains differ from other large animal brains ?

"The scientist Herbert Haug carried out a detailed comparative study of the anatomy of the elephant, dolphin and human brains to see if he could find out how the brains might relate to the intelligent behavior of these creatures. The brains differ distinctly from one another, but all are large. The elephant has the largest brain of all land animals; an adult elephant's brain weighs on average between nine and twelve pounds. But, of course, the elephant also has the largest body of all land animals. The elephant's brain makes up about 0.08 percent of the total body weight, while a horse's makes up about 0.25 percent of its total body weight. The human brain weighs three to four pounds and is also relatively large, making up 2 percent of our body weight.

The Nature Institute - Elephantine Intelligence

Brains of human being, pilot whale and elephant, viewed from the side, and below. Drawn to scale  (1) cerebrum.  (1a) temporal lobe of cerebrum.  (2)cerebellum.  The Nature Institute -  Elephantine Intelligence

The brains of elephant, dolphin, and the human being are all highly convoluted, which increases the surface area of the brain. These brains exemplify the well-known correlation between the degree of brain folding and the degree of intelligent, flexible behavior found in mammals.

Haug's study (Haug, H. (1970). Der Makroskopische Aufbau des Grosshirns. Berlin: Springer-Verlag) led him to be skeptical about any claims that correlate intelligence and the brain too closely:

"From a qualitative point of view, the human being does not possess—compared to elephants and dolphins—a particularly high grade of cerebral differentiation that would provide the morphological basis for such a great difference in intelligence as is actually present.... The question must be asked, whether brain differentiation must necessarily be equated with human productive intelligence."

The Nature Institute - Elephantine Intelligence

Growth in size and importance of the associative areas of the brain, from rats to cats to humans. Green = sensorimotor area, red = visual area, blue = auditory area. THE BRAIN FROM TOP TO BOTTOM

Was the evolution of intelligence inevitable ?

Undoubtedly our minds have much in common with those of the great apes, but that's to be expected given our evolutionary proximity. More surprising is the probably even closer similarity between our mental capacities and those of dolphins. These cetaceans, of course, have remarkably large brains, which were only outstripped in size by the hominids about 1.5 million years ago as the cranial capacity of early Homo burgeoned. But the dolphin brain differs from the primate brain in many significant ways, ranging from microstructure to the emphasis and development of particular regions and lobes. As Lori Marino from Emory University, Atlanta, has shown, this makes the similarities in the intelligence and social behaviour of dolphins and apes, including humans, all the more remarkable.

Dolphins recently demonstrated beyond doubt that they are capable of recognising themselves in mirrors - joining an elite group that includes the chimp and African grey parrot who have passed this test of self-awareness. Dolphins have a sophisticated communication system. Using signature whistles they can recognise at least a hundred other individuals. They are also accomplished mimics. Dolphin social structure falls into a type known as fission-fusion where groups of individuals form fluid alliances, joining and splitting in a medley of associations, some temporary, others quite stable. The same arrangement is found in chimp societies. Despite a lack of manual dexterity, dolphins have been known to use tools. One group has learned to fit conical sponges onto their rostrum so that they can root about in the seabed without being harmed by stonefish and other venomous animals. Some experts even believe that dolphins are capable of abstract thought, based on their remarkable ability to understand and act upon communications with humans.

Even more extraordinary is the convergence between two groups of freshwater fish, one from South America and the other from Africa. Both have evolved an effectively identical mechanism to generate and receive electrical signals, a similarity that even extends to the algorithm they use to avoid jamming each other's signals. Interestingly, both groups - but especially the African fish, which are known as mormyrids - have massively enlarged brains. Presumably this is to deal with the complexities of living in an electrical world where both social communication and navigation in the murky waters depend on rapid and precise pulses of electrical information.

In the case of honeybees, there is unequivocal evidence for communication, memory, decision-making, the ability to make "categorical distinctions" (differentiating between same and different) and even sleep. A bee's brain is extraordinarily complex. And we are far from understanding how bees and other social insects coordinate their activities".

We were meant to be - Simon Conway Morris 

- in which he, one of the world's most authoritative palaeontologists, argues that intelligence was bound to emerge one day - even if it took 600 million years, and even if it hadn't happened in humans (and dolphins).

Lots of energy would grow a bigger brain

A human brain's metabolic budget differs from an ape's - a lot

Apes use ~8% of resting metabolism for the brain, other mammals (excluding humans) use 3-4%, but humans use an impressive 25% of resting metabolism for the brain. This indicates that the human "energy budget" is substantially different from all other animals, even our closest primate relatives - the apes.

The human brain is about 2.5% of body weight but takes about 22%* of resting metabolic needs..

At 315 kcal, humans use over 3.5 times more of RMR to maintain their brains than other apes (i.e.,  255% more). Clearly, even relative to other primate species, humans are distinct in the proportion of metabolic needs for the brain. Leonard and Robertson [1992, p. 180]
Intelligence, Evolution of the Human Brain, and Diet
* Here are two different estimates in the same paper - I have seen another - 18% - it doesn't matter too much - it's a lot, anyway.

"The Expensive Tissue Hypothesis

was proposed in 1995 by Leslie Aiello and Peter Wheeler. They found that most of the human basal metabolic rate--more than 70%--goes to fuel the brain, heart, kidney, liver, and gastro-intestinal tract. To find out if any of these organs were reduced to fuel the human brain, they compared the mass of each organ in adult humans with that expected for a primate of similar body size. Only the gastro-intestinal tract was smaller than expected--and it was about 60% of the size expected for a similar-sized primate. 

The human brain appears to be balanced by an almost identical reduction in the size of the gastrointestinal tract".

But see Are we really carnivores?

The size of our gut has lessened, but the gastrointestinal system itself has not changed radically.

"Aiello speculates that we could have reduced our gut size to free up energy for a larger brain with a dietary change happening as brain size expanded. Our ancestors were shifting from a heavily vegetarian diet, which requires a massive gut to digest plants and nuts, to a more easily digestible, nutritious diet that included meat and requires less gut tissue".
Solving the Brain's Energy Crisis - Ann Gibbons

More energy? Then just why do South African rugby players want to be springboks?

Humans' energy use (Resting or Basal Metabolic Rate), for their body size, is very much in line with other mammals' (Kleiber's Law). But humans need a great deal more energy than other mammals to fuel their enlarged brain - so where does it come from? 

Aiello and Wheeler's insight that humans had a much lower gut size than other similar primates made sense, and so did their notion that, at about the same time, humans must have moved on to a diet with higher 'Dietary Quality'. 

Most anthropologists assumed they meant meat-eating, and left it at that. Aiello & Wheeler didn't propose any particular diet - after all, lean meat doesn't have that much more energy than the average plant material, although fat, of course, does. 

But where did early hominids get their fat? Subcutaneous (under-skin) fat is in very short supply in savannah or woodland animals, and internal deposit fat is usually got at by other scavengers first.

We're back to the  'Skull & Bones Club'  school of thought that proposes early human brain growth  was entirely dependent on hunting, or scavenging brains and bone marrow left over after other predators had had their fill.

Katherine Milton says something similar to Aiello and Wheeler: 

"Neither gorillas nor orangutans are as active, agile and behaviorally complex as members of the genus Pan (chimpanzees) nor do they show the high degree of sociality that characterizes chimpanzees. In fact, orangutans are the only extant anthropoids that live solitarily, a social regression apparently dictated by their large size and the distribution patterns of their wild plant foods. Due to features of their almost exclusively plant-based diet in combination with their pattern of gut kinetics, energy input in these two great apes may often be sufficiently limited such that nonessential behaviors are not favored by selection—in other words, orangutans and gorillas may not have sufficient "extra" energy to be more active and social

The Pan (chimpanzee) ancestor may have been somewhat smaller than extant chimpanzees and perhaps not such an extreme ripe fruit specialist. By becoming larger in body size over evolutionary time—extant male chimpanzees weigh some 49 kg and females 41 kg - and increasingly specialized on ripe fruits, which are an unusually high energy food. Chimpanzees and bonobos persist today as highly active and social apes.

Evolving humans able to satisfy their protein and many mineral and vitamin requirements with Animal Source Foods rather than plant foods, would free space in the gut for energy-rich plant foods such as fruits, nuts, starchy roots or honey. It is popularly believed that plant starches need to be cooked before they can be digested by humans, but this is not necessarily the case

Without routine access to Animal Source Foods, it is highly unlikely that evolving humans could have achieved their unusually large and complex brain while simultaneously continuing their evolutionary trajectory as large, active and highly social primates. 

This change in dietary focus in early Homo, which is a clear departure from known diets of other members of the Hominoidea, both fossil and extant, was gradually reflected both in the human brain size (substantial increase) and in the form of the human gut (a shift in gut proportions and overall gut size) as well as features of the dentition (smaller teeth, jaws and muscles of mastication)"  

The Critical Role Played by Animal Source Foods in Human (Homo) Evolution 

Animal Source Foods are not restricted solely to big game meat - they include reptiles, birds, insects, fish, crustaceans and molluscs, and the highly nutritious eggs of all of them. 

If there was, along with the taking up of Animal Source Foods, also a move away from low energy leaves and shoots to high energy fruits, nuts, seeds, roots and honey, as Milton notes, that would remove the 'dependence' that early humans were supposed to have had on obtaining bone marrow for energy, and all the theorising that that assumption has spawned. 

See: Loren Cordain, below

Richard Wrangham compared pigs--animals "rumoured to be quite smart," says Wrangham - with mammals such as cattle, sheep, goats, and deer. Pigs have small stomachs compared with these mammals, but their brains are no larger, showing that the gut-brain trade-off didn't apply to them. Other studies have shown that the theory doesn't hold for birds or bats. In fact, it may apply only to some primates.

Although huge quantities of energy go into a working brain, energy may not be the key limiting factor in brain size, says Oxford University evolutionary biologist Paul Harvey: "There's no reason to suspect that the reason other mammals don't have big brains is that they are energetically limited."
Solving the Brain's Energy Crisis - Ann Gibbons

Extra energy and available body-space gained by replacing gut by brain just wasn't enough to make humans grow a larger brain. They might provide the basic fuel and space, but not the building blocks.

Richard Wrangham has gone further, proposing that cooking underground storage organs (USOs or tubers - almost inedible unless cooked), brought on: 

Tooth reduction

Changed subsequent hominid social systems

Reduced sexual size dimorphism 

Central-place foraging with delayed consumption of food

Pressure on females to form protective bonds with males, so females benefited from being more sexually attractive than previously (ie continuous oestrus)
The Raw and the Stolen

Which seems an awful lot to build onto the theory that very early humans controlled fire and cooking nearly a million years before most people think they did. 

Richard Wrangham is an expert (and direct observer) of the behaviour in the wild of gorillas and chimps - if he doubts that developing into meat-eating hunters played any part in our early development, then his views are more than worthwhile, but it may be that his search for an alternate dietary switch to USOs is ultimately untenable.

His theory pre-supposes that early hominids somehow knew that many tubers are positively poisonous when raw, and must be cooked to make them at all edible. Many of them have enzymes and chemicals, particularly phytates, that positively inhibit brain development by preventing essential mineral uptake. And many of them are no more energy-rich than any other plant foods. 

But Wrangham  would have seen for himself that chimps (and most other animals), with their very small brains, already and instinctively (or culturally?) know how to avoid harmful foods.

Cooking may well have had a bearing on the second brain spurt - from Homo erectus to Homo sapiens, about a million years later, and even more so in the last 10,000 years, when our diets have become heavily carbohydrate-centred, and our brains seem to have actually shrunk.  

Perhaps central-place foraging with delayed consumption of (stored) food did change hominid social systems and female mating strategy, but maybe that happened a lot later than Wrangham would prefer. 

Central-place foraging doesn't necessarily mean temporary or longer residence in the middle of a natural yam field. It could be anywhere there is a readymade, reliable source of food, be it a shoreline (molluscs, crustaceans, fish) or a fruit tree grove (date, palm nut or coconut). Or both.

And then give the brain a bit of room and it'll grow by itself

Carl Zimmer reports (March 24 2004) - Chew On This:

MYH16 is a muscle-building gene that only becomes active in the developing muscles of the jaw. Apes and monkeys all have massive jaw muscles, which pass from the jaw up under their cheekbones, fanning out across the top of their skull and anchoring to a keel-shaped ridge. We have no such ridge, and we have pretty puny chewing muscles compared to apes. And much of the difference between us and other primates in this respect comes down a fatal mutation that hit a single gene: MYH16.

By tallying up the changes each version of the gene underwent, they were able to estimate when MYH16 shut down in our own lineage ~ 2.4 Mya.

This was no ordinary time in our history. Hominids before that age still had big anchoring crests and large jaws. Younger species had smaller jaws and smooth tops on their skulls. And something else happened at the same time: their brains began getting significantly bigger. It's possible that when MYH16 still worked in our ancestors, their chewing muscles acted like a clamp on the evolution of brain size. The architecture of the entire top of the head was so dominated by the muscles and their anchoring that expanding the brain was impossible".

Full report in Nature (Stedman et al 2004) - subscription only

For a bit more of the speculations that Stedman et al tacked onto a discovery of a minor gene change, see:
Sciscoop

and  Science Central, that quotes:

While studying human muscle disease, Stedman, a gastrointestinal surgeon, found a new version of a gene that encodes for a muscle-fueling protein called myosin. "Myosin is the most abundant protein in muscle," explains Stedman. "It's the motor protein that generates all the force. The body is able to make a wide range of different myosins, and each one has a different gene. The surprise came in finding that one of them…winds up having a mutation that cripples its ability to make a functional myosin…in all humans, as far as we can tell" ... They reported finding MYH16 in human jaw muscle, but they found a different version of this gene in other primates.

Changing to a wider diet, humans had to go out and find it - they grew bigger brains to do that - just so. 

"Creatures who feed on scarce foodstuffs cannot afford to sit in a single tree all day as some foliovores do; they must be able to find their food, perhaps in neighbouring (and hostile) territories, which may require remembering where certain landmarks are (e.g., when and where certain trees are in fruit) and the means to navigate to them; they might also need to employ more flexible or ingenious foraging tactics, all of which requires the right sort of neurological equipment. (Aiello and Wheeler 1995; Seyfarth & Cheney 2002) 

Why Are Primate Brains So Big? Robin Prior 

Just read that first sentence again...'Creatures who feed on scarce foodstuffs...etc...all of which requires the right sort of neurological equipment.' Isn't this suggesting, somehow, that when early hominids found they had to eat scarce foodstuffs, they grew brains in order to find them? 

If our brains really evolved to help us mentally map our resources of food and tools, and 'more flexible or ingenious foraging tactics, all of which requires the right sort of neurological equipment', then why are we so bad at mental mapping after 2 million years of evolving brains for such a speciality?

Ask your wife next time she's lost the route on the map because it's gone over the page.

Humans socialised

"Being part of a group offers many advantages over a solitary life, including the possibility of the division of labour, with all the economic advantages that can bring. But group living - at least among birds and mammals - can have its problems too. Unlike the ordered world of insect colonies, the seemingly chaotic (and much more complex) world of warm-blooded group life requires considerably more real-time information processing (Dunbar 1998). The more relationships one has to keep track of, the more one needs a bigger brain to keep track of them".

Co-Evolution Of Neocortex Size, Group Size And Language In Humans

"Dunbar compared the ratio of neocortex to the rest of the brain (or brain part, such as the medulla) in a variety of primate species, and after controlling for body size, showed that neocortex size is strongly correlated with social group size—making brain size the best predictor of group size among primates. This, he says, yields “much the best [statistical] fit, accounting for 76% of the variance in mean group size among 36 genera of Prosimian and Anthropoid primates. (Dunbar 1993)”

Why Are Primate Brains So Big? Robin Prior 

- just like baboons 

But Robin Dunbar also says: "For many years, people thought that the ability to hunt or forage better was what drove the evolution of our brains. 

But a better diet had to come before we could grow a bigger brain.” 

Molecular Insights into Human Brain Evolution

And sex comes into the story, of course

"Individuals with defective brains would likely be readily identifiable by their strange behaviour and afflicted males in particular could well be denied mating opportunities. Male brains, in other words, may act as fitness indicators—which in highly social species may translate as a sign of the status of an individual within the group (Miller 2000, p134).

Sexual selection is a factor that plays a central role in the evolution of brain encephalization (Geoffrey Miller and Peter Todd)   

The peacock’s tail was long considered an oddity; one of the many puzzles of nature. Large brains are, according to Miller, Todd, Wills and others, no different. Mating choices are important—get them wrong and a lineage comes to an end—so it is not surprising that individuals (especially females, who generally have so much more at stake) are picky when it comes to choosing a mate

"Miller (2000) observes that traits formed through sexual selection follow the same pattern—they rapidly evolve to a local maximum.  Any noticeable trait, he suggests, can be operated on by mate choice, whether through sensory bias, as a fitness indicator, or even as pure ornament. He even speculates (convincingly) on the origin of human aesthetic sensibilities in connection with this process. He notes that males greatly outnumber females in their production of ornament, song, combat and so on (in humans this translates into, amongst other things, music, literature and art), and that it is hard to see any survival advantages in aesthetic behaviour: “Since there are no plausible survival benefits for music production, reproductive benefits seem worth a look.” 

...human art . . . [is] a biological adaptation: it evolved through sexual selection to serve the same courtship functions as almost all other examples of organic beauty and complex behavioral signals observable in nature. Such ornamentation often evolves as a reliable, costly indicator of the signaller’s good health, good brain, and good genes.” (Miller 2001)." 

Why Are Primate Brains So Big? Robin Prior 

Geoffrey Miller also says: "I suggest that the neocortex is not primarily or exclusively a device for tool-making, bipedal walking, fire-using, warfare, hunting, gathering, or avoiding savannah predators. None of these postulated functions alone can explain its explosive development in our lineage and not in other closely related species...The neocortex is largely a courtship device to attract and retain sexual mates; its specific evolutionary function is to stimulate and entertain other people, and to assess the stimulation attempts of others" Miller 1992

"Surveys consistently place intelligence, sense of humour, creativity and interesting personality above even such things as wealth and beauty in lists of characteristics  in both sexes". Buss 1989

Both quotes from " The Red Queen" - Matt Ridley, Penguin 1993

If you don't believe that, just take a look at your local newspaper's 'lonely hearts' columns - you'll see 'SOH - sense of humour' in every other entry. 

But his 'sociological' idea of 'why brain growth?' is obviously similar to Robin Dunbar's. 

When I first wrote this, I didn't know that Robin Dunbar had actually done a study of Lonely Hearts Columns.

See: Discover: Love by the line

David Waynforth and Robin Dunbar study evolutionary factors of mate selection in personal ads

Sexual selection, social development skills, etc, can only push or pull brain growth once the brain has started to grow. The human brain, like the peacock's tail, grew so extraordinarily that there must have been some major input to begin with. The peacock's tail, though singular and unusual, has plenty of precedents in other bird tails - the human brain size does, it's true, have precedents in other primates' EQs, but brain growth in our species has been remarkable, and must have had peculiar dietary components - a peacock could grow its tail  on the same old food as usual.

Did larger brains really matter anyway?

Did larger brains really matter anyway? - Tooling through the trees

Tooling Through the Trees

Carel van Schaik observed orangutans using tools in the Suaq Balimbing swamp in Sumatra in 1993 & 1994

"Toolmaking was once considered a hallmark of the unique intelligence of human beings. Researchers began to question that notion, however, as they discovered that wild chimpanzees, our closest relatives, use a number of tools, including sticks to get termites out of nests, and stones to crack open nuts. Moreover, they don't just use whatever happens to be lying around—they tailor their tools to fit a particular job.

The last common ancestor of chimps and humans lived 6 million years ago. Gorillas split off from the ancestor of chimps and humans 8 million years ago, and while they sometimes play with tools in captivity, they never use them in the wild.

Well, actually, they may. A wild gorilla has recently been filmed using a stick to test the depth of the water as it waded across a swamp.

...sometimes an [orangutan] will break off a branch about a foot long, snap off the twigs, fray one end, and put the other end in its mouth. Holding on to a tree trunk with its arms and legs, the orangutan rams the stick into a hole containing a termite nest. It then flicks out the broken-up chunks—full of delectable larvae and pupae—and eats them. The Suaq orangutans also use sticks to scare out ants from tree colonies. “But the most common use is when they go for honey,” says van Schaik. “They put a stick in and poke through some nest wall and move it around and catch the honey, pull it out, turn it around and stick the other end in their mouth, and then go back in." If the stick is too long to use comfortably, they snap off one end.

Orangutans also use tools to eat fruit. When the fruit of the Neesia tree ripens, its hard, ridged husk softens until it falls open. Inside are seeds that the orangutans love, but they are surrounded by fiberglass-like hairs that, as van Schaik can personally attest, “hurt like hell.” A Neesia-eating orangutan will select a five-inch stick, strip off its bark, and then carefully collect the hairs with it. Once the fruit is safe, the ape pops the seeds out with the stick or with its fingers.

It's possible, of course, that the very first invention of tools among orangutans happened relatively recently in Suaq. Van Schaik thinks that's unlikely. Of the four types of great apes, he points out, three—chimps, orangs, and humans—have now been observed to use tools in the wild. The simplest explanation, says van Schaik, is that the apes' common ancestor did, too. Gorillas, the fourth great ape, lost the skill because they developed an easy diet of leaves and nettles that made tools pointless. But by the time humans evolved, toolmaking was a long-established art. “We don't have to see early hominids as these heroes that invented 10,000 different things in five minutes,” says van Schaik. “There may have been little that was new there—they already had the capacity and most likely the skill.”

Van Schaik's observations complement a growing body of research that suggests

the real quantum leap of intelligence did not happen 6 million years ago among the earliest hominids but 16 million years ago among the earliest great apes.

It was then, in this view, that animals first evolved insight—an ability to make connections between concepts, recognize cause and effect, and plan actions. The sort of thinking required for using tools, for symbolic communication, for empathy—for all these supposedly quintessentially human gifts—may be far older than we imagined".

Discover Magazine, November 1995 - Microsoft Encarta

What fuelled human brain growth?

Almost everyone agrees that basic changes in diet kick-started our brain growth 

Brain growth didn't start, just so, when men started walking upright, 'freeing their hands for tool use', etc - that happened a million or more years earlier. 

Dry climatic conditions didn't 'cause' it - ostriches, gnus and rhinoceroses haven't grown themselves bigger brains. And if you've ever closely encountered a camel, you'll know its brain is well behind - probably in its hump.

Human brain growth was even more extraordinary if it occurred in an environment (the 'savannah' or the 'woodlands') where every other animal that has evolved a larger body has reached a bottleneck in available nutrient supplies and has had to accept a reduced proportion of brain. 

This is due solely to the lack of suitable 'brain nutrients' in that environment:

Why savannah & woodland animals have small brains

In mammals, the polyunsaturated (PUFA) acid content of brain ethanolamine phosphoglycerols is virtually identical among varying species and is dominated by 22:6ˆ3 (docosahexaenoic acid, DHA) and 20:4ˆ6 (arachidonic acid, AA). Whether a mammalian species has a high encephalization quotient or a low encephalization quotient, the relative percentage of DHA and AA of the total brain phospholipids remains constant, hence all mammalian brain tissue appears to have an invariant structural requirement for these two fatty acids without which, normal neural function cannot occur. Limitations to the supply of either one of these fatty acids will determine limitations to brain growth.

As mammals evolve larger bodies, encephalization quotients (brain mass/body mass) generally decrease, consequently evolving mammalian brains were not able to maintain their relative mass with greater and greater evolutionary increases in body mass. This limitation occurs because the supply of the fatty acid building blocks for brain tissue (AA and DHA) is constrained by the limited ability of the liver (primarily) and other tissues to synthesize these fatty acids from their dietary precursors. 

Numerous studies in mammals, including humans, have shown that the elongation and desaturation of linoleic acid (18:2ˆ6) to AA and of alpha-linolenic acid (18:3ˆ3) to DHA are inefficient pathways with low product to substrate ratios. Hence, the limited availability of these two fatty acids from endogenous metabolic synthesis may have represented the evolutionary ‘bottleneck’ impeding the encephalization process in all herbivorous mammals.

Encephalization quotients decrease with increasing body size because there literally may be insufficient long chain fatty acid product (AA and DHA) to build more brain tissue. 

Cats and other obligate carnivores* represent a notable departure from the metabolic and evolutionary considerations that limit brain size in herbivorous animals because they obtain virtually all of their AA and DHA as preformed product in the flesh and organs of their prey and are only minimally reliant upon elongation and desaturation of 18 carbon fatty acids as their source of AA and DHA. Throughout evolutionary history, carnivorous mammals have always maintained a proportionately larger brain size relative to body size when compared to their herbivorous prey. The dietary availability of preformed AA and DHA is exclusive to meat eaters, since these fatty acids are not present or present only in trace quantities in plant food sources.

In a similar manner, increasing consumption of animal food products also provided early hominids with a dietary source of preformed AA and DHA, substances that may have opened the evolutionary ‘window’ for encephalization. 

Cordain L, Watkins BA, Mann NJ. Fatty acid composition and energy density of foods available to African hominids:

* If you go back to Jerison's chart,  you might note that lions, at least, have a below-average EQ. You can't increase your DHA intake much if the animals you eat don't have a lot.

Loren Cordain applied this factor, and the energy density requirement (see Aiello & Wheeler above)

to the supposed big game diet of early humans, and ended up proposing that it was the brains and bone marrow of scavenged animals, alone, that kick-started early human brain growth. 

(Hence the title of my web page - The Skull & Bones Club)

There are a number of gaping holes, the largest of which are the unavailability of scavengeable carcasses, and the paucity of brains and bone marrow left in them, that render this theory worthy, but untenable. 

Cordain concentrated narrowly on comparing particular items, such as game meat, fish, and plant foods, in comparing energy and DHA content, while omitting to consider their combination with other energy-rich foods in the diet of an very omnivorous and opportunistic hominid.

See: Fat & The Brain

Conclusions

This page is still a hotch-potch. It was supposed to summarise all the usual explanations of human brain development, most of which posit that the brain evolved to become larger 'for' something. 

It didn't. For some quite unknown reason, it gradually grew larger. At some stage, it was sufficiently luxurious as an organ that it was 'pre-adapted' for Early Humans to make better use of its larger-sized 'hardware' to evolve and grow, with the benefit of positive feedback, the extraordinarily complex software of neurones, ganglia, synapses, etc, that make us exclusive in the animal world. 

Michael Crawford and Stephen Cunnane have a persuasive scenario for the development of human brains without the need to consider the pressures of survival and food-gathering. 

See: Fats & The Brain 2 - Born Fat

But the 'How?', the simple nutritional mechanics of it, is much simpler to tackle.

If you're still with me, go on to read:

New Pages as at May 2006