Chapter #: Neurodiversity in Ancient Human Ancestors
Our ancient ancestors had to adapt to a constantly changing environment, which required the development of a diverse range of mental abilities. Early humans had skulls with different shapes than modern humans suggesting varying patterns of cognitive strengths and weaknesses. Some, such as Neanderthals, even had larger brains than modern humans and seem to have possessed advanced mental capacities, such as excellent spatial reasoning and the ability to create complex tools. Through the study of fossils, artifacts, and DNA, researchers can begin to reconstruct the cognitive and social lives of these people, including their potential differences in cognitive functioning. Overall, studying hominids and early humans can provide a broader perspective on the range of cognitive abilities that have existed throughout human history and can help to generate assumptions about how neurodiverse individuals would have functioned prehistorically.
We have seen that modern humans have a wide range of brain sizes, and that some can be relatively small. When the size is very small, it can come with a decrement in cognitive function. Individuals like these, with intellectual disabilities, may have inhabited an ecological niche like that occupied by ancient hominid groups. Both ancient hominins and intellectually disabled modern people may have hunted and gathered in a way similar to hunter-gatherer children today. They may have used more simplistic food procurement and processing strategies. As the tables below evince, there is tremendous variation in brain size among primates, yet all are capable foragers and survival experts.
Studying hominids and early humans can provide valuable insights into neurodiversity, or the natural variation in human cognitive and neurological functioning. Our distant ancestors had to adapt to a constantly changing environment, which required the development of a diverse range of cognitive abilities. Through the study of fossils, artifacts, and DNA, researchers can reconstruct the cognitive and social lives of these early humans, including their potential differences in cognitive functioning.
One example of this is the study of the Neanderthals, a group of hominids that lived in Europe and Asia over 30,000 years ago. Recent studies have suggested that Neanderthals had larger brains than modern humans and exhibited some unique cognitive abilities, such as excellent spatial reasoning and the ability to create complex tools. These findings suggest that Neanderthals may have had a different pattern of cognitive strengths and weaknesses than modern humans.
Overall, studying hominids and early humans can provide a broader perspective on the range of cognitive abilities that have existed throughout human history and can help to challenge societal norms around neurodiversity. By understanding the natural variation in human cognition, we can better appreciate the unique strengths and challenges of individuals with different cognitive profiles.
| List of Primates by Brain Size in Cubic Centimeters | ||
| Species | Brain Size (cm3) | Time Since Divergence (years) |
| Gray mouse lemur | 2 | 63,000,000 (lemur) |
| Pygmy marmoset | 5 | 40,000,000 (new world monkey) |
| Ring tailed lemur | 20 | 63,000,000 (lemur) |
| Howler monkey | 50 | 40,000,000 (new world monkey) |
| Macaque monkey | 60 | 25,000,000 (old world monkey) |
| Capuchin monkey | 70 | 40,000,000 (new world monkey) |
| Gibbon | 80 | 18,000,000 (lesser ape) |
| Spider monkey | 100 | 40,000,000 (new world monkey) |
| Baboon | 170 | 25,000,000 (old world monkey) |
| Bonobo | 360 | 6,000,000 (great ape) |
| Orangutan | 380 | 14,000,000 (great ape) |
| Chimpanzee | 400 | 6,000,000 (great ape) |
| Gorilla | 500 | 7,000,000 (great ape) |
It is difficult to uncover details about brain anatomy from fossils. This is because soft tissues like nervous tissue leave no fossils traces behind. Skulls; however, are made of bone and do fossilize. Even the interior of the skull is often fossilized. Measuring the interior of the skull gives anthropologists the endocranial volume, which is a good proxy for brain size.
| List of Hominins by Brain Size in Cubic Centimeters | |||
| Species or Group | Brain Size (cm3) | Time Since Extinction (years) | Average Height |
| Homo floresiensis | 400 | 50,000 | 3’3’’ to 3’7’’ |
| Australopithecus | 450 | 1,400,000 | 3’3’’ to 4’11’’ |
| Human w/ microcephaly | 450 | NA | |
| Paranthropus | 550 | 1,200,000 | 3’7’’ to 4’7’’ |
| Homo habilis | 700 | 1,650,000 | 3’4’’ to 4’5’’ |
| Homo ergaster | 850 | 1,300,000 | 4’9’’ to 6’1’’ |
| Homo erectus | 1,000 | 110,000 | 4’10’’ to 6’2’’ |
| Human with Down syndrome | 1,150 | NA | 4’6’’ to 5’2’’ |
| Homo Heidelbergensis | 1,200 | 300,000 | 5’2’’ to 5’9’’ |
| Homo sapiens | 1,300 | NA | 5’2’’ to 5’6’’ |
| Homo neanderthalensis | 1,500 | 40,000 | 5’0’’ to 5’5’’ |
Uner Tan Syndrome
Uner Tan syndrome was discovered by the Turkish evolutionary biologist Uner Tan. People affected by it walk on all fours on the bottoms of their feet and the palms of their hands. This is a form of quadrupedal locomotion very similar to that used by apes. People with the rare syndrome have severe learning disabilities and speech deficits and if you watch one of the documentaries on the subject, the comparison with primates is hard to miss.
Tan postulated that this syndrome is an example of “reverse evolution” or atavism. He said that these people show characteristics of the primate ancestors of Homo sapiens, before the move to bipedalism. News stories on the topic were accompanied by claims about “devolution,” “ape-people,” and “throwbacks.”
However, expert critics pointed out that these were people with rare genetic mutations that resulted in a balance impairment (congenital cerebellar ataxia). Lack of balance forced these people to learn to compensate from an early age by bear crawling with their feet rather than kneel crawling with their knees like most babies. These people are not able to walk on two feet, although monkeys and apes can do this for several seconds at a time before tiring. In fact, they are very slow and exhibit coordination and neurological deficits to an extent that it seems clear that Uner Tan syndrome could not be an adaptive human variant. However, it is possible that the syndrome involves some kind of fundamental disruption of human bipedal walking revealing the older substructure of quadrupedal gait. Psychologist Nicholas Humphrey said this about Uner Tan’s unfounded assumptions.
It’s terribly easy to be led away by some notion of living fossils… I’m not going to make any bones about this. I think that Professor Tan’s description of this family as a “devolution”, as an evolutionary throwback, is not only scientifically irresponsible, but is deeply insulting to this family.
— Nicholas Humphrey, The Family That Walks on All Fours (BBC Documentary)
I think Humphrey’s caution is appropriate. His accusation of irresponsibility gives me pause because it could certainly be directed to topics I introduce in this book. However, I don’t think that Tan meant any deep insult to the people with his syndrome. There is nothing debasing about our origins in natural history and nothing disgraceful about how they might affect us today. Everything about us has been fashioned by our past. The genetic and behavioral studies on Uner Tan syndrome have not led to any firm conclusions but it should not be surprising if certain medical disorders simulate ancient modes of life. It is known that various genetic conditions also cause people to be born with extra nipples (supernumerary nipples), monkey-like tails (vestigial tails), or hair all over their bodies (hypertrichosis).
So now, with appropriate caution and esteem, let’s consider some “ape-people” that are scientifically confirmed to have existed and discuss the implications they may have for our understanding of neurodiversity.
Australopithecus (4.4 to 1.4 mya)
Despite the lack of fossils, researchers generally assume that the common ancestor we shared with chimpanzees 6 million years ago foraged in much the same way chimps do today, in tropical rainforests. However, geological evidence shows that around 6 million years ago the thick African forests, home to multiple ape species, began to thin out. Some apes retreated to deeper forests and others began to lumber around on the savannah floor in search of food. These apes were forced to being experimenting with bipedal walking.
Aside from walking on two legs Australopithecines were not much different from chimps. They would have started walking on two legs because it helped them save energy in their habitat. It would have helped them cover wide, open landscapes quickly and efficiently. It also would have freed their hands for carrying food, tools, or babies. It may have also made them appear larger or more intimidating to predators.
Around 4 million years ago our ancestor, Australopithecus, became very comfortable walking the ground on two feet. Fossil skeleton Australopithecines (which include the famous Lucy) have shown us that they would have stood a little over 3 feet tall and had a brain no bigger than a chimpanzee. This was a tremendous surprise to primatologists at the time because it meant that bipedal walking did not necessitate a big brain. It is now thought that the ability to walk on two feet allowed access to a vastly different ecology, and that this change was pivotal in allowing the brain to expand to its present size. It has been estimated that the australopithecine brain used up 11 percent of the calories they consumed. This is about half as energy-consuming as our own.
“It is clear from the study of their fossil remains that, anatomically speaking, the australopithecines were peculiarly defenseless creatures.”
-William Le Gros Clark
Austalopithecus was small, short, light, and had a small brain. You might question how such a small primate could survive on the African savannah exposed to major predators like hyenas and lions. But remember, vervets, baboons, and patas monkeys are all ground-living species that get along quite well without the protection of trees. They have no trouble finding and eating bugs, lizards, snakes, birds, bird eggs, and rodents. Size and musculature are not prerequisites for survival in Africa. A number of forms of intellectual disability, including disorders like Laron syndrome, congenital iodine deficiency syndrome, and Down syndrome feature short stature and small brains. Extinct hominids and extant monkeys demonstrate that short stature, low muscle mass, and small brains do not preclude survival. In fact, the table below shows that humans are an outlier suggesting that our large weight and big brains may come with certain risks.
| Primates and their Food | ||
| Species or Group | Weight (lbs) | Food |
| Vervet Monkey | Leaves, gum, seeds, nuts, grasses, fungi, fruit, berries, flowers, buds and shoots, and occasionally insects, birds’ eggs, birds, lizards, rodents and other vertebrate prey | |
| Baboon | Grass, berries, seeds, pods, blossoms, leaves, roots, bark, and sap from a variety of plants. They also eat meat including, insects, fish, shellfish, hares, birds, vervet monkeys, and small antelopes. | |
| Chimpanzee | Fruits (e.g. figs, mangoes, bananas, oranges and apples), nuts (e.g. oil palm, kola and panda nuts), leaves, seeds, blossoms, tree bark, plants, mushrooms, flowers, honey, insects (e.g. termites), meat (monkeys) | |
| Gorilla | Stems, bamboo shoots, fruits, termites, and ants | |
| Chimpanzee Diet | Percentage |
| Fruit | 59 |
| Leaves | 21 |
| Seeds | 5 |
| Blossoms | 4 |
| Insects | 4 |
| Meat | 2 |
| Other | 5 |
Goodall (1986) Chimpanzees of Gombe, page 233
| List of Primates by Weight | ||
| Species or Group | Weight (lbs) | Weight (kgs) |
| Gray mouse lemur | 0.125 | 0.06 |
| Ring tailed lemur | 5 | 2 |
| Capuchin monkey | 3 to 10 | 1 to 5 |
| Vervet Monkey | 7 to 20 | 3 to 10 |
| Macaque monkey | 10 to 20 | 5 to 10 |
| Patas Monkey | 10 to 30 | 5 to 15 |
| Baboon | 30 to 100 | 15 to 45 |
| Gibbon | 10 to 20 | 5 to 10 |
| Bonobo | 60 to 140 | 25 to 65 |
| Chimpanzee | 70 to 160 | 30 to 0 |
| Orangutan | 80 to 280 | 30 to 130 |
| Human | 100 to 200 | 40 to 90 |
| Gorilla | 130 to 500 | 60 to 230 |
Table X.3: Some of the groups listed contain multiple species, and the weight given is an average.
Homo Habilis (2.4 to 1.5 mya)
Homo Habilis was the first of our genus, Homo. It was probably meat eating that allowed its brain to grow.
There was a period about 2.5 million years ago when members of each of the three major hominid groups lived contemporaneously. The early ape-like Australopithecines, the larger and more robust Paranthropus, and members of our own genus, Homo, all walked the African plains, savannas, and woodlands together. The first stone tools date back to about this time.
Climatically, 3 million years ago, Africa had a wide variety of different ecological niches. This is one of the reasons that there were so many different ape-people at that time. Each was able to specialize. Like Boisei, with molars four times the size of our own for chewing through fibrous plant matter. But this variety of ecological niches should make us think about how each of these species must have had exotic forms of neurodiversity.
Humans were able to improvise and rethink their menu. We were jacks of all trades being inquisitive scavengers and opportunists.
It takes intelligence to recognize animal tracks and follow them, to understand that vultures gather around animal, carcasses, just hard to understand how the distribution of food changes by season, that particular types of clouds portend rain, for a large brain to be worthwhile. It must be absolutely vital for survival. To place water in an ostrich, egg and bury it in the ground to be drinks later.
The brain areas that control the hands are enlarged in humans compared to apes, giving us more manual dexterity. True stone tool construction probably began with homo habilis. But even modern-day apes use tools. Orangutans use branches to ward off bees and wasps. They dip leaves into holes to drink water, they strip twigs to stick them into ant nests and spiny fruits. Chimps do this as well. They also place nuts on flat rocks and smash them open. Interestingly, gorillas are rarely observed using tools.
Homo Erectus (1.6 mya to 100,000 ya)
About 2.5 million years ago, australopithecines began fashioning stone tools. This led to the evolution of Homo erectus which were nearly twice as large. The bigger bodies and brains probably made them better adapted to chase carnivores from their carcasses, hunt animals by running them to heat exhaustion, and dig deeply for tubers. Increased intelligence permitted them to start using fire. Cooking food frees up nutrients and makes it more digestible.
Homo erectus is what biologists call a chronospecies, one that changes through time. Homo ergaster was erectus’ earlier phase which lived mostly in Africa. The later Homo erectus lived mostly in Eurasia.
Relative to us, they were probably very literal minded, saw things in black and white, had rudimentary communication, and lacked in imagination.
Homo erectus was mostly hairless having lost the fur of its ancestors, was capable of sweating to cool off, and probably had a white sclera surrounding its irises to that its companions could see where it was looking. This last adaptation suggests that they were highly cooperative and had a sense of empathy. Homo erectus developed unusually thick skulls which probably indicates that they fought heavily between one another, perhaps hitting each other over the head with clubs.
500,000 years ago, Homo erectus had an endocranial volume of 1,000 cc, the low end for humans. They stuck around long enough to live contemporaneously with Homo heidelbergensis and Homo sapiens.
Homo Floresiensis (200,000 to 50,000 ya)
One of the more recently discovered forms of archaic human is Homo floresiensis, also known as Flores Man or the “Hobbit.” This small human species lived on the island of Flores, Indonesia, until the arrival of modern humans around 50,000 years ago. Homo floresiensis had very small brains, around 400 cubic centimeters, in the range of chimpanzees. These people may have used fire and simple stone tools.
Homo floresiensis remains suggest they would have been around three and a half feet tall. This is much smaller than any modern pygmies and even smaller than Lucy the australopithecine. This small size is thought to be due to “insular dwarfism,” where animals living on islands can become very small due to the island ecosystem. The island also skewed the size of other animal inhabitants, as Homo floresiensis shared the island with tiny elephants and giant rats.
Because these people were petite, with tiny brains, but lived very recently, many researchers assumed that they might be modern humans with some form of pathology. Some anthropologists even suggested that they may have had Down syndrome. Since then, detailed analyses on the remains have tried to determine whether these skeletons belonged to modern or archaic humans. The researchers were unable to isolate DNA from the specimens but could base their inferences on skeletal measurements. The wrist bones were very similar to chimpanzee wrists, and the feet had several primitive characteristics. Further analyses of the jaw, chin, and teeth have concluded that Homo floresiensis was not Homo sapiens but rather an early species of Homo and a sister species of Homo habilis (Argue et al., 2017).
Homo Heidelbergensis (600,000 to 300,000 ya)
Archaic Homo sapiens (H. heidelbergensis and H. rhodesiensis) evolved from African Homo erectus or Homo ergaster (Lewin, 2005).
Homo Heidelbergensis had a brain size of about 1,200 cm3 and may be the direct ancestor of both Neanderthals and humans. They left Africa 500,000 years ago and traveled north into Europe becoming the Neanderthals. Homo Heidelbergensis individuals that remained in Africa largely evolved into modern humans.
Homo Neanderthalensis (130,000 to 40,000 ya)
In 1865, three years before Darwin published The Origins of Species, German quarry workers unearthed the first Neanderthal remains in a cave on the side of Neander valley, near Düsseldorf. These were, of course, very human-like and did not satisfy Darwin’s critics that were looking for a missing link between apes and humans. Earlier fossil hominids, with more primitive features weren’t found until 30 years after Darwin’s death.
Neanderthals were short, stocky and very muscular to help them retain heat in their frigid habitat. They also had large, broad noses to warm and humidify the air they breathed.
Portions of human chromosomes that contain a high proportion of expressed genes have fewer Neanderthal genes, whereas portions that contain few expressed genes have more Neanderthal genes. This suggests that Neanderthal genes didn’t work well after being introduced into Homo sapiens. However, a few Neanderthal genes are much more common than others indicating that they were valuable and discovering their function has been a high priority for researchers. Some of these genes are expressed in the brain and some of them have been shown by mathematical analyses to be responsible for altering the shape of the brain.
Neanderthal brains were around 1500 cm3, 10% bigger than ours. Neanderthals had bigger brains than we do today, but this difference would have been much smaller if we were comparing them to us 10,000 years ago, when our own brains were larger.
Homo Sapiens (350,000 ya to Present)
Anatomically modern Homo sapiens weighed less than Neanderthal or other early H. sapiens. They had smaller jaws, teeth, noses, and brow ridges. 200,000 years ago the human brain reached 1350 cm3. 40,000 years ago, they began to make tools from bones and shells in addition to stone. They also began to conduct more elaborate burials, adorn their bodies with many types of artifacts, and paint images on rock. Agriculture arose 10,000 years ago. Writing occurred in the last 4,000 years and seems to have arisen independently in different regions of the world around the same time.
Modern humans have a mutation that increases the production of cortical neurons early in fetal development. This mutation results in a single amino acid change that alters the shape of a protein called TKTL1. This mutation is completely absent in other primates including more ancient humans. Scientists have sequenced the full genome of Neanderthals from DNA isolated from fossils. The Neanderthals and the Denisovans have each had their genome sequenced after their DNA was isolated from fossils, and neither group had this mutation. The protein is expressed early in development by progenitor cells that give rise to the cortex. When the protein is added to the brains of mouse and ferret embryos the animals grow more cortical neurons. This TKTL1 gene is a clear example of how natural selection can make a tiny change that possibly leads a species to higher intelligence.
The genes Auts2 and cadps2 make significant contributions to autism. Interestingly the variants have not been found in Neanderthal DNA.
Essentially, there were five big jumps in primate brain size:
- During the shift from prosimian to monkeys 50 mya
- During the shift from monkeys to apes 10 mya
- During the shift from australopithecines to Homo habilis 2.5 mya
- During the shift from habilis to erectus 2 million years ago
- During the shift from Homo erectus to Homo heidlebergensis 500,000 years ago
| Millions of Years Ago | Eon | Era | Period | Notable Events |
| 251 | Mesozoic | Triassic | First dinosaurs and mammals | |
| 200 | Jurassic | Giant dinosaurs dominate | ||
| 145 | Cretaceous | First primates appear | ||
| 65.5 | Cenozoic | Paleocene | Dinosaurs go extinct | |
| 55.8 | Eocene | Early modern mammals | ||
| 34.0 | Oligocene | Grasslands and herds develop | ||
| 23.0 | Miocene | Apes appear | ||
| 5.30 | Pliocene | Human-like primates evolve | ||
| 2.40 | Pleistocene | Humans develop |
Boskop Humans (30,000 to 10,000 ya)
In 1913, two Afrikaner farmers found a large skull in a field in South Africa. The skull had a tremendous braincase and was named Boskop Man. At first, skeptics thought it was an anomaly or perhaps had a disease such as hydrocephalus which makes the skull large without a large brain. However, these assumptions were preempted by the discovery of more skulls. Paleoanthropologists have unearthed many similar skulls since.
Boskops had the largest brains of any hominids or humans on record with a cranial capacity ranging from 1,700 to 2,000 cubic centimeters. This is more than 30% percent larger than ours. Keep in mind that our brains are about 30% bigger, on average, than a homo erectus individual. Many Boskop finds had small childlike faces and slight gracile skeletons. One petite, lithe female of 5’6’’ had a brain around 1750 cm3. This would have been greater, according to Raymond Dart, than the Italian renaissance painter Raphael who supposedly had one of the largest human brains on record.
In their 2008 book, “Big Brain: The Origins and Future of Human Intelligence,” neurologists Gary Lynch and Richard Granger claimed the large brain size in Boskop individuals is probably indicative of high general intelligence. They state that the “human of the future” has already come and gone. The book caused a lot of controversy. Anthropologist John Hawks summed up the quarrel reasonably saying the collection of Boskop skulls is a cherrypicked set of large crania taken from a much larger sample of varied crania. In other words, “any old, large skull was a ‘Boskop’.” Thus, it has become clear that the Boskops were not a race of superintelligent humans. But it is just as clear that there were ancient African humans with skulls much larger than yours or mine.
What would it have been like to interact with a person with a 2,000 cm3 brain? Having a conversation with one might be difficult because they would probably be difficult to understand. Unless they were speaking down to us, they would be likely to converse in abstractions that would go right over our heads. We would be like a child listening to an adult conversation. They would think in difficult-to-decipher analogies and thus much of the meaning in their communication would be implicit. If we could hear them speak, our little brains may not be able to put it all together at once to fully appreciate their message. We may find it difficult to fathom the conceptual leaps between their sentences. Humans with damage to the frontal lobe, like people with intellectual disabilities, think concretely and often in black and white. What they are saying may make perfect sense but is often simplified and exact. Boskops may have been the opposite, and their speech may have seemed flowery and confusing to us.
Highly intelligent children and adults are another form of neurodiversity. Often we praise them, but they’re often depressed, or anxious, they don’t always make a lot of money, they can be more prone to suicide, more prone to certain psychiatric disorders are sometimes they are nutty, or eccentric, they can lack street smarts.
So just as it is clear that hunting and gathering humans could get along with small brains, they could also get along with very large brains.
Hyperfrontality
Costs and benefits of hyperfrontality.
Apathy.
Brain Areas that Shrunk in Humans
Since we descended from the common ancestor we share with chimpanzees, many brain areas have shrunk. These include the olfactory bulb (involved in the sense of smell), the magnocellular portion of the red nucleus (involved in a form of motor coordination) and the dorsal cochlear nucleus, an auditory nucleus in the mammalian medulla, is highly laminated in prosimians, simpler in monkeys and simplest in apes and humans. It is unclear why these brain areas diminished in size but their functions may have been subsumed by the expanding neocortex.
Lewin, Roger (2005). Human Evolution an illustrated introduction (Fifth ed.). Blackwell Publishing. p. 159.
Adaptive Neurodiveristy 