Anyone can see similarities among humans, apes, and monkeys. Those similarities extend to the brain as well. In this section, we consider the brain and behaviors of some more prominent ancestors that link ancestral apes to our brain and our behaviors. Then we consider the relation between brain complexity and behavior across species. We conclude by surveying leading hypotheses about how the human brain evolved to become so large and the behavior that it mediates so complex. The evolutionary evidence shows that we humans are specialized in having an upright posture, making and using tools, and developing language but that we are not special, because our ancestors also shared these traits, at least to some degree.

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Humans are members of the primate order, a subcategory of mammals that includes apes, Old World monkeys, New World monkeys, tarsiers, and lemurs as well (Figure 1-9). We humans are but one of about 275 primate species. Primates have excellent color vision, with the eyes positioned at the front of the face to enhance depth perception. They use their highly developed visual sense to deftly guide their hand movements.

Female primates usually have only one infant per pregnancy, and they spend a great deal more time caring for their young than most other animals do. Primate brains are on average larger than those of animals in other mammalian orders, such as rodents (mice, rats, beavers, squirrels) and carnivores (wolves, bears, cats, weasels).

Humans are members of the great ape family, which includes orangutans, gorillas, and chimpanzees. Apes are arboreal animals with limber shoulders that allow them to brachiate in trees (swing from one handhold to another), a trait retained by humans, who generally do not live in trees these days. Nevertheless, freeing the arms at the shoulder is handy for all sorts of human activities, from traversing monkey bars on the playground to competing in the Olympic hammer toss to raising a hand to ask a question in class. Apes are distinguished as well by their intelligence and large brain, traits that humans exemplify.

Among the apes, we are most closely related to the chimpanzee, having had a common ancestor between 5 million and 10 million years ago. Between that common ancestor and us over the past 5 million years, many hominids—primates that walk upright—in our lineage evolved. During most of this time, a number of hominid species coexisted. At present, however, we are the only surviving hominid species.

One of our hominid ancestors is probably an Australopithecus species (from the Latin austral, meaning southern, and the Greek pithekos, meaning ape.) Figure 1-10 shows reconstructions of the face and body of one such animal, Australopithecus africanus. Many species of Australopithecus lived, some at the same time, so it is uncertain which is our common ancestor.

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These early hominids were among the first primates to show distinctly human traits, including walking upright and using tools. Scientists have deduced their upright posture from the shape of their back, pelvic, knee, and foot bones and from a set of fossilized footprints that a family of australopiths left behind, walking through freshly fallen volcanic ash some 3.8 million years ago. These footprints feature impressions of a well-developed arch and an unrotated big toe—more like humans' than other apes'. (Nevertheless, australopiths retained the ability to skillfully climb trees.) The bone structure of their hands evinces tool use (Pickering et al., 2011).

One of the first finds to be designated as genus Homo (human) dates to about 2 million years ago. The remains were found by Mary and Louis Leakey in the Olduvai Gorge in Tanzania in 1964. The primates that left these skeletal remains had a strong resemblance to Australopithecus but more closely resembled modern humans in one important respect: they made simple stone tools. The Leakeys named the species Homo habilis (handy human) to signify that its members were toolmakers. To date, the earliest member of the genus Homo, found in Ethiopia, dates to about 2.8 million years ago. The fossils reveal a jaw and teeth much smaller than in any Australopithecus species but characteristic to humans (Villmoare et al., 2015).

The first humans who spread beyond Africa migrated into Europe and Asia. This species was Homo erectus (upright human), so named because of the mistaken notion that its predecessor, H. habilis, had a stooped posture. Homo erectus first shows up in the fossil record about 1.6 million years ago. As shown in Figure 1-11, its brain was bigger than that of any preceding hominid, overlapping in size the measurements of present-day human brains. The tools made by H. erectus were more sophisticated than those made by H. habilis. An especially small subspecies of H. erectus, about 3 feet tall, was found on the Indonesian island of Flores. Named Homo floresiensis, these hominids lived up to about 13,000 years ago (Gordon et al., 2008).

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Modern humans, Homo sapiens sapiens, appeared within about the past 200,000 years. Most anthropologists think that they also migrated from Africa. Until about 30,000 years ago in Europe and 18,000 years ago in Asia, H. sapiens sapiens coexisted and interbred with other H. sapiens species, collectively called archaic humans. In Europe, for example, H. sapiens sapiens lived alongside another subspecies of modern humans, H. neanderthalis, named for the Neander Thal (Valley), Germany, where the first Neanderthal skulls were found. As the first fossil ancestral humans to be discovered, Neanderthals have maintained a preeminent place in the study of modern human ancestors.

Neanderthals had brains as large as or larger than those of modern humans, used similar tools, and wore jewelry and makeup. They lived in family groups similar to modern human ones, made music, cared for their elders, and buried their dead. From these archeological findings, we can infer that Neanderthals probably communicated using language and held religious beliefs.

We do not know how modern humans completely replaced archaic human species, but perhaps they had advantages in toolmaking, language use, or social organization. Contemporary genetic evidence shows that modern European humans who interbred with Neanderthals acquired genes that adapted them to the cold, to novel disease, and possibly to light skin that better absorbs vitamin D (Sankararaman et al., 2014). Reconstructions such as that in Figure 1-12 show how similar to us Neanderthals really were.

One possible human lineage is shown in Figure 1-13. A common ancestor gave rise to the Australopithecus lineage, and one member of this group gave rise to the Homo lineage. The bars in Figure 1-13 are not connected because many more hominid species have been discovered than are shown, and exact direct ancestors are uncertain. The bars overlap because many hominid species coexisted until quite recently. The last of the australopith species disappeared from the fossil record about 1 million years ago.

Scientists who study brain evolution propose that increased brain size and complexity evolved in different species to enable more complex behavior. Having a large brain clearly has been adaptive for humans, but many animals have large brains. Whales’ and elephants’ brains are much larger than ours. Of course, whales and elephants are much larger than humans overall. How is relative brain size measured, and what does brain size signify? Two ways of estimating relative brain size are to compare brain size to body size and to count brain cells.

Estimating Relative Brain–Body Size

Harry Jerison (1973) developed an index that compares the ratio of brain size to body size across species. He calculated that as body size increases, brain size increases at about two-thirds the increase in body weight. Jerison’s underlying assumption was that even if one knew very little about an animal’s behavior, its brain size could provide some clues to its behavioral complexity. The idea is that species exhibiting more complex behaviors must possess more brain than species whose behaviors are fewer and less complex.

Using the ratio of actual brain size to expected size, Jerison developed a quantitative measure, the encephalization quotient (EQ). He defined an average animal (a domestic cat is Jerison’s pick) as having an EQ of 1. The diagonal trend line in Figure 1-14 plots the expected brain–body size ratio of animals with an EQ of 1. Some animals lie below the line: their brain size is smaller than would be expected for an animal of that size. Other animals lie above the line: their brain size is larger than would be expected for an animal of that size.

The lower an animal’s brain falls below the trend line in Figure 1-14, the smaller its EQ. The higher an animal’s brain lies above the trend line, the larger its EQ. Notice that the rat’s brain is a little smaller (lower EQ) and the elephant’s brain a little larger (higher EQ) than the ratio predicts. A modern human, farther above the line than any other animal, has the highest EQ.

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Jerison’s EQ provides a rough estimate of comparative brain size, but body size and brain size can vary independently (Figure 1-15, top). To get around this problem, scientists have devised ways to count the cells in the brain.

Counting Brain Cells

Consider the roundworm Caenorhabditis elegans. C. elegans has 959 cells. Of these, 302 are neurons. In contrast, the blue whale—the largest animal that has ever lived, weighing as much as 200 tons—has a brain weighing 15,000 grams (33 lb). In cell number, 30 percent of C. elegans is nervous system, whereas in terms of body weight, less than 0.01 percent of the blue whale is nervous system.

Based on EQs, we would predict that C. elegans has a more complex behavioral repertoire than a blue whale. But it makes no sense to suggest that a worm’s behavior is more complex than a whale’s. Obviously a whale has a lot more neurons than a worm. Our estimate of brain size and behavioral complexity line up better if we simply compare cell counts.

Karina Fonseca-Azevedo and her colleagues (2012) have devised such a method, using a counting machine. Not only can they estimate the number of cells in a brain or a part of the brain but they also can estimate the brain cells’ packing density. For example, two similar-sized brains could consist of either diffusely distributed large cells or closely packed small cells. It turns out that the packing density of cells differs by a lot among species and brain regions. Rodents have larger, more loosely packed cells, for example, and primates smaller and more densely packed cells.

Packing density is relatively constant in the primate lineage, and so EQ provides a good comparison of their brain sizes (Figure 1-15, bottom). Brain cell counts support the EQs Jerison calculated in the primate lineage: Australopithecus had about 50 billion to 60 billion neurons, Homo habilis about 60 billion, Homo erectus about 75 billion to 90 billion, and modern humans have about 86 billion neurons.

In terms of brain size and cell counts, then, what makes us humans unusual (along with archaic humans such as Neanderthals) are our large brain and many neurons. Although researchers have not made similar neuron counts in all other animal species with brains larger than the humans', if neurons are not densely packed, resembling the somewhat less-dense packing of a rodent, then they may well have far fewer neurons. For example, despite their large brains, dolphins’ 30 billion neurons is similar to the number in chimpanzees, many fewer than in humans, because dolphin neurons are not densely packed. As detailed in Comparative Focus 1-3, The Elephant’s Brain, on page 24, pachyderms have an enormous number of brain cells, but most are in the cerebellum, while the number in the elephant cerebrum is equivalent to that of dolphins and chimpanzees.

The evolution of modern humans—from when humanlike creatures first appeared until humans like us emerged—spans more than 4 million years. As the relative size differences of the hominid skulls pictured in Figure 1-16 illustrate, much of this evolution was associated with increases in brain and body size and by changes in behavior. These changes were probably driven by many influences. Among the wide array of hypotheses that seek to explain why the modern human brain enlarged so much and so rapidly, we examine four ideas.

One hypothesis suggests that numerous drastic climate changes drove adaptation by hominids and led to more complex behavior. Another hypothesis contends that the primate lifestyle favors an increasingly complex nervous system that humans capitalize on. A third links brain growth to brain cooling. And a fourth proposes that a changed rate of maturation favors larger brains. Likely, a combination of all of these factors was influential.

Climate and the Evolving Hominid Brain

Climate changes have driven many physical changes in hominids, ranging from brain changes to the emergence of human culture. Evidence suggests that each new hominid species appeared after climate changes devastated old environments and led to new ones.

About 8 million years ago, a massive tectonic event (deformation of Earth’s crust) produced the Great Rift Valley, which runs through the eastern part of Africa from south to north. The reshaped landmass left a wet jungle climate to the west and a much drier savannah climate to the east. To the west, the apes continued unchanged in their former habitat. But the fossil record shows that in the drier eastern region, apes evolved rapidly into upright hominids in response to the selective environmental pressures that formed their new home.

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Thereafter, the climate in East Africa did not remain static. It underwent a number of alterations (Maslin et al., 2015). The appearance of Homo habilis 3 million years ago and that of Homo erectus 1 million years ago were associated with these climatic alterations. Climatic changes also track the disappearance of other members of the human family. The warming in Europe that ended the ice age as recently as 30,000 years ago contributed to modern humans migrating to the continent and to the Neanderthal and other archaic European and Asiatic human species disappearing.

What makes Homo sapiens the survivor? One suggestion is that we modern humans evolved to adapt to change itself and that this adaptability has allowed us to populate every region on Earth (Antón et al, 2014). The caution is that modern humans have been around only a short time relative to the millions of years that other hominid species survived: our adaptability has yet to be severely tested.

The Primate Lifestyle

British anthropologist Robin Dunbar (1998) argues that a primate’s social group size, a cornerstone of its lifestyle, is correlated with brain size. His conclusion: the average group size of about 150 favored by modern humans explains their large brains. He cites as evidence that 150 is the estimated group size of hunter-gatherer groups and the average group size of many contemporary institutions—a company in the military, for instance—and coincidentally, the number of people that each of us can gossip about.

Consider how group size might affect how primates forage for food. Foraging is important for all animals, but while some foraging activities are simple, others are complex. Eating grass or vegetation is an individual pursuit: an animal need only munch and move on. Vegetation eaters such as gorillas do not have especially large brains relative to their body size. In contrast, apes that eat fruit, such as chimpanzees and humans, have relatively large brains.

Katharine Milton (2003) documented the relation between fruit foraging and larger brains by examining the feeding behavior and brain size of two South American (New World) monkeys of the same body size. As illustrated in Figure 1-17, spider monkeys obtain nearly three-quarters of their nutrients from fruit and have a brain twice as large as that of the howler monkey, which obtains less than half its nutrients from fruit.

What is so special about eating fruit? Good sensory skills such as color vision to see it, good motor skills to reach and manipulate it, good spatial skills to find it, good memory to return to it, and having friends to help find it and ward off competitors are all useful fruit-harvesting skills. Having a parent who can teach fruit-finding skills and being a good learner are also useful. The payoff in eating fruit is its nutritional value for nourishing a large, energy-dependent brain that uses more than 20 percent of the body’s resources. These same skills are useful for obtaining other temporary and perishable types of food, such as those obtained by scavenging, hunting, and gathering.

A neuron’s metabolic (energy) cost is estimated as relatively constant across different species but also high relative to that of other types of body cells. So any adaptive advantage to having more neurons must support that energy cost. Fonseca-Azevedo and Herculano-Houzel (2012) suggest that cooking food is a unique contribution to hominid brain development. Gorillas must spend up to 8 hours of each day foraging for vegetation and eating it. Chimps and early hominids, with a more varied diet, could support more neurons provided that they also spent most of their waking time foraging.

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The use of fire by Homo erectus and later hominids allowed for cooking, which predigests food and thus maximizes caloric gain to the point that much less time need be devoted to foraging. A high degree of male–male, female–female, and female–male cooperation in food gathering and cooking, characteristic of the hominid lifestyle, further supported the evolution of a larger brain.

Changes in Hominid Physiology

Cooking might foster genetic mutations associated with marked size reductions in individual muscle fibers in the face and entire masticatory muscles in hominids (Stedman et al., 2004). The Stedman team speculates that smaller masticatory muscles paved the way for smaller, more delicate bones in the head. Smaller bones in turn allowed for changes in diet and access to more energy-rich food.

Another physiological adaptation may have given a special boost to greater brain size in our human ancestors: changes in the morphology (form) of the skull. Dean Falk (Kunz and Iliadis, 2007) developed the radiator hypothesis from her car mechanic’s remark that to increase the size of a car’s engine, you also have to increase the size of the radiator that cools it. Falk reasoned that if the brain’s radiator, the circulating blood, adapted into a more effective cooling system, brain size could increase.

Brain cooling is so important because the brain’s metabolic activity generates a great deal of heat and is at risk for overheating under conditions of exercise or heat stress. Falk argued that, unlike australopith skulls, Homo skulls contain holes through which cranial blood vessels pass. These holes suggest that, compared to earlier hominids, Homo species had a much more widely dispersed blood flow from the brain, which would have greatly enhanced brain cooling.

Altered Maturation

All animal species’ life history can be divided into stages. Heterochrony (from the Greek meaning different times) is the study of processes that regulate the onset and end of life stages and their developmental speed and duration. Several proposals suggest that altered heterochronicity accounts for the large human brain and other distinctive human features.

In neoteny, juvenile stages of predecessors become adult features of descendants. Neoteny is common in the animal world. Flightless birds are neotenic adult birds, domesticated dogs are neotenic wolves, and sheep are neotenic goats. Many anatomical features link us with the juvenile stages of other primates, including a small face, vaulted cranium, unrotated big toe, upright posture, and primary distribution of hair on the head, armpits, and pubic areas.

Because a human infant’s head is large relative to body size, neoteny has also led to adults with proportionally larger bodies and larger skulls to house larger brains. The shape of a baby chimpanzee’s head is more similar to the shape of an adult human’s head than to an adult chimpanzee’s head (Figure 1-18). Along with this physical morphology, human adults also retain some behaviors of primate infants, including play, exploration, and intense interest in novelty and learning. The brain processes that support learning thus are retained in adulthood (Zollikofer, 2012).

Critics of neoteny offer an alternative view in which all of the stages of development still occur, but their onset and duration change (Workman et al., 2013). Evidence for this idea is that each stage of human development—gestation, infancy, childhood, adulthood—is prolonged relative to such ancestral species as macaques and chimpanzees. Prolonged infancy allows the birth and development of more neurons, more time for their growth to produce a bigger brain. Prolonged childhood enhances learning time; and prolonged adolescence allows for the growth of a bigger body.

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Evolution of the Human Brain and Behavior

Before you continue, check your understanding.

Question 1

Question 2

Question 3

Question 4

Question 5

Answers appear in the Self Test section of the book.