The Making of the Mind: The Neuroscience of Human Nature (3 page)

Chimpanzees and their close sister species, bonobos, are the species alive today with the closest genetic relationship to modern human beings. An interest in comparing the mentality of these apes with humans is therefore natural and of great interest in cognitive social neuroscience. Even so, if the calculation of shared ancestry is correct, human beings and chimpanzees are the unique products of six million years of independent evolution. With
Ardipithecus ramidus
, paleontologists get a glimpse of a species removed from the branching point by only two million years or so, and it is apparent that the hominid branch had already diverged considerably from the branch that led to modern chimpanzees. It is thus important to keep in mind the limitations of comparing modern chimpanzees with modern humans. Despite their striking genetic similarity, each is the product of 6 million years of potential evolutionary change.

Out of Africa

A second lesson from the human genome project is that all modern human beings belong to a single species with a single origin. Scientists have proposed two hypotheses regarding human origins and the spread of
Homo sapiens
across the face of the earth. One is the out-of-Africa scenario. Paleontologists discovered a cluster of skulls of early
Homo sapiens
from an earlier time period than those found outside of Africa.
¹⁴
The oldest known fossils of our species came from Ethiopia and have been dated to 130,000 years ago. Other early fossils of modern humans were found in a cave near Nazareth in Israel; these are estimated to be ninety thousand to one hundred thousand years old. All fossils of
Homo sapiens
found outside Africa and the Middle East so far have been identified as more recent in origin—no more than about forty thousand
years old. One interpretation of these age differences is that modern humans originated in Africa and later migrated to the Middle East and eventually to Asia and Europe.

Genetic diversity provides a clock by which biologists can calibrate the antiquity of human populations. The more time to accumulate mutations, the more differences will be observed among individuals of the population. According to the out-of-Africa hypothesis, the primordial group of
Homo sapiens
grew in population and eventually began to migrate to other locations. The paths of migration can be modeled by comparing the genetic diversity of groups living in various regions of the world today. Put differently, the variations that are found in the DNA of populations around the world today can be traced back to the stock of possible genetic diversity established first in ancient Africa. For example, as human beings migrated to the Middle East from Africa, they carried in their cells the nuclear DNA of their ancestors with the subset of mutations that had accumulated at that point in time. The more diversity found in a population, the longer it has been there. Conversely, the less diversity found, the more recently the population had migrated to the region. By developing models of the patterns of migrations from the genetic evidence, scientists conclude that groups of people migrated from Africa first to other parts of the Middle East. Later, they migrated to Asia and northern and eastern Europe. The Americas were populated much more recently by a migration across the Bering Strait from eastern Asia to the region of present-day Alaska.
¹⁵

The alternative hypothesis argues that there may once have been a migration from Africa, but it was not
Homo sapiens
who migrated but rather an earlier ancestor, possibly
Homo erectus.
From ancestral populations of an earlier ancestor, such as
Homo erectus
, modern human beings evolved independently of one another in multiple regions across the planet. That is to say, modern human beings in Asia evolved from the Asian variant of
Homo erectus
whereas modern Europeans evolved from the European variant. This alternative view is known then as the multiregional hypothesis. The two hypotheses have long competed to explain the skeletal fossils and cultural artifacts discovered in different regions of the world. However, human-genome research offered fresh evidence favoring the out-of-Africa over the multiregional view.

One approach involves analyzing mitochondrial DNA. This is of interest because mitochondrial DNA is passed on from one generation to the next solely from the egg cells of mothers. The father's sperm cells do have a few mitochondria, but to the great fortune of geneticists, all of the sperm mitochondria are discarded in the fertilization process. This means that geneticists can trace the variability in mitochondrial DNA of today's women back to their mothers, which in turn can be traced back to their grandmothers, then their great-grandmothers, then their great-great grandmothers, and so on. In theory, one could reconstruct the family tree backward in time to identify a female who was the source of all mitochondrial DNA existing today, sometimes referred to as mitochondrial Eve. Mitochondrial DNA offered a picture of the family tree undistorted by the blending of maternal and paternal genes that occurs in the DNA of the cell nucleus. Furthermore, mitochondrial DNA mutates at a much faster rate than nuclear DNA, so it can provide a clock capable of tracking comparatively short passages of evolutionary time. The family tree constructed from these data had two major branches—one contained all mitochondrial DNA variations today found only in Africa; the second branch contained a mixture of all types. This outcome was thus consistent with the out-of-Africa theory of human origins.
¹⁶

The variability in mitochondrial DNA is largest among African populations, as would be expected if they have been around the longest and accumulated the most mutations over time. The study of nuclear DNA confirmed this conclusion by showing that of the total genetic variations on a particular chromosome found in populations around the world, most all of them are represented in the African populations. Relatively few were found in Europeans, Asians, and Americans. In sum, Peter Raven and George Johnson concluded, “Taken together with fossils of early H. sapiens from Africa and Israel, these results strongly support an out-of-Africa model of human origins.”
¹⁷

The view that the human mind originated with an ensemble of five distinctive parts would have no credence at all if the multiregional view of human origins were correct. It is inconceivable that such an ensemble emerged not just once but independently multiple times in different populations in different geographical locations. Thus, the ensemble hypothesis is worth entertaining only because modern human beings arose and only once.

THE HUMAN BRAIN

The genome shows our common origin as human beings and so does the anatomy and physiology of our brain. Our conceptions of the mind, and ultimately society, can be fruitfully viewed through the lens of neuroscience. That is not to say that scientists have solved the problem of how the mind and brain are linked. Our perceptions, thoughts, feelings, memories, and dreams—the makings of mind—are qualitatively different from the neural and other bodily cells that make up the organ of the brain. Understanding this duality of mind and brain remains a profound philosophical and scientific challenge that we are not yet near resolving. Still, at the beginnings of the twenty-first century, scientists take for granted that the neurobiology of the brain informs our descriptions of the mind and that, in some fashion, the duality must be bridged.

Cognition is assumed to be a function of the brain, just as breathing is a function of the lungs or blood circulation is a function of the heart. The human brain may well be the most complex structure in the known universe. Consider just a few of the brain's properties to understand this point. A neuron includes dendrites for receiving signals from other neurons, a cell body, and an axon for transmitting a signal to other neurons via a synaptic connection. The dendrites of a single neuron may receive thousands or even tens of thousands of synaptic connections from other neurons. With about a trillion (10
¹²
) neurons in the brain, there are at least 1,000 trillion (10
¹⁵
) synaptic connections among these neurons.
¹⁸

Structurally, the neurons of the brain are organized in terms of the hindbrain, the midbrain, and the forebrain, each developing from distinct portions of the neural tube that forms after conception. The forebrain eventually divides into two parts during maturation. The first part is known as the telencephalon, with its massive left and right cerebral hemispheres and associated structures that make up much of the human brain. The cerebral hemispheres are visible to the eye on inspection. The associated structures include the basal ganglia that underlie the cerebral cortices and the structures of what is known as the limbic system. Lying beneath the telencephalon is another “hidden” interior region of the forebrain known as the diencephalon, which includes the
thalamus and its smaller partner the hypothalamus. All of this rests on top of the brain stem. The brain stem itself is composed of a midbrain (mesencephalon) at the top and, below that, a set of structures known as the hindbrain (the metencephalon and myelencephalon).
¹⁹

The human brain is notable for its large size, especially when compared with the size of the body. The typical endocranial volume of a human being is 1,355 cubic centimeters, and this is about three times the estimated brain size of the hominid species known as
Australopithecus africanus
(457 cm
³
), which paleontologists date as having lived 2.6 to 3.0 million years ago.
²⁰
Although the skull size of a modern chimpanzee looks from the exterior to be about comparable in size to that of a modern human, its cranial capacity is much smaller. As Richard Klein explained in
The Human Career
, the typical chimpanzee brain size is about 400 cm
³
, or about the same size as that of the ancient genus
Australopithecus
. Within the advent of the genus
Homo
, the fossil record supports the conclusion of an increase in brain size from very early to more recent species, but the “long-term increase in endocranial volume in
Homo
was not due simply to increased body size,” as “it was accompanied by a less dramatic, but still conspicuous decrease in the relative size of the cheek teeth (premolars and molars).”
²¹

The remarkable increase in cranial capacity documented in the field of human paleontology is arresting. The large brain of modern
Homo sapiens
—and that includes all of its diverse populations—is fundamental to understanding who we are as a species. The massive expansion of the brain in the modern human being undoubtedly was a necessary condition for the ensemble of capabilities posited as unique to the mind within us. If not one but five significant cognitive systems distinguish the human brain, then only a massive increase in size could accommodate all five parts of the ensemble.

The size of the brain relative to body size—what is known as the encephalization quotient—provides a way to take into account that large animals are likely to have large brains and small animals, small brains. Whales, dolphins, and elephants, for example, all have larger brains than do human beings because they have larger heads and bodies in general. A plot of the mean brain and body weights of a large number of primate species on a log-log scale enables one to look at this relationship over a diverse range of body
sizes. The plot turns out to be a straight line that shows a very strong positive correlation (+0.97), almost a perfect relationship of 1.00. That is to say, the plot shows this without including
Homo sapiens.
Tellingly, the data point for human beings lies well above those of the chimpanzee and the gorilla. It is in fact about three times higher than it should be according to the straight line of regression found among all other primates.
²²

In addition to the relative size of the human brain, changes also apparently occurred in its organization across the course of hominid evolution. That is to say, the relative size of specific brain regions have increased or decreased as opposed to an overall expansion in size. Comparative neuroscience has revealed several differences among living species.
²³
For example, primates dedicate more cerebral cortex to visual functions in comparison to rodents, which show relatively larger areas dedicated to sensing odors rather than seeing. Primates in general have relatively large prefrontal regions that are anterior to the motor areas of the frontal lobe. Among the functions of these prefrontal areas are social behaviors that are known to be complex in great apes and in human beings. The left hemisphere of most human beings is dominant in controlling motor behavior such that we are more often than not right-handed due to contralateral control, with the left brain controlling the right side of the body. Chimpanzees show some similar asymmetries in the frontal and temporal lobes, but they lack the strong degree of asymmetry in handedness.

Yet trying to account for differences in cognitive capabilities between chimpanzees and human beings in terms of brain reorganization—as opposed to sheer brain expansion—has not been straightforward. For example, as will be discussed in
chapter 4
, language in the human brain depends on a neocortical region in the frontal lobe of the dominant left hemisphere. It is known as Broca's area, and the region is known to be relatively larger than its counterpart in the right hemisphere. If this left-right asymmetry were absent in chimpanzees, then one could reasonably hypothesize that a reorganization of the left frontal lobe played a role in the origin of language in the human brain. Yet neuroimaging studies have documented a similar left-right asymmetry in chimpanzees, bonobos, and gorillas.
²⁴
If there are differences between the language areas of the human brain and their homologous counterparts in the chimpanzee, then it must be in the microanatomy of local neural circuitry or
the connections between cortical regions rather than at a more obvious level. The ensemble hypothesis stresses that the reorganization from nonhuman to human of any one cognitive system—such as working memory—might be relatively modest. It is the sum of all five parts and their nonlinear interactions that result in a qualitatively different kind of mind.