The Making of the Mind: The Neuroscience of Human Nature (2 page)

When considered in isolation, it should not be surprising to see substantial evidence of the precursors of these five distinctive parts in species with which we share a genetic history. Because the modern chimpanzee is the only living species with a relatively high degree of similarity in its genome to
Homo sapiens
, neuroscientists are especially interested in comparisons between the two species. As will be seen later in the chapter, such comparisons are of limited value because human beings are not descended from modern chimpanzees.
Homo erectus
is hypothesized to either be a direct ancestor of modern human beings or a close side branch of the human lineage; it is in any case extinct and known only from the evidence of paleontology and archeology.
Homo neanderthalis
is another extinct hominid, but one thought to have coexisted with modern human beings recently enough to have left behind DNA samples for geneticists to study. Neither DNA nor fossilized skulls tell us how the brain was organized, however. It is impossible to compare, then, the brain of an immediate ancestor or close hominid relative with the brain of a modern human being.

So, for better or worse, comparative neuroscience does what it can and contrasts the brain and cognitive abilities of chimpanzees with those of human beings. If the ensemble hypothesis is correct, then it should be expected that, say, working memory ought to show continuity between a chimpanzee and a human being. Although the executive functions of human working memory are more advanced than those of a chimpanzee, the differences ought to be a matter of degree rather than of kind. Conceivably, the summation of all five parts yielded a human mind that
is
distinct in kind from a chimpanzee's mind, but that is debatable. According to the ensemble hypothesis, however, the summation of five parts is only the beginning of what sets the mind within us apart. It is the interaction of two or more parts that yields a qualitatively different kind of mind.

The ‘human revolution” came as a result of a nonlinear discontinuity with ancestral species, according to the ensemble hypothesis. Advanced working
memory is only one of the five parts in the ensemble. The other parts each added their own distinctive features, and critically, they interacted with each to yield a new
kind
of mind in
Homo sapiens
. Thus, the human mind is more than the summation of an advanced working memory plus an advanced social intelligence. The interaction of these two parts alters mental functioning beyond their linear combination. As language, the interpreter, and mental time travel add to the ensemble, the human mind diverges sharply and discontinuously from all nonhuman forms, despite the fact that each individual part shares similarities with that of the nonhuman counterpart. Even though each of the brain reorganizations underlying the components of the ensemble could have been relatively minor compared with an immediate ancestor, the whole of their ultimate effect would be unpredictable from looking at each of the parts. Thus, when considered as an ensemble, the human mind inhabits a mental world on earth with one and only one member.

What exactly is meant by
nonlinear discontinuity
? The critical concept at the heart of the ensemble hypothesis is a statistical interaction of two or more independent factors that have an effect on brain or cognitive complexity. Advanced working memory increases cognitive complexity. So, too, does advanced social intelligence. Both of these operating at once in the modern human brain could increase cognitive complexity in an additive way—the net result may be simply a linear combination of each factor taken separately. Yet, according to the ensemble hypothesis, there is a two-way interaction of factors that yields a nonlinear combination of their independent effects. The power of an advanced social intelligence to boost cognitive complexity is enhanced by the presence of an advanced working memory. Similarly, the power of symbolic thought and language to boost cognitive complexity is impressive on its own merits. Even so, when combined with an advanced social intelligence at the same time, the result is a nonlinear discontinuity from nonhuman minds that possess less powerful social intelligence and modest to minimal symbolic thought and language capabilities. Analogous cases can be made from pairwise interactions of all five parts of the ensemble hypothesized here.

A simple visual metaphor for grasping the idea behind the ensemble hypothesis is to think of five jigsaw puzzle pieces. When looking at any one piece, one can readily see the similarity between it and another piece, a close
relative that is slightly or even moderately changed from it. Yet, when the five pieces are assembled, they together portray a picture that is qualitatively different from the appearance of any single piece. In other words, while a family resemblance can be detected with ease at the level of an individual piece, a qualitatively new entity emerges from the ensemble of parts.

An alternative metaphor is to think of the ensemble hypothesis in terms of emergent properties in the physical science of chemistry. Neither the properties of hydrogen (H) nor the properties of oxygen (O) reveal the emergent properties of water when the constituents are appropriately bonded as H
₂
O. The wetness experienced in the warm waters of a bath or the cool refreshment experienced in drinking water to quench a thirst emerges from the ensemble. Neither experience with hydrogen nor experience with oxygen prepares one for the experience of the warm wetness and cool refreshment of H
₂
O. The whole is unrecognizable from the parts.

In the remainder of the chapter, foundational facts about the genome and brain of
Homo sapiens
are presented as background for what follows in the book. The thesis that five distinctive features of human cognition emerged in the modern human brain implies that the advent of our species was a highly unlikely event that occurred once and only once. Archeological evidence on this point has long been controversial, with some scholars arguing for a multiregional model of human origins in which the transition to modernity occurred independently in different regions of the world. Recent evidence from human genetics appears to have settled the question in favor of a single origin. It is also important to understand the rationale of comparative neuroscience as an investigative tool into human origins. Comparisons are often made between human brains and behaviors and those of other primates, particularly the common chimpanzee and bonobo. Macaque monkeys, and even rats, commonly provide animal models useful in the neuroscience of human cognition. The basis for, and limitations of, these comparisons can be found in similarities among species with respect to genomes and brain architectures.

INSIGHTS FROM THE GENOME

Recent advances in human genetics have provided scientists with a new tool for understanding the origins of modern humans. To review some of the basics, with the exception of the egg and sperm cells, the nucleus of every cell in the human body contains twenty-three pairs of chromosomes. The final pair is significant because it distinguishes males and females. This twenty-third pair consists of an X-chromosome and a Y-chromosome in males, and two X-chromosomes in females. The egg and sperm cells possess twenty-three single chromosomes rather than pairs that then combine during sexual intercourse and fertilization. Egg cells contain an X-chromosome whereas sperm cells contain either an X- or a Y-chromosome. Whether the offspring is a genetic male or female, then, depends on which kind of sperm cell fertilizes the egg, creating either an XX or XY in the twenty-third pair.

The information needed to make proteins and enzymes in the cells of the body is coded by a molecule of DNA within each chromosome. A remarkable amount of DNA is packed into each chromosome. The DNA extracted from all forty-six chromosomes and laid end to end would extend six feet.
⁹
The building blocks of DNA are called nucleotides, and there are four types: adenine, thymine, cytosine, and guanine. What geneticists have learned from studying the DNA sequences is that all humans, everywhere on the planet, have exactly the same set of genes. But some of these genes come in slightly different versions. If you compare two individuals, taken at random, you will typically find about two differences in their nucleotides for every one thousand examined.
¹⁰
It is from these differences in the DNA sequences of our genes that the differences emerge between the two individuals. An observed characteristic or trait of an individual is referred to as a
phenotype
, whereas a pattern of nucleotides in the genes comprise a
genotype
. Differences in the genotype, then, provide a path for differences in the phenotype. Our skin color, eye color, hair color, head shape, and body dimensions in general are examples of such phenotypic differences. Susceptibility to diseases is another example. One person might be more susceptible to a particular disease than is another person. A disease may be virtually preordained by the genotype, such as Huntington's disease, or the genotype may only increase the chances of contracting a disease,
such as a heart attack or stroke. Lastly, some of our behavioral or cognitive characteristics, such as temperament and intelligence, can also be influenced by the genotype. For these in particular, more than one gene is responsible, and the influence of the gene is always probabilistic rather than certain.

Comparisons with Chimpanzees

Phylogeny refers to the evolutionary history of the different kinds of animals or plants. The human genome has been compared with the genome of our closest living primate relatives as part of the study of phylogeny. Strikingly, the genome of the common chimpanzee (
Pan troglodytes
) has been found to be remarkably close to that of
Homo sapiens.
In terms of single-nucleotide substitutions, the two genomes differ by only 1.23 percent; insertions and deletions (3 percent) and duplications (2.7 percent) reveal somewhat greater differences.
¹¹
As a simple summary of the similarity of the two species, human beings share roughly 98 percent of our DNA with chimpanzees.

Although the genes of humans and chimpanzees largely overlap, there are numerous obvious differences in body structure in the two species. Compared with the chimpanzee, human beings are habitually bipedal, larger brained, taller, and less muscled, for instance. The cognitive differences between human beings and chimpanzees are even more profound, as will be argued throughout the remaining chapters of the book. Notably distinctive are the human capacities for planning, thinking strategically, and resisting the impulses of the moment; for imitating and reading the minds of others with respect to their intentions and beliefs; for producing and comprehending language; for interiorizing language and linking it to causal interpretations of perceived events and the actions of others; and for mentally time traveling back into the past or forward into the future to experience events outside the world of immediate perception.

Despite the major phenotypic differences between the two species, evolutionary biologists interpret the strong genotypic similarity as evidence that
Homo sapiens
and
Pan troglodytes
once shared a common ancestor in the tree of life. Approximately five to seven million years ago, it is theorized, the branch of life including the African great apes split into two different
smaller branches.
¹²
One of these eventually led to the development of hominids, including modern human beings. The other branch led to a different set of species that eventually culminated in modern chimpanzees. According to evolutionary theory, it is a fallacy to say that human beings evolved from chimpanzees. On the contrary, each species alive today resulted from a different line of descent. The two species are related only because they presumably shared a common ancestor alive approximately six million years ago and thus inherited many common genes.

The postulated branching of the primate family tree is calculated to have occurred approximately six million years ago by using the small 2 percent difference in genomes to estimate the time elapsed since the divergence from a common ancestor. The calculation starts with a determination of the rate at which mutations occur in the genome, and this rate is assumed to be constant over time. Then, by knowing the current number of DNA differences between the two species, biologists calculate an estimate of the time that has passed since the split from a common ancestor.

The first lesson taken here from the recent discoveries regarding the human genome is that human beings and chimpanzees have followed separate paths for millions of years. It is thus not surprising that the two species differ in body and mind, given the accumulation of six million years of evolutionary changes in each line of descent to the present. One branch of the tree led to
Homo sapiens
and an independent branch led to
Pan troglodytes
, if the inferences from DNA analyses and models of evolutionary biology are correct. The separation and individuality, rather than commonality, of the two species has been underscored by the discovery of fossils from the earliest known hominid species,
Ardipithecus ramidus.

Dated at 4.4 million years old—relatively close to the hypothesized branching point—the skeletal remains of
Ardipithecus ramidus
provide the best approximation scientists have to the bodily structure of the last common ancestor.
¹³
Ardipithecus
may have been an ancestor that led to the genus
Homo
or instead may have been part of a side branch that eventually went extinct. In either case, the fossils offer evidence of the skeletal structure of a hominid that is more ancient than any previously discovered species, and they revealed surprising differences from today's apes. Unlike modern chimpanzees, bonobos,
and gorillas,
Ardipithecus ramidus
appeared to be bipedal with a pelvis and foot capable of upright walking. Also, the large projecting canine teeth of male chimpanzees were absent in the earliest known hominid species; instead, the males possessed small canines similar to the females in
Ardipithecus ramidus
. In fact, the overall body size of males was only slightly larger than that of females in early hominids, in contrast to the sexual dimorphism seen in chimpanzees, with males markedly larger than females.