Genetics of Original Sin (17 page)

The manner in which this extraordinary structure arose and the nature of its earliest manifestations are also unknown. It is certainly not a human innovation. The first vertebrates already show the beginning of a cortex, and unformed conscious experiences—of pain, pleasure, fear, anger, or desire—most likely developed way down the animal line. As the animal brain increased in volume, so did the cortex in surface area. In mammals and, perhaps, in birds, conscious experiences may be quite rich. As we saw in the preceding chapter, many animals, especially those closest to humans, exhibit behaviors, such as the manufacturing of simple tools or the use of different signals in communication, that imply fairly complex mental underpinnings.

And so, over some six hundred million years, brain size slowly increased to a volume of about 21.4 cubic inches (350 cm
³
), which was the volume of the brain of the last ancestor humans have in common with chimpanzees, whose brains are about that size, whereas the cortex surface area expanded to about 197 square inches (500 cm
²
), which is more than a smooth shape would allow and was achieved by the infoldings responsible for cerebral convolutions.

About six to seven million years ago, that is, after animals had gone through 99 percent of their evolution, something stupendous happened, probably the most extraordinary event in the entire history of life on Earth, certainly the most momentous. In an evolutionary line that detached from the chimpanzee line and ended up leading to humans, brain volume started growing at an increasing pace, to more than three times its volume, while the surface area of the cerebral cortex expanded fourfold, producing even deeper infoldings. This dramatic history is pictured in
figure 10.2
.

By and large, humans are what they are and do what they
do thanks to this epoch-making transformation. Between plucking termites with a denuded branch and splitting the atom, between calling the group together under a tree with a howl and singing
Saint Matthew's Passion
in the Sistine Chapel, the difference is one of brain size, 100 billion neurons instead of 25 billion, with a quadrupling of interneuronal connections, from 250,000 billion to 1 million billion. I neglect here the rather trivial point about absolute brain size being only a coarse measure of mental potential. Body size and internal brain structure are also important. We saw this with the Cro-Magnons, who did better than the Neanderthals, even though they had slightly smaller brains. The fact remains that
absolute size
is
critical. The richness and complexity of the operations a brain can achieve depend on the number of connections among neurons, which, in turn, is limited by the number of neurons. In the present case, it is indisputable that the increase in brain volume and, particularly important, in cortical surface area, went together with greater mental abilities and hence greater accomplishments of all sorts.

Fig. 10.2. Brain size and the duration of existence of various prehuman groups. The height of each horizontal line represents the brain volume of the corresponding species, as deduced from fossil cranial measurements (real measurements in the case of contemporary chimpanzees and
Homo sapiens
). The length of the lines represents the duration over which fossils of the species have so far been found. Graph constructed using data from S. B. Carroll, “Genetics and the Making of
Homo sapiens
,”
Nature
422 (2003): 849–857.

The manner in which this fateful process took place raises fascinating questions. As an introduction to the problem, let us start with some known facts, as presented graphically in
figure 10.2
. In this graph, each horizontal line corresponds to one of the groups mentioned in the preceding chapter. The height of the lines represents the average brain volume of the individuals in the group, as deduced from cranial measurements, and the length of the lines gives the time range over which fossils of the group have been found.

The most striking feature of this graph, in addition to the rapidity of the climb, is its stepwise pace. By all appearances, brain size “jumped” from one level to another, subsequently remaining at the new level, with little change, for very long times, exceeding one million years in some cases. In the meantime, new jumps occurred elsewhere, so that several groups with brains of different sizes often co-existed (at least in time, if not in location). Two and a half million years ago, for example, four different species coexisted—
Paranthropus boisei,
Homo habilis,
Homo ergaster,
and
Homo erectus
—with brain sizes ranging from 30.5 to 61 cubic inches (500–1,000 cm
³
).

This picture is incomplete, depending as it does on fossils found so far. New groups may be discovered in the future
and thus fill some of the gaps in the figure. But the main trend is unmistakable. It goes by way of apparently stable levels of considerable duration, separated by periods of rapid increase of which no trace has yet been found. This, incidentally, is a common finding in evolution. Links are rare or missing, probably because their existence is fleeting, in comparison with the durability of the groups that the links connect.

With evidence lacking, it remains for us to imagine the missing connections by educated guesswork. Two extreme possibilities are illustrated in
figure 10.3
. Both models assume, as seems likely, that the process of brain expansion was unidirectional and that regressions from an upper to a lower level did not occur.

In model A of
figure 10.3
, the jumps are pictured as taking place as late as the data permit, that is, at the onset of each new level. In model B, I have assumed that the steps represent horizontal branches that extend laterally from a single, uninterrupted, ascending line. To satisfy this requirement, I had to assume that some branches started earlier than the age of the oldest fossils found, which is not implausible in view of the scarcity of fossils and the role of chance in their discovery. The two models have in common rapid jumps from one level to another, followed by prolonged stabilization of the new level. The difference lies in the timing of the jumps: mostly at some stage within the lower plateau in model A; before the start of the plateaus in model B.

As far as I know, model A represents the generally held view. Its shape corresponds to that most often given in treatises for the human “tree” or “bush” (shown here lying down). Model B, first proposed in 2005 in my book
Singularities,
has not, to my knowledge, been considered before. I tend to favor it because it assumes a single genetic propensity toward bigger brains, with a number of stops on the way, whereas, in model A, a new drive toward brain expansion is initiated at each
level. Model B is thus the more economical of the two in terms of the number of required genetic changes. This could be a point in its favor. Indeed, an impressive aspect of hominization is the extraordinarily small number of individuals involved at any stage, as opposed to the importance of the changes that characterize the process.

Fig. 10.3. Two models of human brain expansion. These two models are drawn using data from
fig. 10.2
. (A) Model based on the hypo thesis that jumps from one level to the other occurred as late as the data permit. (B) Model based on the hypothesis that all levels extend laterally from a single ascending curve. Note that stone tools started to be manufactured by creatures with brains half the size of the modern human brain. Model B adapted from Christian de Duve,
Singularities
(New York: Cambridge University Press, 2005), 222.

We saw in
chapter 9
that the group, comprising mitochondrial Eve and Y Adam, ancestral to the entire, present-day human population may have included no more than ten thousand individuals. Studies of the Neanderthal genome have suggested an even smaller number—on the order of three thousand—for the common ancestral population from which Neanderthals and Cro-Magnons diverged more than half a million years ago. That the appropriate genetic change could have taken place at each stage in such small populations is flabbergasting. It indicates that the changes must have been either exceedingly probable or very few in number. The latter condition is better fulfilled by model B of
figure 10.3
, but the argument is not decisive. A single tendency toward expansion that was repeatedly stifled and reawakened could account for model A.

We must leave it to the experts to decide which of the two models—or any model intermediate between the two—is more likely to be the correct one. What I wish to examine here is the manner in which the “jumps” from one level to another took place, whether in one or the other model. In my graphs, rather than connecting the levels by abrupt steps, in staircase fashion, I have rounded the angles to give S-shaped, or sigmoid, curves, which seem to me more realistic.

Mathematicians have long been familiar with this type of curve, known as the logistic curve. It reflects two opposed phenomena: one, an exponential increase such that every increment enhances the rate of the following one; and two, a
braking effect that increasingly slows down the process as it progresses, until it stops at a limit value that generates an unchanging plateau. What natural processes could account for these two phenomena in the case of brain expansion?
Figure 10.4
illustrates a possible answer to this question.

Fig. 10.4.
Logistic curve, as applied to the expansion of the human
brain in the course of evolution. The curve combines an exponential increase with a braking effect. The exponential part is attributed to neuronal multiplication and the braking effect to anatomical constraints, either the size of the fetal cranium or that of the female pelvis, or both.

For biologists, cell division is the iconic example of an exponential process. One cell divides into two, which divide into
four, which divide into eight, sixteen, and so on. Cell division happens also to be the indispensable mechanism behind brain expansion. One is thus naturally led to the hypothesis that the exponential rise from a lower to a higher level reflects cell division. This assumption is particularly attractive, as it ascribes all the successive rises to a single phenomenon.

The simplest factor that could account for the braking effect responsible for causing the plateau is some kind of anatomical constraint. Here, several factors may come into play. First, there is cranial capacity. As the brain enlarges, it is increasingly hindered by its bony box. This leaves two possibilities. The cranium does not yield, so that only individuals in which neuronal division is genetically programmed to stop at a certain stage are allowed to survive and produce similarly programmed progeny, accounting for the plateau. Or the cranium yields to the outward pressure exerted by an expanding brain, a distinct possibility as the cranium is made of a number of plates that, in the fetus and even in the newborn, are linked by membranous connections that could indeed stretch, as required. If this happens, the drive toward a bigger brain is allowed to proceed further, until a new obstacle is encountered, generating the next plateau. A second factor that could increasingly limit brain size, as it expands, is the dimension of the female pelvis, which may oppose the birth of young with bigger heads. A possible way of overcoming this kind of obstacle would be by way of a change in the brain's developmental program, so that the brain achieves a lesser degree of maturity in the womb and completes a greater part of its maturation after birth.