What happened to the Neanderthals?
Although the Cro-Magnons represent the main heroes of our story, they are not the only ones. They were preceded by an earlier branch that sprouted before them from their common African trunk and also developed considerably, spreading to Europe and other parts of the world. Fossil remains and other traces of this line were unearthed in the early part of the nineteenth century in Belgium and other sites in Europe, but their significance was not recognized until the accidental discovery, in 1856, of some old bones by workers who were cutting a quarry out of the banks of a small river called Neander, not far from Düsseldorf, in the German Rhineland. It is intriguing that the name of this river actually means “new man” in Greek and was given to it in honor of a local musical celebrity, Joachim Neumann (German for “new man”). So much for those who believe in predestination. First mistaken for bear bones, the remains fortunately came into the hands of a local teacher, who was inspired to show them to an expert anatomist, who identified them as human, though significantly modified. All kinds of weird hypotheses were put forward to explain these modifications, until the proposal, inspired by Darwin’s book, which was published just three years after the find, was made, and, eventually, accepted, that the observed features may be those of a long-extinct human ancestor. Today, the Neander Valley has become a major landmark, under its German name of Neanderthal, which is itself perpetuated in the name of its most illustrious denizen, Homo neanderthalensis.
Neanderthals probably moved out of Africa before their younger kin, settling first in many parts of Europe and the Middle East. There, they were joined later by the Cro-Magnons, with whom they coexisted for some time. Much has been written concerning the similarities and differences between the two. In particular, there has been a great movement toward rehabilitating the Neanderthals from the reputation of brutishness that has long been given to them, to the point that it has become politically incorrect to use the term in a pejorative sense.
It is generally agreed that the two groups were sufficiently close to be ranked under the same label of Homo sapiens, with the Neanderthals named Homo sapiens neanderthalensis, and the Cro-Magnons Homo sapiens sapiens. The artifacts and living conditions of the two groups seem comparable in many respects, except culturally. There is virtually no evidence of Neanderthal art, jewelry, or rituals, at least comparable to those of the Cro-Magnons. They did sometimes bury their dead, but with little ornament. Their brains, nevertheless, seem to have been somewhat larger than those of the Cro-Magnons. But this is not necessarily a sign of mental superiority. Brain size is only a rough index of intelligence. The development of certain specific areas, in the neocortex, for example, is a decisive factor. The shape of the skull in Neanderthals and Cro-Magnons is very different, indicating a different organization of certain centers.
A major question is whether the Neanderthals were able to speak. Casts of their lower cranium suggest that their larynx may have been too high for true speech, which could possibly explain their relatively crude cultural development. On the other hand, the shape of a hyoid bone—a horseshoe-shaped bone that surmounts the larynx—found in Israel suggests otherwise. The matter remains subject to debate.
Another unanswered question refers to the relationships between Neanderthals and Cro-Magnons. Did they live alongside largely ignoring each other, as do many kindred species today? Or, did they fight one another, perhaps even to the point that the more clever Cro-Magnons exterminated the other species? Or, on the contrary, did they fraternize and even interbreed?
We may soon have answers to some of these questions, thanks to the wonders of modern technology. Samples of mitochondrial DNA and, more recently, nuclear DNA have been retrieved from Neanderthal bones, and their sequencing has begun. Present results already allow an estimate of the time when Neanderthals and Cro-Magnons separated from the last ancestor they have in common: a minimum of half a million years, perhaps as much as eight hundred thousand years, which is at least three hundred thousand years earlier than the time when mitochondrial Eve and Y Adam (see above) started the Cro-Magnon line and when some key human traits, perhaps including speech, were acquired. These traits may thus have been lacking in the Neanderthals, unless they were acquired by convergent evolution. The DNA results also make it unlikely that much genomic interchange occurred be tween the two. They probably did not interbreed, or if they did, their offspring was infertile, as is the case for many hybrids, such as the mule.*
Modern humans remain the only survivors from the adventure out of which they were born
The Neanderthals disappeared about thirty thousand years ago, for reasons not yet clarified. They may have fallen victim to environmental hardships they were unable to survive. Or they may have been driven out of existence by the more successful Cro-Magnons by a mechanism variously surmised to have been extermination, forced migration to climatically unfavorable regions, deprivation of vital resources, infection with deadly diseases, or hybridization leading to sterility. Whatever happened left the Cro-Magnons as sole inheritors of this long, extraordinary saga that started somewhere in Africa some seven million years ago, when a primate line first diverged from the line leading to modern chimpanzees. After the disappearance of the Neanderthals, the Cro-Magnons went on developing over many millennia, producing remarkable cultural achievements, such as the cave paintings of Lascaux and Altamira, but continuing to rely on hunting and gathering as their main means of survival. Then, some ten thousand years ago, the advantages of planting crops, rather than gathering them, and of raising animals, rather than hunting them, started to be appreciated by some people in the Near East, perhaps also elsewhere, leading to the first permanent settlements. The rest, as they say, is history.
*As this book was going to press, it was announced that certain features found in fragments of Neanderthal DNA are shared with that of modern Asian-European humans, but not of Africans, suggesting possible late interbreeding (Science 328 [2010]: 680–684).
10
Making the Human Brain
O f all the wonders of life on Earth, the human brain is no doubt the most wondrous. Forming this wonder, there is a special kind of cell, the neuron. Like all other cells of the organism, neurons have a body, with a nucleus and all the characteristic structures and organelles of animal cells. But, in addition, they are specialized in the reception, processing, and emission of signals. They do this by means of two kinds of extensions, the longer axons, doing signal emission, and the shorter arborescent dendrites, doing signal reception.
The brain is constructed with neurons
In their most primitive form, neurons probably connected a sensitive spot with some motor or secretory system. As a simple example, imagine a light-sensitive spot connected to a contractile fiber: spark a flash of light, and contraction occurs. Single cells may already show reactions of this sort; but the value of neurons is that they can relay messages over distance, thanks to their extensions. Those distances may be considerable, up to a dozen feet in the nerves of large animals.
But simple relays were only a starting point. What has given neurons their astonishing power is their ability to join by connections, or synapses, that allow coordination among the connected cells. Thus, in primitive jellyfish, which are probably among the first animals to have possessed such cells, the neurons surrounding the hole that permits exchanges between the alimentary cavity and the outside world are linked into a ring, so that the muscles that command the opening and closing of the hole operate in a coordinated manner.
As animals evolved toward greater complexity, neurons increased in number and importance. It is remarkable that, of the 959 cells that compose the body of the small nematode (roundworm) Caenorhabditis elegans, one of the most intensely studied animals, almost one-third are neurons.
Two other neuronal phenomena took place in the course of evolution. First, neuronal cell bodies started to congregate in special centers, called ganglia, while the neuronal fibers joined into bundles, called nerv
es. Furthermore, as animals began to acquire a head (cephalization), ganglia tended to become grouped in this part of the body, inaugurating the beginning of a brain, which, as we know, went on growing in size and complexity throughout animal evolution.
The cerebral cortex is the mysterious site of conscience
A crucial event in this history was the development of the cerebral cortex, a thin, sheetlike structure, about eight hundredths of an inch thick, that, in humans and in higher animals, envelops the entire brain. Typically consisting of six superimposed layers of neurons interlinked vertically and horizontally by a thick jungle of intermingled connections (fig. 10.1), the cortex is the seat of consciousness. Its boundary with the remainder of the brain serves as a screen between the unconscious and the conscious. On the inner side of this boundary, the complex assemblage of nerve centers and fibers that constitute the body of the brain and the rest of the central nervous system receives and processes innumerable incoming signals and sends off innumerable outgoing orders. Most of this activity takes place without our being aware of it, regulating heartbeat, blood pressure, digestion, eye movement, balance, and a host of other physiological phenomena. Some brain circuits cross the boundary and pass through the cerebral cortex. Those that do so elicit awareness, feelings, emotions, impressions, thoughts, dreams, imaginings, reasonings, decisions, the whole gamut of mental phenomena that fills our heads.
Fig. 10.1. “Forest” of neurons in the cerebral cortex. This remarkable computer-generated reconstruction illustrates the dense network of neuronal arborescences in a thin transverse section through the human cerebral cortex. This structure, repeated side by side millions of times, forms what is by far the most complex assemblage in the entire known universe, the generator of consciousness. Courtesy of Javier DeFelipe.
How this mind-generating machinery functions is utter mystery. Neurobiologists have accumulated a considerable amount of evidence purportedly showing that the neurons do it all—“You are just a pack of neurons,” as Crick put it—with, as conclusion, that consciousness is but an epiphenomenon, some sort of aura that emanates from neuronal activity but has no control over this activity, contrary to our feeling of being in charge, which, it is claimed, is a mere illusion, a trick played on us by natural selection. The fact remains that the nature of consciousness has so far eluded objective characterization—it is a purely subjective phenomenon—and the mechanism whereby it is generated by the cortical networks is not understood. Looking at the tangle illustrated in figure 10.1 and reflecting that millions of them are joined side by side in the human cortex, with, in a single human brain, more interneuronal connections than there are microchips in all the computers of the world put together, one cannot help suspecting that this amazing assemblage is the seat of phenomena of a different order from all those described and explained by conventional physics. Perhaps it will take brains of even greater complexity to comprehend the secret of the human brain.
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.
It took six hundred million years for the animal brain to reach, in chimpanzees, a volume of 21.4 cubic inches
And so, over some six hundred million years, brain size slowly increased to a volume of about 21.4 cubic inches (350 cm3), 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 cm2), which is more than a smooth shape would allow and was achieved by the infoldings responsible for cerebral convolutions.
In the human line, it took two to three million years for the brain volume to expand from 21.4 to 82.4 cubic inches
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 expansion of the human brain went through a number of successive plateaus
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 cm3).
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.
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