Biology 202
1999 Third Web Reports
On Serendip

Does Size Really Matter?: The Evolution of the Human Brain


Deborah Silvis

Although my previous two papers concerned the interplay between neurobiology and genetics, I have not quite worked the issue out to my satisfaction nor to the depth which I think the topic warrants. Therefore, I will again tackle this complex set of biological questions pertaining to the ways in which our genes shape our brains. My first paper dealt with the nature-nurture debate and its relation to the brain-behavior problem raised in class. Then, in the second paper, I moved on to a narrower issue in neurogenetics; I wrote about Fragile X Syndrome and the ways in which a specific genetic mutation can drastically change behavioral output. I would now like to enlarge the scope of this outlook on genes and the brain to encompass the topic of the evolution of the human brain. Throughout the semester, as we covered sensory input and motor output, a single neuron and complex motor symphonies, car sickness and dreaming, I have left class wondering: how are these behaviors, from the micro-actions of a neuron to the macro-actions of a human being, adaptive? How did large brains and extensive nervous systems come to be selected for? And why have humans, alone, acquired them? Some aspects of these questions seem to reside in the realm of the paleontologists, others, in the realm of the neurogeneticists. They do, however, seem to me to be central to neurobiology. For it is drilled into us that form connotes function, and, perhaps, if we come to understand how and why the human nervous system was formed, we will have a richer understanding of how and why it functions as it does.

The work and thoughts of Richard Dawkins, author of The Selfish Gene, have been useful to me in working out the issues of my previous papers, and I will again employ his theory that people are merely survival machines for the genes they carry. This is, I think, a logical argument with which to begin a discussion of the evolution of the brain, as it reduces evolutionary processes down to the bare bones of living things, that essential material: human genes and the DNA comprising them. This viewpoint excludes the complicated semi-philosophical questions pertaining to consciousness, higher thought, and the Self experienced by human beings via their neural processing; it primarily addresses the usefulness to human beings of the inordinately large organ contained within the skull. Dawkins believes that the brain, like all other parts of the body, exists to enable the perpetuation of the species; that is, we live and breath solely so that our genes may be transferred to other future individuals. He envisions a sort of symbiotic relationship between a genome and its carrier-human wherein the genes give the human form and function, and, in return, the human preserves the genes and reproduces them. All modifications made to the human brain over millions of years of human evolution, then, must have been attributable to the adaptive needs of the genes. In this light, the brain serves no purpose other than to ensure the survival of the individual in order that that individual may reproduce. Indeed, Dawkins writes: The main way in which brains actually contribute to the success of the survival machines is by controlling and coordinating the contractions of muscles (Dawkins, 49).

While this theory satisfactorily answers questions of why the human brain developed at all, it does not offer any explanations about why it attained the uniquely sophisticated form it has today. In his book The Evolution of Consciousness, Robert Ornstein attempts to answer this question with reference to human skills and the evolutionary need to increase brain organization (and, therefore, brain mass) which arose with the development of such skills. He purports that two million years ago, when the brain ballooned radically in size, it was not because of language, tools, or bipedalism (which had occurred two million years earlier)... some of the physical changes necessary to adapt to the upright position lit a kind of fire within the brain, igniting the modern mind (1). Other internet-authors would contend with theories like Ornsteins which make evolution appear goal-directed. John Stevenson, for instance, puts forth that evolution is a reactive process, it does not plan ahead nor does it build in excess of requirements (2). He believes that as humans became more physically complex, their neural processing necessarily became more complex, and that the matrices used for pattern generation in the brain were modified (over millions of years) in order to preprocess this sensory data into a more compact symbolic form (2).

These latter two viewpoints do not seem to me to deviate too far from the point Dawkins makes. For, if homo sapiens would not have been able to make that physiological leap in brain size away from homo erectus, whether they did so in order to compensate for the new challenge of walking upright or using advanced tools, the genes of our species would surely not have evolved to be transferred the way they have. At this point, perhaps it is relevant to discuss the magnitude of this modification in brain size- just how much of a change, had it not occurred, would have left us all Neanderthals today? It is said that h. habiliss brain ranged from 600 to 750 cc when h. erectus (the evolutionary predecessor to h. sapiens) experienced a ballooning in brain mass to 775 to 1255 cc. This increase brought h. erectuss brain within shouting distance of the modern humans 1,000 to 1,400 cc (1). Stevenson reminds the reader, however, that the h. erectus brain still exists under layer upon layer of neocortex and millions upon millions more neurons. Evolution, he says, did not eliminate the early human brain, it simply added to it. Evolution patches over. It does not do housecleaning (2). And it does so through mutation.

I discussed mutation in detail in my second paper in the context of a genetic disorder resulting from a mutation of the X chromosome. It is not necessarily chromosomal mutations, though, which are the vehicle of evolutionary change. The accumulation over time of even single base mutations in the DNA and/or RNA of an organism can lead to large-scale changes such as the one experienced by h. erectus 2 mya. In order for evolution to move forward, organisms must try on for size certain mutations which may or may not be to their evolutionary advantage. Such a mutation, when it moves through a population across generations, will be propagated if it enables the survival of the organism (i.e. the species) or does not increase the death rate of the species. Conversely, the mutation will not be propagated if it disables the survival of the organism or increases the death rate of the species. It is exactly this principle which led to the increase in human brain size.

Imagine, for example, a member of h. erectus who had to daily contend with tests of survival unheard of for h. sapiens sapiens. One such challenge was the need to escape from or avoid the predation of the large cats which roamed the land in h. erectus day. The posture, speed, and general extent of mental ability of our ancestors left them at the mercy of these animals, and early hominid populations declined until some mechanism was found for outwitting or out-surviving the cats. The mechanism came in the form of mutation to the DNA governing neural development. It is important to note that mutations are always applied to already-existing DNA- they do not create something out of nothing. Or in Stevensons words, If a bear becomes distressed in a given environment, it does not sprout wings and fly (2). Rather, changes are made to structures such as the brain or the muscles in the legs and arms, and these changes are tested for survival. If a mutation which increases brain size is favorable or adaptive, it will be allowed, by natural selection, to remain in the gene pool and even to be transferred to the next generation. Mutations, however, are both rare and random, so how did mutation alone cause incremental and large-scale change in brain physiology? Well, it is proposed that the type of mutation which would have led to the dramatic increase in brain size experienced between h. erectus and h. sapiens must have had the ability to encourage other mutations (those which increase brain size) to occur at a more rapid rate. Such an accelerating mutation is said to have been the cause of the increase in human brain size (2).

Thus far, I have addressed why and how the human brain increased in size and complexity. It remains to discuss the benefits that are actually afforded by a large brain and how, if at all, these are adaptive. For it is one thing for mutations over millions of years to have provided human beings with a large brain, and it is quite another to delineate what advances a large brain confers to humans and how we, alone, use these advances to further our species. One related concept which is thrown around by scientists and philosophers is that of the soul. On this subject, Stevenson has this to say, The mark of man is the ability to understand the meaning of behavior and to know right from wrong. All other animals are instinctive, but a human is a human to the degree that it uses its intellect to control its behavior. The totality of this ability, engraved in its neural system, is the soul of man (2). The soul, according to Stevenson, involves the coordination of non-instinctive, intelligent, controlled behavior by the nervous system. And the degree of sophistication of this coordination is to be found exclusively in the human brain.

Another buzzword pertaining to the uniqueness of the large human brain is consciousness. This word has many meanings for people in a variety of disciplines, but I use it here to connote the awareness and understanding (of which human beings are capable) that there is real-ness, a substance to the life we experience, and that this real-ness can be questioned and investigated in a scientific way. Francis Crick has long been on a scientific search for the soul and consciousness, which has led him to conclude that, if these esoteric elements exist, they can be found in the neural networking of every human brain. One source states that scientists have not been able to agree on the evolutionary stage at which consciousness appeared-- yet they postulate that consciousness is limited to living organisms and that it requires a highly developed central nervous system (1). Human beings surely have such a nervous system. However, while modern science has indeed demonstrated the close connection that exists between consciousness and the brain, it has not necessarily proved that consciousness is produced by the brain (1). If scientists are still far from knowing whether consciousness is produced by the brain at all, how can one expect to explain why consciousness is associated only with humans and higher mammals. Can what is referred to as consciousness really result simply from the intricate folding of the human neocortex or the higher density of neurons found in the cortex of the human brain? As I have neither the know-how to provide a scientific answer to this question nor the philosophical bent to provide a philosophical one, I will leave the subject of consciousness.

Let us return briefly to the example of our friendly h. erectus and his skilled nemeses the dangerous cats. When we left him, evolution was designing a mechanism by which he could outsmart/ out-survive these dangerous predators through the process of natural selection. This process, we found, acts at the genetic level through mutations- sometimes chromosomal, but more often single base mutations. The question now becomes: how do the mutations which lead to a larger, more complex nervous system serve h. erectus in survivalist terms? Why is the large brain adaptive, and how does it benefit the carrier? The answer to this is clear-- a larger brain (which in terms of human evolution implies a larger cortical region) allows the human to run faster and in a more upright posture, to plan in advance to avoid attack, and to use tools to ward off the attacker. Of course these abilities hinge as well on other physical adaptations such as longer legs, more nimble fingers, and a straighter spine; however, the physical changes sans the large brain would fail to accomplish all that the brain enables. Thus, the large brain is adaptive because it increases a human beings chance of survival into the reproductive age. And so, once again, the explanation for a neurobiolgical question returns to genes. The transmission of genetic information from generation to generation over evolutionary time has been influenced by the expansion of the human nervous system. Conversely (and mutually), the modifications to the expanding human nervous system have influenced the transmission, as well as the content, of genetic information. Dawkins would be happy with the conclusion that the large human brain is as essential to the survival of human genes as the genes have been to the evolution of the large human brain.

WWW Sources

1) The Dolphin Institute

2)One Life: The Human Brain

1)Wisdom in the Eye of the Frog