The irony of the theory of evolution is that evolution’s spectacular success in creating complex biological systems was historically the reason that this theory was so difficult to accept. It is difficult to imagine how a random sequence of miscues, of genetic mutations, could be the source of such complexity. Charles Darwin acknowledged the difficulty of imagining such a process operating over the vastness of geological time, but also clearly believed that reason demands its acceptance.
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; and if further, the eye does vary ever so slightly; and if any of the variations be inherited; and if any of the variations be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.
from The Origin of Species (1859)
The evolutionary process did in fact produce the brain, and studying how this happened is an important part of understanding the brain’s form and function. Before considering the evolution of the brain in particular, however, we must first understand some of the mechanisms of evolution itself.
A Sampling of Evolutionary Mechanisms
The process of evolution by natural selection is deceptively simple at first glance: genes that give their carrier, the individual animal, a better chance of surviving and reproducing are more likely to be delivered to subsequent generations. But like most natural processes, this process is layered with subtlety.
Genes can be turned on and off. Genes that are normally off have little impact on the day-to-day survivability of an organism. This absence of selective pressure might allow these genes to exist within a population for many generations without making a single contribution (but also without costing the animal much to maintain). It has been suggested that such genes gather in well-adapted animals until times of stress, at which time the diversity within the gene pool becomes more fully expressed in a variety of individual characteristics, the composite of which is called the phenotype of the individual. We often think of natural selection as picking the “better” phenotype — survival of the fittest. But a particular phenotype may not have an inherent goodness or badness outside of the context of its current environment. Imagine two phenotypes: in one, the release of stress hormones increases metabolic activity, and in the other, the release of stress hormones reduces metabolic activity. If the cause of the stress is the arrival of a new active predator, the high-metabolism hyper animal with its additional energy reserves may have a distinct advantage. If the stress is due to a shortage of food, then the low-metabolism lethargic animal may survive. Being the fittest sometimes means being in the right place at the right time with the right set of genes turned on.
Now suppose that predator arrivals and famines becomes much more frequent, so that both occur before an individual has a chance to reproduce and create self-sufficient offspring. Which phenotype would be preferred? Neither phenotype may be fit. A powerful mechanism that may help prevent such situations is the duplication of DNA which sometimes results in the duplication of genes within the genome. Two coexisting genes might perform identical tasks to begin with, but the presence of both means that one gene can mutate and lose its original function without having disastrous effects on the organism because there is still a gene remaining to perform that function. The genes may diverge so that one performs the original function and the second performs a completely new function, or they may split the original crude function into two more-refined functions. Returning to the example above, a complex of hunger and stress hormones might activate the lethargic gene whereas a complex of fear and stress hormones might activate the hyper gene. An animal containing such genes would have the advantage of appropriate responses to two distinct environmental conditions.
The Impact of Genes on Development
The most powerful influence of genes is within the process of embryological development. Embryological development begins with a single cell, and through repeated cell divisions and specializations, creates an entire organism. A miniscule change in this process can result in dramatic changes in the organism. A minor set of mutations might result in the organism being twice as large, which might solve the predator problem but worsen the food problem. The animal’s niche would be entirely transformed. Alterations of the developmental process can result in rapid evolutionary changes. Of course, the changes cannot be so rapid in a single individual that there is no longer anyone to mate with, but we are using “rapid” in the sense of geological time, when anything less than 100,000 years is considered virtually instantaneous.
The few mechanisms we have reviewed here only hint at the breadth of mechanisms available to evolution, which makes the field so fascinating. The study of evolution is central to our understanding of biology. Researchers in this area must be extremely creative to provide logical bridges that span gaps between facts where the fossil record is incomplete. They must be patient in acknowledging that some unknowns will remain so forever. They must also, of course, be scientific, which means that each new fact must be consistent with existing theories, or the theories themselves must be scrapped. Research on the evolution of animal nervous systems has provided us with insight into how the brains of mammals, primates and humans came into being. Some of the mechanisms described above were likely to have been instrumental in this evolution. The story is continuously unfolding as more data is collected and new theories are tested, but already a remarkable account exists of where we came from, and thus, who we are.
The Emergence of Neocortex in Mammals
Most elementary school students learn that there are five characteristics of mammals: they have hair, they breathe air, they are homeothermic, they bear live young, and they nurse their young. In fact, there are many characteristics that make mammals unique. The most remarkable characteristic, from our undoubtedly biased point of view, is the possession of neocortex, also called isocortex, which covers the brain. Neocortical tissue is derived from the telencephalon and is organized into six layers with neuronal processes and cell bodies distributed within the layers in a stereotypical fashion. More primitive cortices, including the olfactory cortex and hippocampus in mammals, are more loosely structured with a three-layer appearance.
Neocortex and Metabolic Tradeoffs
The neocortex varies greatly from one mammalian species to the next. John Allman estimates in his book Evolving Brains that, even after correcting for body size, the neocortex varies in area by a factor of 125 from the least cortical to the most cortical mammals. He also identifies a number of metabolic characteristics that are tightly linked to the evolution of the neocortex. Much like homeothermy, neocortex can confer upon an animal a significant advantage, but it comes at a significant metabolic cost. Consequently, neocortex coevolves in many species with adaptations for more efficiently obtaining food. This tradeoff is evident today in that animals that digest easily obtained foods, such as leaves, have smaller brains but longer guts. Animals that use their larger brains to obtain more nutritious but harder to find foods, have shorter guts.
Neocortex and Predation
Predatory behavior in particular is associated with larger brains. Though predation is difficult, success results in a highly nutritious, easily digested meal. Catching prey requires highly tuned sensory and motor systems. The neocortex is made up of both primary sensory areas that receive input from the thalamus, and primary motor areas. In mammals with more neocortex, the primary areas are connected to many other areas that further process sensory input and produce motor plans. Much like the duplication of genes, it has been suggested that cortical areas have been duplicated as well. Like the duplicated gene mechanism, such a change would have the advantage that one area could evolve rapidly while the other provides the original function, or they could coevolve, each providing important and possibly refined aspects of the original function. Such duplicated areas most likely arise due to mutations in genes that control processes during development. In a chapter in Brain and Mind – Evolutionary Perspectives, Leah Krubitzer emphasizes this point when she writes, “We consider that the evolution of the neocortex is the evolution of cortical development.”
The Emergence of Prefrontal Cortex in Primates
One mammalian line thought to have been initiated by small predators living in the terminal branches of trees is that of the primates. This lifestyle resulted in a number of adaptations that were critical to the eventual evolution of humans. Like most mammalian predators, early primates had forward facing eyes which allowed for focused pursuit with good depth perception in a large part of the visual field. The arboreal life required grasping hands, and prey were probably captured by visually-guided hand and mouth movements. Consequently, the visual system of primates dominates the sensory cortices, and its structure, even within the brainstem, is unique to primates.
Another important difference between the neocortex of the primate and that of other mammals is a large prefrontal cortex which is the rostral or front part of the cortex not devoted to motor function. These areas seem to be instrumental in higher cognitive functions such as planning and social behavior. Much of what is known about the physiology of the prefrontal areas is based on research on non-human primates. It is believed, however, that many of the general principles that have been discovered apply to humans as well.
The Implications of Brain Expansion in Human Evolution
Humans last shared an ancestor with existing ape species, the chimpanzees and gorillas, about 5-7 million years ago. Since that time, the brain volume has increased dramatically from about 400 cubic centimeters (cc) to about 1350 cc today. Not surprisingly, the higher cognitive areas, those of the prefrontal cortex that are characteristic of primates, are especially pronounced in the human brain and represent the majority of the expansion that has taken place. Whether this expansion led to refinement of existing capabilities or the emergence of completely new capabilities is a source of continuing debate.
Shared Characteristics with Apes
A challenging aspect of evolutionary science, and particularly of human evolutionary science, is determining which characteristics are shared between species, though possibly in highly adapted forms, and which characteristics are truly unique. In the case of humans, such determinations are likely to always be controversial because of their perceived religious and philosophical implications. Questions as to whether apes have culture or primitive language capabilities continue to be researched and debated (see June 25th, 1999 issue of Science). Other questions, such as whether apes and other animals experience consciousness, are even more difficult to resolve. In future articles in this Evolutionary Series, we will revisit some of these issues and explore how and why these perhaps uniquely human characteristics may have evolved.
Driving Cortical Expansion
A more straightforward question is, why did the brain expand so rapidly in our ancestors? This expansion was likely driven by increased social behavior. The cooperative benefits afforded by communication could have produced easier access to food since collective information about accessing food could be shared across individuals and across time. More accessible food meant that larger brains could be supported metabolically.
Plasticity Enables Expansion
The primate brain may have been developmentally well-suited to making use of increases in size of cortical areas and of newly emergent areas. Evidence suggests that cortical areas can adopt new functions based on the input that they receive. Plasticity in the cortex can result in an area of cortex changing its function depending on the frequency and type of tasks that an animal is required to perform in its daily life. Improved performance on a task is thought to be in part a direct result of increased cortical area devoted to the task. Therefore, increased cortical area presumably provided more resources to be devoted to a wider variety of complex functions.
A fact of ecology is that members of a species are always, at some level, each other’s most direct competitors because they share the exact same habitats and food sources. The up side of this harsh reality is that this competition drives speciation, the development of new species, and is responsible for the diversity of life that we enjoy on our planet. Subspecies are rewarded for diverging from one another in that they are then less likely to compete for the same resources. In the past 6 million years, at least 11 hominid species have become extinct, and only one persists. It is possible that the speciation process was too slow, and that the competition between and within hominid species was great. Our large brains, with their social and communicative capabilities, may have arisen as mechanisms with which to compete against ourselves and our nearest relatives. We may have exceeded a threshold where the most pressing selective pressure was from other clans of hominids, and a battle of wits was fought generation after generation, favoring the most agile and powerful cortices. Such speculations are easy to make, but difficult to prove. We will have a better idea of what transpired as more data from the fossil record of early humans is uncovered.
Stephen J. Gould, who studies evolution and writes popular accounts of it, believes that if we started over at the beginning when life first emerged on Earth and let another 4.5 billion years pass, life on Earth would be extremely different from what it is today. We cannot of course start over, but this theory has strong implications of what we might expect to find on other Earth-like planets should they exist.
Daniel Dennett, a philosopher who wrote Darwin’s Dangerous Idea, disagrees with Gould’s theory. He sees the phenomenon of convergence, where different lineages create similar adaptations, such as the eye in mammals and cephalopods, as indicative that there are constraints within the natural world that will limit the number of possible outcomes under selective pressure.
The eye provides a good example of convergence, but it may not be the best example of adaptation in general. The eye is an adaptation for processing a physical phenomenon, light, which has properties that have remained constant throughout the course of time. Most adaptations, however, develop in response to pressures imposed by other life forms. Some of the most elaborate adaptations arise from two species heavily interacting with one another, either in an “arms race” or a symbiotic relationship. Our unicellular distant ancestors are thought to be extreme examples of the latter, when a symbiotic relationship between these one-cell nucleated organisms and certain bacteria became so intertwined that the bacteria became part of the cells and now act as organelles (tiny organs) within the cells. In this radical series of adaptations, two species are thought to have merged into one.
Such tightly interwoven interactions can be highly unstable and are the basis for the butterfly effect, which is more formally described within chaos theory. The gist of the theory is that highly interacting systems are extremely sensitive to initial conditions, so a butterfly flapping its wings in Sydney can cause hurricanes in Miami. If evolution is so sensitive to initial conditions, then Gould may be right, and recreating the initial life conditions of Earth would not lead to intelligent beings 4.5 billion years later. Such logic paints a grim picture for the search for extra-terrestrial intelligence. On the other hand, if Dennett’s theory is correct and natural selection eventually leads to a species that is self-competing in terms of cognitive adeptness, then intelligent life may more or less be an inevitability.
We will continue to learn more about these issues as scientists study intelligence in humans and other species and compare this intelligence with that of their nearest relatives. From such data, scientists can postulate what forces drove descendents of the common ancestor to develop different levels of intelligence within the different species. Abstracting this process may lead to new insight into the probable ubiquity of intelligence in the universe.
Darwin’s theory of natural selection changed biology forever. His insights led to the development of a theoretical aspect of a field that had previously been for the most part descriptive. The theories that have since developed help us to comprehend how biology works. They do not simply explain how systems evolved, but clarify and constrain plausible explanations of their function. As for the brain, evolutionary theories of its origins are likely to be essential to its eventual understanding.