6 Evolution: Adaptation through Natural Selection

The current chapter reviews the evolutionary processes and hereditary mechanisms which resulted in the structure of the human body. We will see how our brain and nervous system transmit and interpret sensory information relaying it to parts of the body capable of responding. All of the inferences and conclusions drawn regarding how our internal processes enable us to adapt to our environment are based upon behavioral observations. At times you may wonder why so much detail is provided regarding human anatomy. You were expecting a course in psychology, not biology. I have done my best to emphasize those parts of the nervous system crucial to our survival and achievements as a species. We will start with a discussion of how human anatomy evolved.

Darwin’s Theory of Natural Selection

Between December of 1831 and October of 1836, Charles Darwin took one of the most important geographic and intellectual journeys in recorded history. On this voyage he collected fossils and observed many forms of wildlife. Upon returning to England and examining his evidence, Darwin detected patterns in variations of animals seemingly related to environmental conditions. For example, he observed that the size and form of a species of birds’ beaks appeared related to the types of available foods (see Figure 2.1). Eventually he published The Origin of Species (1859), arguably the most influential book in the history of biological (if not all) science.

Figure 2.1  Darwin’s finches

Darwin was familiar with the selective breeding practices of farmers designed to result in improved stocks. He reasoned that a similar selective process could occur as the result of natural causes (i.e., natural selection). That is, if environmental factors resulted in some animals having an adaptive advantage relative to others, those animals would be more likely to survive long enough to reproduce. With respect to the birds, those possessing the type of beak best suited to eating the available food type would be able to consume more food and be more likely to survive. Similarly, if some animals possessed a characteristic resulting in their being more attractive to potential mates, they would be more likely to breed (Figure 2.2).

Figure 2.2  Sexual selection

The most controversial aspects of Darwin’s theory of natural selection relate to human evolution. The implication is that our physical structure, and therefore our behavioral potential, is the result of a natural process related to arbitrary environmental factors. Darwin did not at first relate natural selection to human evolution but eventually did so in his later publication, The Descent of Man (1871). It was not until a century later that a substantial number of fossils suggesting very gradual changes in human structure were discovered. This provided physical evidence for human evolution through natural selection.

You might be wondering if the human being is still evolving. In fact, there are several examples of relatively recent adaptive biological modifications apparently resulting from environmental changes. For example, tens of thousands of years ago, humans started moving to higher altitudes. DNA samples from cultures with a history of living in the mountains included genes impacting upon the amount of hemoglobin in the blood (NY Times, May 30, 2013). Individuals with these genes would be better able to cope with the low oxygen levels characteristic of high altitudes. For example, Tibetans tend to have broader arteries and capillaries than nearby Chinese populations living in low-lying areas. These broader vessels permit greater blood flow and a corresponding increase in delivery of oxygen to the cells of the body.

Heredity and Genetics

Darwin did not possess our current knowledge of the mechanisms involved in heredity, the transfer of characteristics from parent to child. He thought that half the characteristics of each parent were combined and transmitted to the next generation. This was a plausible hypothesis given the obvious similarities and differences between parents and offspring. However, if nothing else was involved, natural selection could not occur. Darwin had specified a process for selection but not for the potential variation in genes necessary for change to occur over time. The possibility of genetic mutation was yet to be discovered.

Although occurring at about the time that Darwin published The Origin of the Species (1859), Gregor Mendel’s genetic research with peas did not attain influence until early in the twentieth century. Mendel’s findings enabled Theodor Boveri (1904) to demonstrate the role of chromosomes (tiny threads contained within a cell’s nucleus) in heredity. Cells are the basic building blocks for plants and animals. The human body is comprised of trillions of cells. In 1910, Thomas Hunt Morgan, studying heredity in the fruit fly, demonstrated that genes were located on chromosomes in the cell nucleus. Genes are the basic units of heredity and ordinarily occupy constant positions on chromosomes. Genes are comprised of DNA (deoxyribonucleic acid), which includes all the information required for cell replication. Nearly every cell in the body has the same DNA. In 2003, the Human Genome Project reported that the entire human genome (i.e., all the genetic information characteristic of our species) consisted of approximately 20,000 genes. We now know that most genetic variation results from mutation, a permanent chemical change in the composition of a gene’s DNA. Mutations are rare, and ordinarily provide no adaptive advantage. Thus, it is not surprising that evolution through natural selection is very slow, taking millions of years in humans. First, an adaptive mutation must occur in a member of a species which must be fortunate enough to survive and successfully mate. Then, multiple generations would be required for this difference to show up in a significant number of other surviving individuals. It should be emphasized that the adaptive value of a mutation depends upon the environment in which it occurs (Figure 2.3). For example, one third of the indigenous inhabitants of Sub-Saharan Africa carry the gene for sickle cell disease. This gene increases immunity to malaria, making it adaptive in Africa while being maladaptive elsewhere.

Figure 2.3  Explanation of evolution

Genotypes and Phenotypes

The inherited instructions contained within an individual’s genes are referred to as its genotype. Your DNA includes all the information required to create an individual with your exact physical characteristics, susceptibility to specific diseases, and even some of your temperament. At fertilization, humans inherit 46 chromosomes, 23 apiece from our biological mothers and fathers. The 23rd male chromosome determines the sex of the child. This is because females carry two of the same sex chromosome (called X) whereas males carry one X and one Y (male) chromosome (see Figure 2.4). The chromosome pairs and DNA sequences are structurally similar. Traits are passed on from generation to generation through the DNA contained in genes on these chromosomes.

Figure 2.4  The Y-chromosome

Mendel discovered that when crossing plants with different characteristics (e.g., white or purple) the result was not a blend. Rather, one of the initial characteristics would occur (e.g., the next generation would be purple). Mendel referred to this as a dominant trait and to the other as a recessive trait. For example, in humans brown hair is dominant over red hair. This means that if a child inherits a brown gene from one parent and red gene from the other parent, her/his hair will be brown. However, since that child carries both gene colors, it is possible that offspring will inherit red hair. Traits can skip generations.

The observable physical and behavioral characteristics of a species are referred to as its phenotype. We saw in Chapter 1 that complex human behavior is the result of an interaction between hereditary and environmental variables. It is also true that environmental factors interact with genetic factors with regard to physical traits. For example, an individual’s height and weight can be influenced by nutrition, infection, and other variables.

Human Evolution

It is estimated that the universe is 13.8 billion years old and that the earth is 4.54 billion years old (Dalrymple, 2001). The human being is the most complicated animal on earth. Physical evidence suggests life in the form of simple cells first appeared 3.6 billion years ago. Half a billion years ago, animal life emerged from the sea. Over time, closer approximations to a modern human being appeared. In terms of biology’s family tree, humans are considered primates, a branch including apes, monkeys, and lemurs for which we have discovered fossils dating back approximately 55 million years. The human being’s closest existing primate “relatives”, chimpanzees, bonobos, and gorillas, diverged approximately four to six million years ago. Fossilized evidence suggests the first bipedal animals (i.e., standing on two legs) appeared approximately seven million years ago with the earliest documented evidence for humans (i.e., members of the biological genus “Homo”) dating back approximately 2.3 million years. It is at this time that we observe the first indication of use of stone tools by Homo hablis. This and other transformational events are depicted in Figure 2.5.  Note that the brains of early humans were comparable in size to those of chimpanzees and gorillas. This size more than doubled over the course of 2 million years until the appearance of Homo sapiens (modern humans) approximately 200,000 years ago.

Figure 2.5  Human timeline

None of the transformational discoveries listed in Figure 2.5 was inevitable. Evolutionary biologist Jared Diamond (2005) wrote a wonderful Pulitzer prize-winning book tracing the history of the human being leading up to and after the last ice age, approximately 13,000 years ago. He describes how features of the climate and environment impacted on the course of development of humans on the different continents and why some cultures eventually became dominant over others. For most of our time on earth, variations of the human species survived as nomadic small bands of hunter/gatherers in Africa. Diamond (2005, 36-37) describes fossilized evidence that humans migrated to Southeast Asia approximately 1 million years ago, with Homo sapiens reaching Europe ½-million years ago. Existing evidence suggests that modern humans reached the Americas between 14,000 and 35,000 years ago. Diamond (2005, 87) provides an overview of the causal factors impacting upon the human condition. Geographic and climatic conditions affecting the availability of wild plants and animals determined the possibility of development of agriculture and animal domestication. Localized food production enabled establishment of more permanent residences and larger communities. Food storage permitted surpluses, freeing people from survival demands on a day-to-day basis. This resulted in development of new “occupations” and technologies, dramatically altering the human condition.

My students readily admit that where they were born was an extremely important yet arbitrary event impacting upon the course of their life. If they were born and lived in the rain forest they would not be prepared to attend college. Alternately, if at their present age they were dropped in the rain forest without modern technologies or assistance from natives, they would probably not survive.

The terms “human evolution”, “ human condition”, and “human potential”, must be carefully analyzed to be most meaningful. Our potential as individuals and as a species starts with the physical structure (i.e., anatomy) that evolved through the process of natural selection. The Mostly Nature section of this book describes how our anatomy permits us to sense specific sources of physical stimulation originating from outside (Chapter 3) and within (Chapter 4) our bodies. The current chapter describes how the anatomy of our brain and nervous system enables us to process and coordinate this physical stimulation and transmit information to those parts of our body capable of responding. Our anatomy places limits on the types of physical information we are able to sense and the types of responses we are able to make. These limitations meant that we were not always capable of surviving and reproducing under all the geographic and climatic conditions that existed on earth. Our physical structure, however, included the potential to touch, manipulate, and change the planet. The development of tools and technologies magnified this capability, eventually resulting in transformative changes in our environmental conditions. We refer to the interface between our physical structure and environment as the human condition. Over the millennia, humans acquired the capability of surviving anywhere on this planet, traveling to the moon, and exploring the universe. The fascinating story of the past, present, and future potential of our species starts with an understanding of human genetic potential

Human Genetic Potential

When I was a child I faithfully watched the Superman TV show. I would play with my friends, pretending to be Superman, and try to leap off a step while wearing my cape (actually a towel). Despite doing my best to imitate my hero I never took off. I wasn’t a bird or Superman. Flying was beyond my genetic potential. We saw that the human brain significantly increased in size over the course of evolution. A convenient way of considering what constitutes the genetic potential for human behavior is to examine the human motor homunculus (little person). This is a representation of the amount of “brain space” in the cortex allotted to different parts of our body for acting upon our environment (see Figure 2.6).  Consistent with the distinctive human DNA described in Figure 1.2, a disproportionate amount of the cortex is allocated to organs related to speech (lips, jaw, tongue, and voice box) and to the hands (particularly the thumb). The ability to manipulate our facial muscles, tongue, and larynx provides the potential to emit an enormous variety of vocalizations. Initial attempts to teach chimps to speak (Hayes and Hayes, 1952) were unsuccessful, primarily due to limitations in the use of these body parts. Our ability to manipulate our fingers and thumbs to form the precision grip enables us to grasp and hold objects of different sizes and shapes.

Figure 2.6  The motor homunculus

Millions of years of evolution resulted in an animal with the genetic potential to learn complex behaviors, speak and create tools. This potential took a very long time to emerge. However, once realized, the combination of imagination, communication, and manipulation resulted in humans dominating and changing our planet. Theodosius Dobzhansky (1960), a noted Russian genetic biologist, stated

“Mutation, sexual recombination and natural selection led to the emergence of Homo sapiens. The creatures that preceded him had already developed the rudiments of tool-using, tool making and cultural transmission. But the next evolutionary step was so great as to constitute a difference in kind from those before it. There now appeared an organism whose mastery of technology and of symbolic communication enabled it to create a supraorganic culture . Other organisms adapt to their environments by changing their genes in accordance with the demands of the surroundings. Man and man alone can also adapt by changing his environments to fit his genes. His genes enable him to invent new tools, to alter his opinions, his aims and his conduct, to acquire new knowledge and new wisdom.” The “supraorganic culture” Dobzhansky describes, results from the multiplicative effects of human’s shared imaginings, communications and manipulations. Throughout this book we will seek to understand our potential to transform the world, the human condition and our individual selves. We start with the human genotype.

Attributions

Figure 2.1 “Darwin’s finches” by Avrand6 is licensed under CC BY 4.0

Figure 2.2 “Sexual selection” by Sizzlinggreg is licensed under CC BY-SA 4.0

Figure 2.3 “Explanation of evolution” is licensed under CC BY-SA 3.0

Figure 2.4 “Y-chromosome” is licensed under CC BY-SA 4.0

Figure 2.5 “Human timeline” by Drbogdan is licensed under CC BY-SA 4.0

Figure 2.6 “Motor homonculus” by Albert Kok is in the Public Domain