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The Evolution Of Human Behavior Essay, Research Paper

Charles Darwin’s theory of natural selection was widely rejected when it was first introduced in 1859 in On the Origin of Species. The Church was reluctant to accept it since it contradicted its teachings. Even the scientific community hesitated to embrace such a revolutionary, yet logical, new idea. Since then, much evidence has been uncovered that lends support to natural selection. The evidence has come primarily in the form of fossils, which lends support to the tenets of natural selection. These tenets put forth by Darwin are as follows:

-Characters vary within populations.

-Populations increase at an exponential rate.

-Exponential increase is prevented by struggle for existence; not all individuals can survive and


-During the struggle for existence, those individuals with the most favorable traits for their

respective environment are more likely to survive and reproduce. This differential survival

and reproduction constitutes natural selection.

-These favorable characters are passed on to the next generation, and increase in frequency over


-(Thompson 10)

The fossil record shows a gradual change in morphology compatible with the theory of natural selection. In other words, the evidence supports the idea that the physical form of organisms gradually change over time in response to selection pressures from its environment. However, very little evidence has been found showing that the behaviors of organisms also evolve in the same Darwinian manner. Because behaviors do not fossilize, most of the evidence supporting the evolution of animal behavior has come about through deductive reasoning based on fossils as well as current primate behaviors. For example, brain size, which can be deduced from skull size, is used to determine certain behaviors. Large brains correlate to large body sizes. This means that larger brained animals will have larger foraging areas, which entails a tendency to exploit new environments. Large brains also must do more to sustain themselves, meaning that they have functions which are more numerous and complex. Given these correlations between brain size and behavior, it might surprise one to learn that larger brain size has actually been linked to decreased species survival. Australopithecus afarensis, an early primate closely related to man, had a slightly larger brain than chimpanzees (approximately 500 cc) and lived about a million years. Homo erectus, which had a brain of about 1000 cc lived 750,000 years; Neanderthal had a brain of about 1450 cc and lived 300,000 years. Homo sapiens have brain size of approximately 1350 (which is larger than the Neanderthal when put into proportion with body size), and have only existed for a relatively short period of time with an uncertain future (Thiessen 250).

In addition to studying brain size, efforts to explain the behavior of humans by studying the behavior of man’s closest living relative also exist. These include the great apes, in particular, the chimpanzee. People studying the great apes work under the premise that “in related species, the more widespread a trait is, the phyligenetically older it is likely to be” (Peters 192). Therefore, comparing behaviors of humans and chimpanzees give scientists a way of tracing the origin of a particular behavior. Once the origin has been found, they can then go forward in time to plot the evolution of the particular trait. Scientists also heavily rely on deductive reasoning to help explain why certain behaviors came about. This involves speculation as to why behaviors benefit certain organisms. These efforts to link behavior to evolutionary biology has spawned a new field of study called sociobiology.

Prior to Darwinism and sociobiology, people explained human behavior according to two opposing views. The first view, supported by Marx and Rousseau, was referred to as the optimistic view of human nature. It proposes that humans are cooperative by nature, with selfishness arising as a result of private property, class, or reaction to selfishness of others. The opposite view, advocated by people like Thomas Hobbes and Emile Durkheim is called the pessimistic view of human nature. It suggests that humans are selfish and antisocial by nature. People become civilized only through exposure to culture; without social and governmental controls, cooperation would not occur. Natural selection seems to favor the pessimistic view of human nature as the more correct view. All living things, including humans, are born selfish. After that, they remain selfish throughout their entire lives. Organisms behave the way they do because they want to better their chances of propagating their genes. To understand why insist so vigorously on passing on their genes, one must look back to the time on earth leading to the creation of life. The following description, explains how selfish came about. Richard Dawkins’ account of the sequence of events in The Selfish Gene is perhaps the most concise and easy to understand of all acounts given thus far.

Before the existence of life, the earth primarily consisted of randomly scattered atoms and small molecules. Darwin’s theory shows how these strewn out, “unordered atoms could group themselves into ever more complex patterns until they ended up manufacturing people” (Dawkins 13). The laws of physics and chemistry say that a group of atoms which aggregate into a stable structure will tend to remain stable, unless some outside force destabilizes the molecular interactions. In other words, atoms which happen to come together in a particularly stable structure will persist longer than atoms which do not fall into any type of stable configuration. Experiments have shown that mixtures similar to those abundant in pre-life earth (these mixtures consisted mostly of molecules of water, carbon dioxide, methane, ammonia, and other organic and inorganic compounds) produce amino acid residues when exposed to ultra violet light and electric sparks. Since amino acids still exist today, one may assume that these molecules posses high stability, especially when compared to other molecules in pre-life earth. Because amino acids form proteins, their presence generally foretells the coming of life.

Once synthesized, these proto-amino acids proceeded to drift through the earth’s primordial ooze. In the process, their chemical properties were altered by other stray atoms and molecules also in the ooze. One of these chemical changes eventually ended up producing a proto-amino acid with the unique ability to replicate itself. The mechanism for replication probably involved some sort of affinity between molecules with opposite properties. This is analogous to the positive and negative poles of magnets which attract each other. In the case of the molecules in the ooze, substances protruding from their surface had a specific affinity for atoms containing the opposite substance. A new molecule was constructed once the original molecule attracted enough atoms in this manner. Thus, one molecule served as a template for the building of a complementary molecule. After joining, these two molecules then separated to once again expose their surface substances, which then went on to attract other atoms building even more molecules. This replication also exist in DNA. The DNA molecule consists of two complementary strands running anti-parallel to each other. When these strands separate from each other, their nitrogenous bases become exposed. The two separated strands then each base pair with complementary strands resulting in two new DNA molecules. Here, the surface substances are the nitrogenous bases adenine, guanine, cytosine, and thymine. Adenine base pairs with thymine, while cytosine pairs with guanine.

Like the DNA molecules of today, the replicating molecules of pre-life earth occasionally made mistakes during replication. This resulted in mutations which often altered the characteristics of the molecule. Some of these changes hindered to the molecule’s ability to further replicate, while other changes actually enhanced these abilities. Those molecules which received detrimental mutations were unable to further replicate, and ceased to exist. On the other hand, those molecules which gained enhanced replicating abilities became more abundant in the primordial ooze. Those molecules that were not lucky enough to receive the enhanced replicating abilities were driven into non-existence by the enhanced replicators who were perhaps able to out compete them for the necessary building block molecules in the ooze. In addition to stability and better affinity for building block molecules, a replicator’s ability to duplicate further increased with a rise in fecundity. This refers to the rate of reproduction. Faster replicating molecules then eventually grow to outnumber slower replicators in the primordial ooze.. To summarize, a molecule’s replicating ability increases due to mutations which increase stability, affinity for building block molecules, and fecundity. As mutations cause molecules to become more effective replicators (hence, better collectors of essential substances), the building block molecules in the ooze become more scarce. Thus, the introduction of a mutation which gives molecules a tendency to aggregate with other molecules might dramatically change the characteristics of the primordial ooze. Because aggregating replicating molecules secure more building block molecules for itself when in a group as compared to by itself, these molecules tend to become more abundant in the ooze. These aggregations then became more complex over time, eventually aggregating into bacteria, tissues, organs, and ultimately, entire organisms. Thus, organisms merely act as receptacles for selfish genes that function for the sole purpose of replicating themselves.

Many argue that humans cannot be considered as mere selfish gene receptacles since they posses consciousness. Because consciousness spawns emotion and morality, humans have the tendency to look out for their fellow human. However, it may also be argued that consciousness gene selfishness caused the evolution of consciousness. Neurobiologist Nicholas Humphreys has proposed an outside-inside hypothesis for the evolution of consciousness which shows that “natural selection tends to maximize mental representation of motor actions that increase reproductive performance” (Thiessen 252). According to Humphreys, the evolution of consciousness occurred in three steps. In the first step, simple reflex arcs were adopted by organisms in order to better sense their surroundings. These simple reflex arcs involved nothing more than responses directly caused by some peripheral stimulus. An example of this in humans is the leg jerk reaction caused by tapping a particular spot on the knee. In the second step, a central ganglion (a collection of neural cell bodies) was placed somewhere within the simple reflex arc. This allows the organism to store incoming neural information as a neural charge. In the third step, consciousness finally emerged as information within the reflex arc reverberated repeatedly–even without an outside stimulus. These reverberations had the potential to reactivate at any time if coerced by adjacent and non-related neural activity. Thus, elementary consciousness and anticipation evolved. Genes contained within conscious organisms have a reproductive advantage because they have the ability to “assess the qualities of their environment and anticipate future events” (Thiessen 252). This may mean that the organisms adept at avoiding predators and securing food also survive and reproduce very efficiently.

Evolutionary biology cannot fully explain the phenomenon of mortality. However, it has been suggested that morality likely evolved as a “characteristically human form of induced altruism” (Badcock 121). If a selfish individual coerces another individual into providing him with some resource, then the coerced individual has shown a form of morality. In other words, it appears as if the coerced individual has concern for the well-being of another individual, perhaps even at a cost of reducing his own ability to reproduce. Obviously, this type of behavior hinders the ability to produce offspring. One would then expect that genes coding for moral behavior should disappear from the population. However, if the gene which codes for morality also codes for another trait which increases the ability to reproduce (for example, increased metabolism), then morality may persist within a population.

People also exhibit other behaviors that seem purely altruistic when considered at the individual level. However, this altruism becomes purely selfish when one looks at natural selection at the molecular level. For example, mothers who risk their lives to save their children clearly contradict the idea that individuals look only to increase the chance of reproduction. Yet, recall that organisms serve only as receptacles for selfish genes. Once the genes replication has taken place, the gene has essentially completed its task. When applied to the mother/child example, one can see why saving the child makes sense. The mother’s genes have already passed on into the child. Because the child is younger than the mother, the child has a better chance of producing more offspring in the future than the mother. So, if the mother perishes in the process of saving her child, the gene still benefits because it has already replicated, and it has a good chance of replicating again in the future.

The selfish gene theory also applies in explaining aggressive behavior in males. Differences between male and female behavior results from differences in relative parental investment. This refers to “any investment that a parent makes in offspring at the cost of that parent’s ability to produce more offspring” (Thompson 21). Human females have higher relative parental investment than males because they nourish the developing fetus within its uterus for nine months before birth finally occurs. During this time, the female cannot produce additional offspring because her one and only womb is occupied. In addition, complications may occur during the pregnancy which may prevent the mother from having children in the future. On the other hand, do not have an obligation to nurture the growing fetus. Furthermore, male sperm is plentiful and inexpensive to produce. Thus, human males have low relative parental investment. So, if a female wishes to produce the most viable offspring possible, must ensure that the male impregnating her has qualities associated with successful reproduction, so that the offspring inherit those qualities. Conversely, males adopt a different mating strategy. Because females choose mates more selectively than males, and because males produce many sperm at very low cost, males attempt to mate with as many females as possible. This gives them a greater chance of passing on their genes since availability of the female egg occurs unpredictably only once a month. As a result, males compete with each other for females. Because aggressiveness often correlates to better ability to compete, this behavior evolved in males.

Interestingly, male competition for females has also been used to explain the evolution of male homosexuality. Christopher Badcock explains male homosexuality in the following way. Men who do not have the physical build to compete with other men develop cryptic behaviors such as homosexuality to capture females. Homosexuality fools both sexes into believing that the individual has no interest in mating. Homosexuals scathe off other male competitors who do not want to waste their energy competing with someone who has no interest in securing females. They also fool females into believing that they have no interest in sex, thus causing the female to drop her guard. As a result, males exhibiting homosexual behavior gain mating opportunities without competing with other males.

Most other human behaviors have also been explained through the eyes of evolution. For example, impulsiveness may have once been helpful in escaping predation. Imagine early man in a confrontation with a charging animal. If he took the time to analyze all possible options for escape, he might die before he puts his plan into motion. On the other hand, if he simply runs without giving it any second thought, he increases his chances for survival. In most cases, human behavior has been described as a result of manipulation by selfish genes. However, this explanation falls short theory falls short in explaining certain human behaviors such as morality and volunteerism. For reasons stated earlier, sociobiology is a relatively new field of study, with much more discoveries still to come. Thirty years from now, sociobiology may have a more complete answer to the question of why humans act the way they do. On the other hand, this field of study might fade away like several other branches of psychology which fell to the wayside after their explanations of human behavior were found inaccurate.

Works Cited

Badcock, Christopher. Evolution and Individual Behavior. Oxford: Basil Blackwell Ltd, 1991

Dawkins, Richard. The Selfish Gene. New York: Oxford University Press, 1976

Peters, Calvin B. Understanding Behavior. New York: Oxford University Press, 1991

Thiessen, Del. Bittersweet Destiny: The Stormy Evolution of Human Behavior. New Brunswick:

Transaction Publishers, 1996

Thompson, John N. Botany/Zoology 405 Coarse Packet. Pullman: Students Books Corporation, 1996

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