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Essay on Human Evolution
Essay Contents:
- Essay on the Introduction to Human Evolution
- Essay on the Human Evolution into the Historical Era
- Essay on the Theories of Human Evolution
- Essay on the Mistakes in Evolutionary Thinking
Essay # 1. Introduction to Human Evolution:
The discovery of new fossils, the sequencing of the human genome, and the completion of the Human Genome Diversity Project will provide new insights that alter currently cherished beliefs. This has been the history of science in human evolution, so there is no reason to suspect that the trend will change. Still, we are stuck in the present and must do the best we can with the available data.
Most biologists suspect that humans and chimpanzees split off from a common ancestor as recently as 4 to 5 million years ago. The split occurred in Africa. One of the first evolutionary developments that distinguish human ancestors from chimps was upright posture.
One of the earliest of the human genera, the Australopithecines, who walked upright and had modified hands, but in many other ways resembled a chimp. They were small (between 3 and 4 feet tall), had curved fingers, and a skull with a protruding jaw, a recessed cranium, and heavy ridges behind what are now the eyebrows.
The reason for the development of upright posture is unknown, but it certainly permitted the Australopithecines to travel long distances, freed their hands to carry objects, and may even have allowed more efficient thermoregulation.
Because of these advantages conferred by upright posture, many anthropologists suspect that the early Australopithecines were adapting to life on the savanna while the ancestors of today’s chimps remained in the forested areas of central Africa.
Over the course of 2 million years, several varieties and species of Australopithecines may have cohabited together and possibly even competed with one another in the savanna and woodlands that border the savanna. Little is known about the behaviour of the Australopithecines other than they probably used tools and were a highly social species as chimps and we are today.
About 2 mya, fossils begin to appear from our own genus, Homo. Although several species of Homo are recognized, we will lump them all together for didactic purposes and refer to them as Homo erectus. The upper cranium of the skull expanded and assumed a more rounded shape, permitting brain size to increase from about 450 cc to between 800 and 1200 cc.
Other evolutionary trends continued—the protruding jaw receded a bit, teeth became smaller, and height increased.
Essay # 2. Human Evolution into the Historical Era:
Irrespective of whether amh replaced and/or interbred with Homo erectus and Neanderthals, there is considerable agreement on the particulars of very recent human evolution. The slanted forehead of H. erectus gave way to the large, vertical forehead of modern humans, permitting the brain to increase in size.
The skeletal structure attained a gracile form very close to modern humans. Tool use—or at least the evidence of tool use—suggests that it developed into an art. Spearheads were invented, bone instruments were fashioned to sew, pictorial drawings appear in caves, and some implements show evidence of engraving.
But human evolution was not finished by 50,000 ya. The skeleton continued its gracile development and cranium capacity still increased to give its present day range of 1000 to 2000 cc, the average today being somewhere between 1300 and 1400 cc.
Most scientists believe that early amh were foraging hunter-gathers. They lived in small, cooperative groups that would settle in a single location and hunt, dig roots, pick fruit, and possibly harvest grain until the immediate resources were depleted. Then they would move on.
Many hypothesize strong sex-role differences during this period— the guys hunted, the gals gathered. The small human groups—like virtually every other mammalian omnivore— adapt to seasonal change, migrating to areas of optimal foraging and hunting at the appropriate time of year.
Somewhere in the history of this—and whether it started 2 mya or 20,000 ya is anybody’s guess—the mating structure changed. Some form of Homo eventually recognized a relative permanence in mating that said something to the effect that this guy have a recognized relationship with this gal that permits them to mate, call them “their own,” and transfer property and prestige to their offspring. Early Homo also became cognizant of genealogy. Barak was not just Barak. He was also Thrug’s and Amalog’s son.
Everyone agrees that the increasing human cranial capacity was accompanied by an increase in intellect— memory, symbolic manipulation, learning capacity, etc. The largest anatomical differences between human and chimp brains are in the frontal lobes—those areas associated with executive functioning, evaluation, and reason.
The increase in frontal material permitted our hominid ancestors to develop culture beyond the simple social learning cultures of macaques, chimps, and bonobos. Our monkey and ape cousins have only the “monkey see—monkey do” cultural transmission. Homo’s ability to transmit culture includes simple imitation but expands into symbolic instruction.
At some point Homo could communicate the idea “don’t do it that way, do it like XYZ” without ever physically demonstrating the “XYZ” behaviour. Barak is no longer just Barak and is no longer just Thrug’s and Amalog’s son. He is also Gortog’s grandchild, even though Gortog, dead for several years, is a person unknown to the listener.
The reasons behind the evolutionary increase in brain size are not known, although there is no shortage of speculation. The need to fashion better tools, the requirements for sophisticated social interaction with conspecifics, the benefits of symbolic thought and language for competition between human groups have all been postulated as reasons for the intelligence of hominids.
It is also possible that the causes for increased brain size shifted over time, say from social communication to symbolic and rational thought to competition. Whatever the reason(s), they must have been quite important. Metabolically, the brain is a very expensive organ. Although it comprises only 2% of body weight, it consumes about 20% of the body’s metabolic resources.
Such an expense does not come without important evolutionary tradeoffs. Also, the increased brain size pose (and still does pose) difficult problems for mothers who must squeeze such a large structure through the pelvis and vagina during childbirth.
Two cultural inventions altered the environment for amh. The first was the domestication of certain animal species. A few human populations no longer had to hunt for meat. They could simply tame the “meat,” lead it to green pastures, slaughter it at will, and use its milk, wool, etc.
The second invention was agriculture. It is thought that agriculture-was developed sometime around 10,000 ya, probably independently in several different areas of the world. But the pattern of its discovery and diffusion is unclear. No matter. The end result was the same— agriculture limited the nomadic wandering of some human populations. They had to stay in a single geographical area to plant, tend, and harvest crops.
It is assumed that agricultural populations increased in number. This had two important effects. First, some agricultural populations migrated into the adjacent areas occupied by hunter-gatherer societies. Because the agriculturists grew in size while the population size of their hunter-gatherer neighbours remained stable, the number of agriculturists would eventually overwhelm the hunter-gatherers. Through interbreeding, population growth, cultural assimilation, and/or competition, the agricultural societies would become dominant in many fertile areas of the world.
The second effect of population growth under agriculture was an elaboration of social roles. As the technology of raising crops improved, it was no longer necessary for everyone to toil in the fields. Some people could become what today’s economists call service and manufacturing employees while others became supervisors. The result was an integrated web of codependent roles and occupations, leading to the development of cities and what we now call civilization, the first evidence of which appeared 5,000 ya.
The archeological record clearly shows that civilization did not start in one place and then spread unchecked throughout the world. Civilization appeared here and there in a series of starts and stops and not from a slow, inexorable diffusion from a single central origin. In a manner still obscure to science, civilizations develop in an area, flourish, and disappear.
To the best of our knowledge, the actual humans do not change, at least in any dramatic way—the cranial capacity of those who start and develop a civilization appear to be no different than those who disperse and engage in less sophisticated social, political, and occupational roles after the civilization’s demise. Indeed, the reasons for change in civilizations are some of the greatest mysteries facing social science.
Essay # 3. Theories of Human Evolution:
The first theory is termed the multiregional hypothesis (regional continuity hypothesis) and is espoused by some physical anthropologists. This view holds that Homo erectus populations in Africa, Europe, and Asia underwent convergent evolution and with sufficient gene flow among the geographically separated populations, jointly evolved into amh.
Convergent evolution occurs when different populations face similar selection pressures that lead to the same adaptive response. Development of the fin in fishes and in whales is a classic example in which two very different types of organisms evolved a similar mechanism for swimming through water.
Applied to humans, the need to seek shelter from temperature extremes is the same as the world-over and could—in theory at least—lead to selection for the increased cognitive skills to build those shelters.
Few, if any, advocates of the multiregional hypothesis hold that convergent evolution alone is responsible for the worldwide anatomical similarity of modern humans. Some gene flow is required among geographically separated populations.
Theorists posit that there was a sufficient amount of human migrations and mate exchanges between adjacent populations to permit H. Erectus populations to evolve in similar directions. In this way, mutant but beneficial alleles that originated in Africa could eventually spread to other regions of the Old World.
The multiregional hypothesis holds that the descendants of Homo erectus are our direct ancestors. Anatomically modern humans are the result of some beneficial mutations that caused an increase in population size. Subsequent migrations and interbreeding with extant groups of Homo erectus in different regions of the world spread these mutations.
According to this view, Neanderthals were not a different species of hominids that became extinct. Instead, generations of matings between the Neanderthals and the more gracile variant, coupled with a selective advantage for the genes of the gracile variant, resulted in a change in the mean of a continuous distribution of skeletal dimensions.
The second theory has been dubbed the Garden of Eden or GOE hypothesis. The name of this theory has little to do with the accounts of creation given in the Judaic- Christian tradition. Instead, the term is a slightly perverse— but humorous—extension of early reports from the genetic literature of a “mitochondrial Eve” and later a “Y-chromosome Adam”.
Mitochondrial DNA and the Y chromosome are in many ways ideal for studying human evolution because they are passed intact from mother to child or from father to son (Y chromosome) and do not recombine as the DNA on the autosomal chromosomes do.
This form of transmission has the mathematical implication that in some very ancient ancestral population, all but one of the mitochondrial variants will eventually die out. After all, the mitochondria of a mother who has only sons will die out as will the Y chromosome of a male who has only daughters. By examining today’s mtDNA and today’s Y chromosome, one can work backwards to arrive at an approximate date for these ancestral populations.
Today’s estimates are between 100,000 and 200,000 ya. Somewhere in this time period, a single woman lived from whom all current mtDNA is derived. A man also lived during this time and gave rise to all variants of the Y chromosome seen today.
Contrary to popular misconception, this Eve and Adam are not the ancestors of all modern humans. They are the ancestors of only our mitochondrial DNA and the DNA on the Y chromosome. Many other individuals contributed to the DNA in our autosomal chromosomes.
According to the GOE hypothesis, amh originated somewhere in Africa between 50,000 and 150,000 ya. The African origin is suggested by the observation that genetic variation is greatest in contemporary African populations. This ancestral population was possibly a new species of Homo that grew in size and migrated—possibly more than once—out of Africa and into the Middle East.
One hallmark of the GOE theory is population replacement. Advocates of this speculate that early Homo sapiens were a completely different species that did not interbreed with the populations of H. erectus and Neanderthals with whom it came into contact. Instead, they competed with those populations and eventually replaced them.
Two types of data are used in support of the GOE theory. First, archeological investigations show noticeable skeletal differences between Neanderthals, recent Homo erectus populations and the gracile form that is assumed to be our direct ancestor.
In parts of Eurasia, the emergence of these fossils also coincides with a marked advancement in technology stretching from Europe to Siberia. The second line of evidence consists of the molecular genetic data. Estimates of the time frame for human origins from these data fit very well with the archeological data.
Essay # 4. Mistakes in Evolutionary Thinking:
As we discuss human evolution and then evolutionary psychology, it is crucial to keep in mind the interactive nature of the forces of evolution. Amateur evolutionists, as well as many of their critics, often fall into certain errors that can colour and even invalidate conclusions derived from data. In this essay, several of the common mistakes will be pointed out.
i. Evolution Has a Goal:
Evolution is a description of a process that lacks consciousness, intentions, morals, and goals. Tigers do not have stripes because evolution wanted them to blend in against a forested background where the sun highlights one area but leaves an adjacent area in deep shade. Tigers have stripes because at some point in their past striped tigers out reproduced other tigers.
If things had gone differently, tigers may well have had spots like leopards and jaguars. Writers speak of evolution as “working towards this” or “acting against that” because it is difficult to speak about a process without attributing some sense of agency to it. “The sun warms the earth” is a descriptive statement; it does not mean that the sun consciously changes its physics in order to keep the earth warm. Similarly, “evolution made us humans smart” is also a descriptive statement. Evolution did not “intend” for that to happen. It just happened.
ii. Evolution Works for the Good of the Species:
Older writings and TV documentaries often spoke of evolution as working “for the good of the species.” Lacking sentience, evolution does not work for the good of anything, even individuals. Species are a necessary consequence of evolution because of genetic transmission and, in sexual species, because of the requirement that one have the anatomy, physiology, and behaviour to allow successful mating with conspecifics.
In The Selfish Gene, Richard Dawkins argues that evolution is really a case of some forms of DNA being able to out replicate other forms of DNA. A chicken’s egg did not evolve for the good of the chicken species. Instead, the chicken is an egg’s way of making another egg. In the process, DNA can develop any type of mechanism that assists in its own replication, even when the mechanism involves inhibiting other conspecifics from reproducing.
A classic example is the mechanism of the copulatory plug that occurs in some insect and reptilian species. After a male mates with a female, he secretes a thick, viscous “glue” into the female that inhibits—and in some cases even prevents— other males from having successful intercourse with her.
This clearly works against the good of the species. To insure the female’s maximal reproduction, she should mate with many different males to insure that each and every one of her eggs is fertilized. Instead, the mechanism evolved because males with the genes for the anatomy and physiology for secreting a copulatory plug out reproduced males without those genes.
iii. Evolution and Optimization:
Herons, pelicans, osprey, and loons all catch fish for a living, but they do so in quite different ways. A heron stands motionless in shallow water patiently waiting for a fish to swim by. When this happens, the heron swiftly grabs the fish with its long, extended beak.
Pelicans fly ten to thirty meters over water looking for fish near the surface. When the pelican spots a fish or a school of small fishes, it goes into an abrupt dive and crashes into the water, attempting to scoop up the prey in its pouch. Ospreys soar high above water.
When the osprey locates a fish near the surface, it goes into a deep dive and grabs the fish with its talons. Loons paddle on the surface like ducks. When the loon spots a fish swimming underneath, the loon dives and swims after the fish.
The different strategies for fishing illustrate that evolution does not find the global optimum. Instead, it finds solutions, some of which may be local optima. To use the shifting balance landscape, evolution has resulted in the heron, pelican, osprey, and loon living in their own adaptive pits.
It does not guarantee that all four species must end up in the deepest pit in the whole landscape. There is no “best” solution for a bird to catch fish. Instead, there are several different solutions and as long as a species develops one of them, it is perfectly okay.
iv. Confusing Relative with Absolute Fitness:
The confusion between relative and absolute fitness can lead to false inferences about evolution. When there is continuous selection against a genotype over time, then that genotype will decrease relative to the more fit genotypes. However, this does not necessarily imply that the less fit genotypes will eventually be removed from the population.
That may be the case, but it is not necessarily the case. Why? Because the extinction of an allele or of a genotype depends on population, size and absolute fitness, not on relative fitness.
Hence, the absolute fitness of a genotype equals the relative fitness multiplied by 1.05. Notice how genotype aa increases in absolute numbers over time. After 100 generations, aa is quite rare relative to AA, but its actual numbers have quadrupled.
As you might guess, the critical variable in all of this is the rate of population growth. When this rate is high relative to the relative fitness of genotypes, then lesser-fit genotypes can increase in absolute numbers.
Once their numbers increase sufficiently to overcome the effects of drift, then the alleles for lesser-fit genotypes can remain in the population. When the rate of population growth is low relative to the relative fitness of the genotypes, then the lesser fit alleles are likely to be removed from the population.
Since recorded history—and probably before that—the pattern of human evolution has been one of high population growth. Although natural disasters, famines, epidemics, and warfare act to the contrary, the reduction in population is local and temporary—they are irregular blips compared to the long-term trend. Hence, the human genome may contain a number of alleles that would have been eliminated had not the population been growing.