Introduction
Evolution is often perceived as a slow process which requires thousands or millions of years. During the last few decades a common consensus has been generated by world’s preeminent biologists that human evolution is over. It is often claimed that evolutionary process in modern humans have ceased because cultural and technological advancements have annihilated natural selection. Since modern Homo sapiens emerged 50,000 years ago, “natural selection has almost become irrelevant” to us, the influential Harvard paleontologist Stephen Jay Gould proclaimed. “There have been no biological changes. Everything we’ve called culture and civilization we’ve built with the same body and brain”. In contrast, recent studies show that selection can be strong in contemporary populations. Growing genomic revolution supports that microevolution, defined as a genetic change from one generation to the next in response to natural selection, can lead to changes in the phenotypes (observable characters) of organisms over just a few years or decade. This likely applies to humans as well because
- (a) natural selection operates on several morphological, physiological, and life-history traits in modern societies through differential reproduction or survival (Pettay et al., 2007 & Stearns et al., 2010)
- (b) a number of these genetic traits present variations, attesting the potential for a microevolutionary response to selection (Milot, 2011 & McAuliffe, 2009).
According to Henry Harpending “It is likely that human races are evolving away from each other and we are getting less alike, not merging into a single mixed humanity.” Harpending theorizes that the attitudes and customs that distinguish today’s humans from those of the past may be more than just cultural, as historians have widely assumed. He says that “We aren’t the same as people even a thousand or two thousand years ago”. “Almost every trait you look at is under strong genetic influence” (McAuliffe, 2009).
Signs of on-going evolution:
Over the past 10,000 years, data shows human evolution has occurred a hundred times more quickly than in any other period in our species history. Geneticists have identified more than two dozen genes that appear to have come under selective pressures since the rise of Homo, and several of them may still be subject to such pressures today. Some of these favoured alleles apparently arose at highly critical periods in human evolution. Such is the case of FOXP2, the so-called speech gene, which is implicated in the ability to talk, shows signs of strong selection, and arose no more than 200,000 years ago, coinciding closely with the first appearance of Homo sapiens. Other genes under selection are linked to cognition and behaviour, and still others are involved in defense against diseases such as hypertension, malaria, and AIDS. The new genetic adaptations, some 2,000 in total, are not limited to the well-recognized differences among ethnic groups in superficial traits such as skin and eye color. The mutations relate to the brain, the digestive system, life span, immunity to pathogens, sperm production, and bones—in short, virtually every aspect of our functioning (Balter, 2005 & McAuliffe, 2009). Following examples clearly show the signs of on-going evolution:
- (a) There is growing evidence that epidemics are exerting selective pressure on our species. New methods for studying genetic variability—which can be used to study long-lived species with long generation times—have demonstrated directional natural selection on human genes by looking for signatures of selection in the genes of present populations. These include the glucose-6-phosphate dehydrogenase (G6PD) gene, which confers resistance to malaria, and the chemokine receptor 5 (CCR5) among Europeans, which confers resistance to the human immunodeficiency virus (HIV). The latter is likely to have evolved within the past 2,000 years, in response to an infectious agent that uses the CCR5 receptor to infect host cells. Numerous other studies have also provided evidence for recent natural selection on the human genome through comparisons of large sections of DNA (Stock, 2008).
- (b) Another most incendiary aspect of the fast-evolution research is evidence that the brain may be evolving just as quickly as the rest of the body. Stronger evidence that natural selection has continued to shape the brain in recent epochs comes from studies of DRD4, a mutation in a neurotransmitter receptor that is linked to attention-deficit/hyperactivity disorder (ADHD). Children with the mutation tend to be more restless than other youngsters and to score higher on tests of novelty-seeking and risk-taking, all traits that might have pushed those with the variant to explore new frontiers. Sequencing studies suggest that the DRD4 mutation arose 50,000 years ago, just as humans were spreading out of Africa. Its prevalence tends to increase the farther a population is from Africa, leading some investigators to dub it “the migratory gene.” At least one allele (or copy of the gene) is carried by 80 percent of some South American populations. In contrast, the allele is present in 40 percent of indigenous populations living farther north in the Americas and in just 20 percent of Europeans and Africans (McAuliffe, 2009).
- (c) Our exodus out of Africa, paved the way for one of the most obvious markers of race, skin hue. People with dark skin have trouble manufacturing vitamin D from ultraviolet radiation in northern latitudes, which makes them more susceptible to serious bone deformities. Consequently, Europeans and Asians over the last 20,000 years evolved lighter skin through two dozen different mutations that decrease production of the skin pigment melanin (McAuliffe, 2009).
- (d) Similarly, the gene for blue eyes codes for paler skin coloring in many vertebrates and hence might have piggy backed along with lighter skin. Clearly something made blue eyes evolutionarily advantageous in some environments and it is well known fact that 10,000 years ago no one had blue eyes on earth.
- (e) It is also claimed that human sperms may also be evolving at high speed, driven by the competition to get to the egg before another man’s sperm. In the modernized cities there is a fiercer competition among sexual partners. Because sperm can fertilize an egg up to 24 hours after being ejaculated in the vagina, a woman who copulates with two or more partners in close succession is setting up the very conditions that pit one man’s sperm against another’s. According to John D. Hawks (Evolutionist, University of Wisconsin—Madison), “sperm today is very different from sperm even 5,000 years ago.” Newly selected mutations in genes controlling sperm production show up in every ethnic group. The selection for such kind of “super sperm,” provides further corroboration that our species is not particularly monogamous—a view widely shared by other anthropologists (McAuliffe, 2009)
Other examples of agriculture and domestication mediated human evolution:
There are some examples of human evolution that occurred subsequent to the invention of agriculture, and that involve the co-evolution of cultural and genetic systems with changes in subsistence strategies.
- (a) The example that is most often cited is the natural selection of heterozygous carriers of the sickle cell gene to maintain sickle-cell anaemia in populations that are exposed to malaria. The disease of Malaria is about 35,000 years old, with the most lethal form of it just 5,000 years old. Yet in sub-Saharan Africa and other regions where it is endemic, “people have already developed 25 new genes that protect against malaria, including the Duffy blood type, an entirely new blood group. This natural selection is particularly visible in regions of central Africa where tropical forests have been cleared for agriculture, which, in turn, has caused the proliferation of mosquitoes that transfer the malaria-causing Plasmodium parasite (Stock, 2008 & McAuliffe, 2009).
- (b) Another example of more recent evolution within the human genome is provided by evidence for strong natural selection on the gene that controls lactase production. Once people began keeping cattle herds, it became an advantage to derive nutrient calories from milk throughout life rather than only as an infant or toddler suckling at its mother’s breast. A mutation that arose about 8,000 years ago in northern Europe allowed adults to digest lactose (the main sugar in milk), and it propagated rapidly, allowing the rise of the modern dairy industry. Today the gene for lactose digestion is present in 80 percent of Europeans but in just 20 percent of Asians and Africans (Bersaglieri et al., 2004 & McAuliffe, 2009).
- (c) As agriculture became established and started creating a reliable food supply, more men and women would have begun living into their forties and beyond—jump-starting the selection pressure for increased life span. Robert Moyzi is currently performing a genetic analysis of men and women in their nineties who are of European ancestry. He has traced many early onset forms of cancer, heart disease, and Alzheimer’s to older human gene variants. According to him people with more modern variants tend to have greater resistance to these chronic illnesses of old age and should be overrepresented in the age 90-plus population (McAuliffe, 2009).
These examples clearly demonstrate that natural selection has recently acted upon humans after the origin of agriculture and the domestication of animals, and independently among different populations.
Barriers to Natural selection: Cultural and technological adaptations
According to many scholars humans are no longer subjected to the random mechanisms of variation and selection and, as a species, they are depended on culture and technology for survival. In this respect, humans have been regarded as a species so dependent on culture and technology that cultural adaptation has replaced biological adaptation. During the past 12,000 years, humans have increasingly used culture and technology—built upon agriculture and animal domestication—to control and modify the natural environment. Therefore, culture has an important role in understanding whether evolution is still influencing the biology of our species.
Culture and technology were clearly crucial to the successful colonization of the world by our species. They allowed us to occupy most regions of the planet through the use of fire, housing, watercraft, versatile tools and cognition, which enormously improved our ability to hunt and forage for food in markedly different environments and, in the process, to occupy more environmental niches than most other species. Since the origins of agriculture, the rate of technological progress has increased exponentially. Agriculture originated independently within the past 12,000 years in various parts of the world, and the surplus of food resulting from agriculture has allowed people to specialize in different tasks, and has provided greater scope for innovation and cultural transmission. The technological achievements of our contemporary and industrialized society still rest on our agricultural production system, and the effective distribution of food resources. In turn, these technologies allow us to modify our environment so effectively that many have argued that we have removed our species from nature. Gene frequencies might still change over time through random factors such as genetic drift, but if our culture effectively removes us from environmental stress, then natural selection will no longer occur. However, it is important to remember that our ability to adapt to environmental stress is contingent on the availability and distribution of resources and energy. Regarding another tier of environmental buffering, there is evidence that humans have physiological characteristics that allow them to adapt efficiently to different or changing environments (Wells and Stock, 2007). The ability to cook food provides humans with a greater dietary flexibility than chimpanzees, gorillas or orang-utans. This dietary flexibility and the extensive use of meat have allowed humans to converge on a common adaptive niche, and to survive in a greater range of environments. Humans also show greater flexibility in growth and have larger stores of body fat than many other species, both of which increase our ability to survive short-term environmental fluctuations. Humans have greater variation in fertility and birth spacing, which allows populations to bounce back quickly after periods of high mortality, and there is increasing evidence that environmental conditions can alter the regulation of specific genes. All of these physiological features allow us to respond to environmental stress without the need for genetic adaptation by natural selection. Considering the strong evidence that our species has a greater range of both technological and physiological mechanisms for buffering the effects of environmental stress, one could argue that genetic evolution is no longer influencing our species. However, it is clear that most of our non-genetic methods for mediating environmental stress depend on our access to the resources provided by agriculture. As a result, these means of environmental buffering might not be sufficient in all circumstances (Stock, 2008).