How did adaptations in humans increase our chances of survival?

Variety of adaptations that the human species came across allowed us to survive and become the predominant species on the planet. An increasing amount of information about genetic variation, together with new analytical methods, is making it possible to explore the recent evolutionary history of the human population. This research will explain different types of adaptations, as well as clarify the selective pressures, the way adaptation promotes survival and how other species would respond to such selective pressures.

Climatic factors like temperature and humidity play an important role in determining species distributions and they likely influence phenotypic variation of populations over geographic space. Form of adaptation which can take moments to weeks to occur and is reversible within an individual’s lifetime no matter if it occurs when one is a child or an adult is called short-term acclimatization. An example of this type would be tanning. Human skin pigmentation is the product of two clones produced by natural selection to adjust levels of constitutive pigmentation to levels of UV radiation (UVR). The association of dark skin pigmentation with intense sunshine and heat was developed by Aristotle and his followers as part of a comprehensive “climatic theory,” which related human features, dispositions, and cultures to the environment. By the mid-18th century, naturalists such as John Mitchell and, later, Samuel Stanhope Smith recognized a pronounced latitudinal gradient of skin pigmentation among the world’s peoples — from dark near the equator to light toward the poles — and related it mainly to differences in sunshine heat experienced by people at different latitudes. Tanning was important to populations settling between roughly 23º and 46º, where levels of UVB varied strongly according to season. Classical studies which examined the global distributions of human physiological traits such as pigmentation suggested that natural selection related to climate has been important during recent human evolutionary history.

Another interesting adaptation which was discovered 10,000 years ago is the lactose tolerance in some human populations. The ability to digest lactose, a sugar found in milk, usually disappears before adulthood in mammals, and the same is true in most human populations. However, for some people, including a large fraction of individuals of European descent (nearly 80% of people of European descent carry this allele), the ability to break down lactose continues because of a mutation in the lactase gene (LCT). This suggests that the allele became common in Europe because of increased nutrition from cow’s milk, which became available after the domestication of cattle. Several explanations have been proposed for how and why lactose tolerance may have been selected. For example, it may simply be that milk is a good source of calories, or specifically an important source of protein and fat. Strong selective pressures on lactose tolerance may have been in sections and occurred only under certain extreme circumstances like drought, an outbreak or shortage of food. For example, milk would have represented an alternative food resource in the middle of farming. When no cereal food was available, for example between harvesting seasons or in periods of crop failure, lactose tolerance individuals would have had an advantage. A common feature of most populations with high frequencies of lactose tolerance gene is a history of dairying activity (Holden C., Mace R. 1997). The availability of fresh milk to some human groups has challenged their niche, thereby creating a potential genetic response for a need of lactose throughout adult life. It is likely that the study of the process of when, how and why some populations kept and exploited ungulates will make it more clear for our understanding of the distribution of lactose tolerance in humans. Indeed, archaeological data can be used to provide evidence for the presence of this cultural behavior in past populations. For example, the analysis of milk residues (Dudd S. N., Evershed R. P. 1998) accounts for an archaeological method that can inform on the disclosure of dairying. Hence, to fully understand the origins and evolution of lactose tolerance, it is necessary to consider archaeological research on the origins of domestication and dairying.

The development of agriculture also changed the selective pressures on humans in another way: Increased population density made the transmission of infectious diseases easier. The already substantial role of pathogens as agents of natural selection expanded and that role is reflected in the traces left by selection in human genetic diversity; multiple loci associated with disease resistance have been identified as probable sites of selection. In most cases, the resistance is to the same disease — malaria. Malaria is one of the human population’s oldest diseases and remains one of the greatest causes of disease and mortality in the world today, infecting hundreds of millions of people and killing 1 to 2 million children in Africa each year. The geographical distribution of the sickle-cell mutation (Glu6Val) in the beta hemoglobin gene (HBB) was limited to Africa and that individuals who carry the sickle-cell trait are resistant to malaria (Allison, 1954). Since then, many more alleles for malaria resistance have shown evidence of selection, including more mutations in HBB, as well as mutations causing other red blood cell disorders, such as a-thalassemia, G6PD deficiency, and ovalocytosis (Kwiatkowski, 2005).

All the examples of selective pressures and different types of adaptations the human species were exposed to allowed us to survive on the planet, however there is one more important factor which is how humans were able to become the predominant species. In a TED talk, the famous author of the book ‘’Sapiens’’, Yuval Noah Harari said, “The real difference between humans and all other animals is not on the individual level, it’s on the collective level. Humans control the planet because they are the only animals that can cooperate both flexibly and in very large numbers”. For example, comparative studies between humans and chimpanzees show that while both will cooperate, humans will always help more. Children seem to be natural helpers. They act selflessly before social norms set in. Studies have shown that they will spontaneously open doors for adults and pick up “accidentally” dropped items. They will even stop playing to help. Their sense of fairness begins young. Even if an experiment is unfairly rigged so that one child receives more rewards, they will ensure a reward is fairly split. We know that chimpanzees also work together and share food in apparently unselfish ways. However, Michael Tomasello of the Max Planck institute for Evolutionary Anthropology in Leipzig, Germany, says they will only cooperate if there is something in it for them.

We have reviewed recent advances in understanding the genetic architecture of adaptive human phenotypes based on insights from the studies of lactase persistence, skin pigmentation and malaria resistance. These adaptations evolved in multiple human populations, providing a chance to investigate independent realizations of the evolutionary process. The outcome of adaptive evolution is often highly variable even under similar selective pressures. As a result, over the past years selective pressures have allowed humans to evolve, adapt and become the predominant species on the planet.