The environment is everything to human evolution. In regards to humans, the concept must encapsulate both physical and cultural aspects of environments. Various environs provide a testing ground which all mutations must face if they are to graduate to the level of a polymorphism. This would be any variation physical, cultural, and behavioral within a population that is expressed 1% or above within a population. If a polymorphism is significantly beneficial in the face of certain pressures it will be selected for. Especially potent advantages could eventually become ubiquitous. An example of such a polymorphism would be the foxp2 gene found in modern humans. This gene is so important to language that a variation causes syntax malfunction. This particular adaptation functions both within the evolutionary environment, where an adaptive communication set is key to survival in areas thick with predators, and is culturally necessary where mate access could depend on an ability to converse meaningfully. Surely our modern environment has only replaced predators with education centers and conversing meaningfully with superficial flirting. Even those without verbal communication depend upon sign language in order to express themselves in all the ways we are familiar with vocally.
At the core of success is the level of selection pressure present within the population. Selection is a mechanism measured by the individual fitness derived from traits and behaviors. On a larger scale, a population can also undergo pressures that shift the direction of allelic distribution. If we are to turn this concept towards punctuated events, such as pathogen driven bottlenecks, we can observe relatively quick changes in the allelic frequencies of a population. Survival of the Sickest outlines just such an event when describing the prevalence of hemachromatosis in northern European nations. Pathogens can create extremely harsh selection pressures which produce a quick change. Evolution does not always take this path though; biological changes are produced over a longer period of time in relation to cultural events. Lactose persistence is a salient example of such an event occurs after the Holocene and the uptake of animal domestication and agriculture.
Lactose is literary milk sugar and the innate ability to gain caloric benefit from this substance generally ceases at a certain age. A high level of the pancreatic enzyme lactase is requisite for this process to occur, and production of this enzyme is under genetic control. The presence of this enzyme in most infants is high until weaning occurs. If milk is consumed after weaning the undigested portion can cause abdominal pain. In mammals it is normal to become intolerant. Cultural centers where milk is derived from domesticated animals this is not the case. Indeed most Western nations tend to supplement their diet with cow’s milk without hesitation.
This genetically determined ability is clinal and clearly a product of evolution within our species. Northern portions of Europe, Africa, and Asia all demonstrate this ability where unprocessed milk is consumed. This pattern a gradually decreases as one moves from these centers into other environments with separate selection pressures. For instance, moving South and East across Europe will result in a decline in lactose persistence. In native populations with no admixture, such as Australia and Oceania, we see a 100% lack of lactose persistence (Mielke et al. 2008). This cline is an example of an evolutionary trend within the past 10,000 years.
as pastoralism activity became increasingly beneficial over the millennia. In other words those individuals who could digest milk were at a selective advantage over those who could not.
This pattern makes sense because milk promotes the uptake of calcium and aids in the normal bone growth. Children that could to digest and use milk would not run the risk of getting rickets in latitudes where the sun is less intense. Those individuals living in desert environments would be better able to make use of the milks water content, thus increasing their fitness. Studies on this ability project that the strength the selection factor would need to be about 4% with a starting frequency of .001% in non-European areas. Current levels of lactase persistence in Europe would need a selection pressure of 5-7% for it to be in line with domestication diffusion 3,000 years ago (Mielke, James et al. 2006). Although this is certainly one of the more widespread evolutionary patters observed in our time, it is not the only one.
Further allelic alterations can be seen in malarial environments. Adaptations of this short are to be found—but not limited to—the Mediterranean. The enzyme deficiency of Glucose-6-Phosphate-Dehydrogenase provides people with a certain amount of protection from the parasite which causes malaria. The function of this enzyme helps prevent oxidization of cells when they metabolize. Lack of this enzyme can result in certain dietary restrictions due to the provocation of negative reactions and has therefore been called favism (Mielke, James et al. 2006). This condition is beneficial in malaria stricken environments because it is thought to expose the parasite within infected cells. This occurs because cells under stress are more sensitive to the hydrogen peroxide being produced by the parasite, which allows for the agents of the immune system to identify and destroy the infection. On an interesting historical note, the Mediterranean region and the condition could be intimately tied to actions of ancient Rome 2,400 years ago.
As O’Sullivan and colleagues point out in their paper, “Deforestation, Mosquitoes, and Ancient Rome: Lessons for Today” an increased population, an open sewage system, and a sudden alteration of a landscape allows for the introduction of new disease vectors. The act of deforestation for agriculture expansion allowed for the build-up of the Pontine marshland which provided the perfect grounds for mosquitoes to breed (O’Sullivan et al, 2008). Although the genetic impacts of this behavior were not outlined within the paper, practices such as these provide a plausible explanation for the levels of the G6PD deficiency we see in the area. The idea that large, ancient population center is found in this region in conjunction with an artificial malarial environment is too coincidental to ignore. This event could have certainly heightened the frequency of an already present polymorphism.
Perhaps one of the more interesting and overlooked aspects of post-agricultural effects on humans is the idea of becoming domesticated. Perhaps it has to do with the stigma of the word “domesticate”. To be a domesticated animal makes us think of penned creatures being raised for consumption. Or it might be tied to the notion that humans are somehow beyond the reach of environmental selection. After all, we often see ourselves as the ones that alter our environments, not the other way around. We do understand the process of domestication through various death assemblages throughout history. In the paper, “Human Domestication Reconsidered” a number of skeletal changes are outlined in regards to domestication:
(1)change in body size, initially to smaller, with decreasing skeletal robusticity; (2) reduction in cranial capacity; (3) shortening of the facial region of skull, including jaws, sometimes associated with tooth crowding and maleruption and/or reduction in size of cheek teeth; (4) reduction in sexual dimorphism; and (5) greater diversity in shape and size of horns (in cattle, sheep, and goats). Domestication changes affecting only soft tissues, body biochemistry, and/or behavior and therefore archaeologically invisible in any direct form may include the following: (6) increasing variation in coat colour and hair structure; (7) increasing fat storage (subcutaneous and intramuscular); (8) enhanced physiological performance, including lactation; (9) precocity, extended breeding seasons, and greater sexual stimulation; (10) retention of juvenile behaviours into adulthood; (11) greater litter size and frequency of multiple births; (12) reduction in motor activity; (13) reduction in information acquisition systems; (14) reduction in intraspecific aggression, especially in males (though this may be attenuated defensive behaviour); and (15) increased docility as part of reduced environmental responsiveness. (Leach, 2003)
With this list in mind, Dr. Leach examines the modern human condition, pointing out that this shift is present within humans during the Pleistocene to Holocene shift. One must only think of the Nubian shift from hunting and gathering to agriculture to find an example of reduced cranial robusticity (Larsen, 1999). Whether this exemplifies domestication is another question all together. An important factor considered within this paper is that human built environments, such as the ones humans have been perfecting since the sedentary lifestyle allowed for permanent domiciles, has acted upon our biology in a way not unlike guided selection. The only hesitation to ascribe domestication to the modern human condition is the need for a significant emphasis on preserving certain stock. As we have observed historically there has been a special emphasis on this with the Eugenics movement in western nations at the beginning of the 20th century. Besides being a perversion of evolutionary theory, the concept is largely bankrupt and we have not seen eugenics in a significant level since these early attempts.
When just a few examples of biological variation are considered, we can easily find results of selective pressures that are surely evolutionary events. These microevolutinary shifts may not be the dramatic evidence we observe in the fossil record from Homo erectus to Homo sapiens, but that make them no less significant. If we keep in mind that the environment determines whether a variant is good or bad, we can place G6PD deficiency in a good category where malaria is found. We can also place lactose non-persistence in the “bad category within a pastorally dependent culture. Whatever the category we ascribe the evolutionary examples provided we must recognize that they are present, and that they are representative of our constantly shifting environments both physically and culturally.
Larsen, C.S. 1999. Bioarchaeology: Interpreting Behavior From the Human Skeleton.
Cambridge University Press, Cambridge.
Leach, H. M. 2003. Human Domestication Reconsidered. Current Anthropology. 44:
349-368.
Mielke, J.H., L.W. Konigsberg., J.H. Relethford. 2006. Human Biological Variation. Oxford
University Press, New York.
O'Sullivan, L., A. Jardine, et al. 2008. "Deforestation, mosquitoes, and ancient Rome: lessons for today”. (Biology in History)(Report)." BioScience 58(8): 756(5).
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