Wednesday, November 17, 2010

From Trees to Cities: Evolution Through Urbanization

Throughout history, there have been a myriad of situations or shocks, which have helped, pave the way for the evolution of modern Homo sapiens.  These include environmental changes; such as drought, an ice age, or volcanic eruption, the development of tools, larger brain size, and various others.  However there is one change that has developed fairly recently in terms of evolution that can be argued as having grand affects on human evolution: urbanization.  One major way in which city life has influenced human evolution is through diseases.   

Cities have long been a part of our culture.  Ever since the days when Catalhoyuk, Suberde, and Tepe Yahya joined Jericho's mesh of intercity trade, and four thousand years before the rise of the Sumerian cities of Ur, Uruk, and Kish, Stone Age metropolises from Anatolia to the edges of India were already rich in challenges and opportunities.  The emergence of these ancient cities has played an integral role in our evolution.  They brought our ancestors out of the caves and other isolated rural establishments they lived in, into densely populated cities.  Bringing all of these people together has influenced our evolution.  With people living close together in these metropolises, it allowed for diseases to be easily spread and much more common than before.  Researchers believe that this cesspool of diseases has allowed us as modern day humans, to evolve and adapt to become resistant to several infections.

Evolutionary biologist, Ian Barnes, has set out to prove this thinking.  In order to do this, he utilized the genetic variant, SLC11A1 1729+55del4 which is associated with the resistance of germs within cells (Charles Q. Choi).  To test this hypothesis, he and his team took DNA samples from 17 urban centers ranging from Catalhoyuk, 6000 B.C. or roughly 8,000 years old, to Juba which was erected in the 20th century and is barely a hundred years old (Matt Kaplan).  One thing that he made sure of was to choose cities that had all been inhabited for different lengths of time.  In other words, a city settled for 200 years compared with one settled for 5,000.   In their findings, the team found that longer populated cities were more likely to have this variant than cities inhabited for only a hundred or so years.  The article specifically states the city of Susa in Iran.  This city has been inhabited for more than 5,200 years and the people here are “almost certain” to have this specific variant, which allows them to resist a number of diseases.  However, in comparison with Yakutsk in Siberia, which was only settled for a few hundred years, only around 70 percent of the people would have the variant (Charles Q. Choi).

According to the article by Charles Q. Choir, Barnes states that “The research shows evolution happening.”  Evolution takes more than a hundred years to take place so it makes complete sense that a population in a city that had been established for more than 5,000 years would have adapted to the conglomeration of diseases through acquiring this genetic variant.  Eventually, through natural selection, only those with this variant would survive.

Thursday, November 11, 2010

Evolution of Dentition

All primates express dietary plasticity, which means that they eat a wide variety of different foods.  In order to keep this varied diet, primates’ dentition, or teeth, have adapted in various ways.  Primates have four distinctive tooth types: incisors, canines, premolars, and molars.  Each has different functions; incisors and canines are for biting and tearing food while premolars and molars are for grinding and crushing food.  Evolution has allowed primates to retain these different teeth because there is a purpose for all of them based on what primates eat.  Primates also have a reduced number of teeth compared to their earlier mammalian ancestors.  This reduction has occurred over the course of primates’ evolution, meaning that higher level primates have fewer teeth than lower level primates.  Old World higher primates, including humans, have the least number of teeth: 2 incisors, 1 canine, 2 premolars, and 3 molars in each quadrant of their dentition.
While human teeth do match primate teeth in many ways, human dentition has also evolved on its own due to dietary changes.  The most prominent difference between human teeth and primate teeth is the fact that humans have nonhoning chewing.  Approximately 5.5 million years ago humans lost their large, projecting canines.  This is a common feature among primates, which allows them to slice and shred their food.  Early humans learned to make and use tools that allowed them to process their food before putting it in their mouths.  These changes in lifestyle lead humans to lose their honing canine and develop a vertical, incisor shaped canine in its place.  Going along with this, humans also lack the diastema, or gap, that primates have between their lower canines and first premolars.  This gap allows room for the large honing canine, as well as a surface on which to sharpen the canine and slice food materials.  Over time, human teeth have also become smaller than primate teeth.
Even more recent changes in human dentition have come about as the result of farming and agriculture becoming humans’ primary source of food.  Within only the last 10,000 years humans have changed from foraging for wild plants and animals to producing domesticated plants and animals.  In short, foods became more processed and softer over time, which lead to less stress on the chewing muscles.  The reduction of stress on the muscles led to a reduction of the jaw bone underneath the muscle.  However, tooth size is based more on genetics than bone size, and environmental changes had little effect on the size of the teeth in the increasingly smaller human jaw.  All of this has lead to two extremely common problems among humans today: tooth crowding and malocclusion (overbites and underbites).  Crowded and misaligned teeth are the direct results of having smaller jaws than our ancestors but still retaining similar sized teeth.  The most common way to correct problems that are brought about by tooth crowding is braces.  Orthodontics has become a multibillion-dollar industry in the United States because of how common teeth straightening has become recently.
             Tooth decay is another recent problem facing humans as a result of dietary changes.  Domesticated plants, which are carbohydrates, produce more tooth decay because bacteria in the mouth digest carbohydrates and then produce lactic acid, which dissolves the enamel of the teeth.  Humans surprisingly have rather thick enamel on their teeth.  Thick enamel is usually associated with eating harder foods, but since humans eat softer foods it is odd that the thick enamel has remained.  Some types of domesticated plants, such as rice, cause very little tooth decay, while others, such as corn, cause considerable amounts of tooth decay.

Larsen, Clark Spencer. Our Origins: Discovering Physical Anthropology. New York: W.W. Norton &, 2008. Print.