Saturday, April 3, 2010
The environment that fostered the rise of Homo Sapiens
On page 209, Coyne discusses the possible selective advantages of walking upright on two feet rather than knuckle-walking. In addition, on page 211 he also discusses the genetic differences inherent between chimp and human, such as enzymes used to break down starch. Taking into consideration these and other imformation given in this chapter, describe the possible environment and niche that could have fostered the growth of such traits (bipedalism, larger brain size etc) among the ancestors of humans. Include climate, mode of nutrition, predation (consider why more cooperative social interaction may have arisen rather than the individualistic tendencies of other apes), and geography.
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Understanding human history is a key factor in determining how many of these different traits described may have occurred. According to genetic evidence, humans most likely emerged from Africa: "analyzing DNA from Asians, Europeans and Africans...found greater [genetic] diversity in African populations...consistent with Africa containing the source population from which modern humans arose 100,000 or so years ago." (G & G) as well as archeological evidence of a skull found that offers "genetic evidence indicating that modern humans originated in sub-Saharan Africa and migrated about this time to colonize the Old World." (Science Daily). Knowing this information and the geography of Africa, we know that outside of rain forests (where apes lived), most of Africa was either desert or savanna, which include open plains with scattered trees, and low rainfall. (BB 1169)Since navigating the open plains would be slow and tedious, it is obvious that the ape-like ancestors of humans would have developed a more energy-efficient form of transportation--bipedalism. These African savannas also had a climate that is warm year round, with little shade from trees. (BB 1169) With the dryer climate, cooling the body via evaporation of water (sweating) would have been much more efficient than in the humid rain forests, and with the increase in sweat, hair would have been a hindrance, and there would have been an evolutionary advantage for humans to have less hair. Another difference, nutrition, brings out the fact that the resources that these ape-like human ancestors had in a savanna would be vastly different than in a tropical forest. Whereas in the tropical forests apes could easily get water, fruits, and other types of nutrition easily, the savanna had few fruit-bearing trees, which would have lead to these early humans to look for other types of nutrition. A study done between humans who live in rain forest environments and agricultural groups shows that the rain forest groups had much lower genes that produced amylase (which digests starch). (Nature Genetics) This shows how different environments in early human development may have effected how the need for other nutrition created room for evolutionary divergence of humans.
ReplyDeletesources:
BB- Biology Book
G & G- http://www.ramsdale.org/dna3.htm
Science Daily- http://www.sciencedaily.com/releases/2007/07/070718140829.htm
Nature genetics- http://www.nature.com/ng/journal/v39/n10/full/ng2123.html
On page 208, Coyne mentions that the evolution of humans can only be inferred through “more or less plausible guesses” based on the emergence of traits such as bipedality, increased brain size and smaller teeth. Scientists have confirmed that “this critical period of hominin evolution” saw a marked decrease in average temperature, erratic glaciations and prolonged droughts that likely led to the expansion of open grasslands and savannas. Homo habilis evolved towards the end of the Pliocene epoch about 2.5 million years ago and the increasingly harsh environment presented selective pressures that promoted a “change in subsistence pattern”. In order to survive the demands of the cold, hunting and foraging necessitated the selection for larger brains and “more sophisticated hunting techniques” that included individual cooperation, increased manual dexterity and bipedality. The development of cooking and hunting techniques was similarly coupled with a diet shift and the high amounts of starch consumed by bipedal hominids compared to that of modern, fruit-eating apes likely accounts for the presence of the salivary enzyme amylase that “acts in the mouth to break down starch into digestible sugar”. (Coyne, 211)
ReplyDeleteIncreased manual dexterity allowed for manipulation of hunting and foraging tools and also offered a defense against predators.
The development of highly social behaviour and cooperative units, as seen in modern-day primates, maximized hunting success and productivity but also facilitated mate selection.
Similarly, the evolution of early hominids saw the development of smaller teeth which is possibly the effect of “chopping, scraping and cutting tools” which required less mastication upon ingestion. (Carey, 2)
The Pleistocene epoch was largely characterized by glaciations interspersed with warm interglacial periods. During the Ice Ages, the demands of the cold selected for larger bodies that had the necessary insulatory capacity as dictated by the “Bergmann rule”.(“Climate change and Human Evolution”)
cont. On page 209 Coyne also mentions that other advantages of bipedality were “to travel more efficiently…freed the hands…[and] could also have helped us deal with high temperature by raising our body off the ground”. The loss of body hair and increase in sweat glands of the hominids were likely evolutionary developments that allowed for more efficient thermoregulation, especially during warm interglacial periods. Nevertheless, there were other advantages to the development of bipedality. Walking upright, it is likely that the human ancestor was able to see over tall grasses and consequently evade predators more quickly and easily. Another theory is that their upright posture allowed early hominids to “keep one’s [their] head[s] above water when wading in rivers and lakes”. The subsequent loss of body hair and layers of insulatory body fat further seems to suggest that humans evolved to spend “an inordinate amount of time in the water” as supported by the development of “downward-pointing nostrils…the way we hold our breath, and even…weeping”. (Boeree)
ReplyDeleteIt is also likely that the development of the larger brain size and highly developed skull of Homo sapiens was coupled with the development of “vocalizations and paralinguistic features for communication” that is considered “a derived characteristic…since the ape-human split” (Cunningham). The emergence of highly specialized and specific communication likely improved the efficiency of hunting, foraging and perhaps even mate selection. Communication and greater intellectual capability increased reasoning and problem-solving skills that proved a selective advantage for survival in the wild.
Sources:
Carey, Gregory. “How Humans Evolved”.1999-2000
http://psych.colorado.edu/~carey/hgss/hgsschapters/hgss_chapter14.pdf
Coyne, Jerry. “Why Evolution is True”. 2009
“Climate change and human Evolution”. http://anthro.palomar.edu/homo/default.htm
Boeree, George. “Human Evolution”. http://webspace.ship.edu/cgboer/humanevol.html
Cunningham, Deborah. “Language and Human Evolution”. Volume 29, 1999.
There were many adaptations that contributed to the evolution of modern humans. First, there was only so much that an animal was able to do to get food in Africa. The competitive environment selected for the ability of hominids to walk long distances to reach a food supply (http://www.guardian.co.uk/world/1999/sep/22/population.timradford). The ability to walk upright made it much easier for our ancestors to access food, as well as what Beatriz mentioned (ability to see over tall grass, keep head above water). This ability can be shown in the fossils of the species Australopithecus, which could walk upright but lacked a large brain and still had body hair ("Human evolution: the fossil evidence in 3D"). Our African ancestors all competed with other hominids for food, and so later on natural selection chose for a more elaborate brain and the ability to make tools was likely to obtain a competitive advantage over other species for getting food (http://bruceowen.com/introbiological/a201-09s-23-ModernHsapiens.pdf). The size of a human brain is about 1,400 cubic centimeters, which is twice the size of a modern chimpanzee. This larger brain allowed for social learning and language development in humans ("How Humans Evolved"). The reason for social beings and language was most likely because animals that could cooperate were more likely to be successful hunters, and thus more likely to survive and reproduce. Eventually our ancestor hominid species began to migrate throughout the world from Africa, and this is where scientists believe humans really diverged from other apes ("The relationships between neanderthal man and homo sapiens"). Once our ancestors migrated to Northern climates with much more seasonality in the weather, as Tianyu and Beatriz said, humans lost hair, developed a larger body and developed sweat glands as adaptations for more efficient thermoregulation in unpredictable climates ("Implications of natural selection in shaping 99.4% nonsynonymous DNA identity between humans and chimpanzees: enlarging genus Homo"). With a more complex brain, hominids became more capable of using tools, and the use of tools was cause for the selection for a power and precision grip, making it much easier to hold and use the tools ("Physical Anthropology"). Tools made many tasks, including predation, much easier to modern humans.
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