Friday, May 11, 2007

Exam 2 Essay Timestamp (sources page omitted)

The rise of modern Homo sapiens is marked by physical and cultural changes, some not ever seen in animals before. We have several tools to look into the past, tools which give us some interesting and sometimes seemingly conflicting answers to the questions: how, when, and where did we originate? When did we migrate? How are we physically different from the species most closely related to us? When did we begin to have a distinctly human way of interacting with the world?
There are three main models as to how modern Homo sapiens originated. The first is the Recent African Origin model in which modern humans evolved in Africa and dispersed throughout the world with little to no mixing with indigenous populations. Differences between human populations are caused by genetic drift in relatively isolated populations. In this model, humans evolved in Africa approximately 100 thousand years ago, with a single main migration arriving in Western Asia about 45 thousand years ago and spreading to Europe by 35 thousand years ago. It does not give dates for the arrival of East Asian or Australasian people as the data are still too scant to set any firm dates. This model argues for humans replacing indigenous populations where they met them but does not say whether this was through violent conflict or merely being better able to exploit resources. Similarities to indigenous people such as the shovel-shaped incisors in Asia which are similar to H. erectus teeth and large noses in Europe similar to Neanderthal noses are simply homologies. This is in line with observations which show African people to be the most genetically diverse people with the most diverse dental and cranial morphology. The second is the Assimilation Model in which modern humans evolved in Africa, but did not yet form a separate species, and mixed with indigenous populations as they dispersed from Africa. It argues the same basic dispersal dates as the first model but it does not argue for humans out-competing indigenous populations but rather assimilating them and breeding with them, accounting for the regional differences seen in humans such as shovel-shaped teeth in Asia and large noses in Europe. The third is the Multiregional Model in which humans dispersed before 200 thousand years ago and developed regional adaptations due to bottlenecking but had enough gene flow to not speciate. (Stringer, 2002; Stringer, 2003; Klein, 1999) The reason for humans not showing more genetic diversity is accounted by heavy influence of African genes through gene flow of all human populations.
Mitochondrial DNA and Y-chromosomal gene analysis place our most recent common ancestor at approximately 200 thousand years ago. (Klein, 1999) Our most recent common ancestor with Neanderthals is placed by mitochondrial DNA data somewhere 317 to 741 thousand years ago. (Stringer 2002) This is too much time for most people to accept them being able to create viable offspring (and therefore be able to leave their mark on our DNA) with Homo sapiens in the window of time in which H sapiens and H. neanderthalensis interacted, assuming they did in fact take notice of each other. This would seem to indicate that the Recent African Origin model of human evolution is the most accurate model we have at this time. This paper operates on that assumption.
So, where do we see evidence of the evolution and expansion of modern humans? The two earliest known sites for fully anatomically modern humans are the Klasies River Mouth in South Africa, and Omo Kibish in Ethiopia which both date to 100 thousand years ago at the latest. (Pearson, 2000) Next we see the Skhul and Qafzeh caves in Israel which have early-modern humans with bones stained with red ochre dating to 90 thousand years ago at the latest. At the time when these specimens were alive, modern-day Israel would have been part of the continent of Africa so this is not evidence of an early migration. (Klein, 1999; Pearson, 2000) The Old Man of Cro-Magnon was important for demonstrating that modern humans had been in Europe for at least the past 35 thousand years. This is evidence of a true migration. (Klein, 1999) Liujiang which was first populated by modern humans between 60 and 30 thousand years ago and Zhoukoudian which was first populated by modern humans around 30 thousand years give minimum dates for the population of East Asia. (Klein, 1999; Stringer, 2002) Monte Verde charcoal dating to over 33 thousand years ago (Klein, 1999) gives an earliest date for the population of the Americas. Lake Mungo had modern humans at least 30 thousand years ago. (Pearson, 2000)
Modern humans have several physical traits that differentiate us from our most recent ancestors. We have a mental eminence which may be related to our less-robust mandible (Wood, Richmond 2000) and genetic drift in our earliest ancestors. We are the first organism to have such a feature. (Klein, 1999) Possibly related to the chin is a more gracile anatomy (Pearson 2000) which allows humans to consume less nutrients to fuel their bodies. A taller, more gracile body form is a more heat-adapted body form (Pearson, 2000) which suggests an African origin. However, it is important to note that while modern humans have a less robust skeleton than our premodern cousins (Kappelman, 1997), this difference shows up the most in femoral measurements between Neanderthals and humans from warm climates. Humans from colder climates show a marked increase in robusticity relative to warmer climate humans in all long bones, although femoral robusticity is still significantly but not greatly less than Neanderthal robusticity. When looking at the humerus, however, Neanderthals plot within the range of cold climate residing modern humans and when looking at radial robusticity, they plot at the less-robust end of modern humans from warm climates! (Pearson 2000)Since we know that physical activity also affects skeletal robusticity (Kappelman, 1997), this would seem to suggest that genetics does not in fact play such a great role in skeletal robusticity as we once thought. This data combined with the Skhul-Qafzeh fossils which plot similarly to warm climate modern humans suggests a shift in resource procurement from primarily relying on how much ground is covered in search of food to more reliance on tool creation and upper body strength. We are much more sure of the significance of the increase in basicranial flexion and lowered larynx (Klein, 1999) which had been gravitating toward what is seen in modern humans. Anatomically modern humans were the first hominins capable of the full range of speech that we enjoy today because the lowered larynx allows for a wider range of vowels and makes for a more resonant chamber. (Klein, 1999) It leaves us more vulnerable to choking, but much more efficient at communicating. Our steeply rising frontal bone, globular skull, face placed below the forebrain, (Klein, 1999) could suggest changes in the brain structure (if it does, it is most likely an expansion of the frontal lobe, which is of course the seat of what are considered to be the highest brain functions) or simply the way the cranium re-shaped itself in response to a more gracile cranial structure and a need to retain relative brain size.
An important question is how intertwined human anatomy was with human behavior. As we have already seen, anatomically modern humans most likely did already have a unique way of acquiring adequate nutrition. However, before about 40 to 50 thousand years ago, human culture changed gradually, with cranial and postcranial anatomy evolving alongside human culture at a similar rate. After the 40 to 50 thousand year ago mark, human cultural change appeared to accelerate while human physical change appeared to achieve some kind of stasis. The evidence for this shift is the sudden appearance of more and more elaborate tools such as fishhooks, decorative items such as beads, and pieces of art such as the Venus figurines. This is also the first time we see significant use of media such as shell and bone for tool use. This could just be another example of exponential growth- innovation compounded upon itself causing innovation to occur faster. But this does not explain how graves made a shift from possibly being only utilitarian to definitely being attached to some sort of reverence for the dead- items which can only be grave goods start showing up. Some people suggest a neural shift in human populations, possibly to do with an increased forebrain. (Klein, 1999) At any rate, human culture as we think of it today with significant investment in art and widely varying tools has been with us for less than half of the time that our species has been around. (Klein, 1999) This is significant because it means that humans had a sort of punctuated event where humans started to become truly human in the sense that we think of it.
We may disagree on exactly when we became modern humans, but one thing is certain- the rise of modern human culture and innovation was one of the most significant things to occur to hominins since habitual bipedalism and paved the way for even more innovation.

No comments: