2ndglance
疾惡如仇，柔情似水。重情感，意境，和哲理
http://anthropology.si.edu/humanorigins/faq/encarta/encarta.htm
Paleoanthropologists are engaged in an ongoing debate about where modern humans evolved and how they spread around the world. Differences in opinion rest on the question of whether modern humans originated in a small region of Africa or took place over a broad area of Africa and Eurasia. By extension, opinions differ as to whether modern human populations from Africa displaced all existing populations of earlier humans in other parts of the world.
Those who think modern humans originated only in Africa and then spread around the world support what is known as the Out of Africa hypothesis. Those who think modern humans evolved over a very broad area, with gene flow between regions, support the Multiregional hypothesis. Researchers have conducted many genetic studies and carefully assessed fossils to determine which of these hypotheses agrees more with scientific evidence. The results of this research do not entirely confirm or reject either one. Therefore, some scientists think a compromise between the two hypotheses is the best explanation.
According to the Out of Africa hypothesis, also known as the Replacement hypothesis, the transition to modern humanity occurred in only one area, which is consistent with the idea that new species usually arise from small, geographically isolated populations. Furthermore, modern anatomical traits evolved relatively recently, within the past 200,000 years or so. Modern-looking populations expanded and divided within Africa, and then they spread to other areas of the world. During this process, populations of migrating modern humans replaced archaic human populations, including the Neanderthals and any surviving groups of H. erectus.
According to the Multiregional hypothesis, also known as the Continuity hypothesis, the evolution of modern humans began when Homo erectus spread throughout much of Eurasia around 1 million years ago. Regional populations retained unique anatomical features for hundreds of thousands of years, but they also mated with populations from neighboring regions, exchanging inheritable traits with each other. This exchange of inheritable traits takes place by the process known as gene flow.
Through gene flow, populations of H. erectus passed on a variety of increasingly modern characteristics, such as increases in brain size, across their geographic range. Gradually this would have resulted in the evolution of more modern looking humans throughout Africa and Eurasia. The physical differences among people today, then, would result from hundreds of thousands of years of regional evolution. This is the concept of continuity. For instance, modern East Asian populations have some skull features that scientists also see in H. erectus fossils from that region.
Some critics of the Multiregional hypothesis claim that it wrongly advocates a scientific belief in race and could be used to encourage racism. Supporters of the idea point out, however, that their position does not imply that modern races evolved in isolation from each other, or that racial differences justify racism. Instead, the idea holds that gene flow linked different populations together. These links allowed progressively more modern features, no matter where they arose, to spread from region to region and eventually become universal among humans.
Geneticists have studied the amount of difference in the DNA (deoxyribonucleic acid) of different populations of humans. DNA is the molecule that contains the inherited genetic code. Differences in human DNA result from mutations in DNA structure. Mutations may result from human exposure to external stimuli such as solar radiation or certain chemical compounds, while others occur naturally at random.
Geneticists have calculated rates at which mutation can be expected to occur over time. Dividing the total number of genetic differences between two populations by an expected rate of mutation provides an estimate of the time when the two shared a common ancestor. Many estimates of evolutionary ancestry rely on studies of the DNA in cell structures called mitochondria. This DNA is referred to as mtDNA (mitochondrial DNA). Unlike DNA from the nucleus of a cell, which is inherited from both mother and father; mtDNA is inherited solely from the mother's egg, since sperm mitochondria are usually discarded during fertilization. The mtDNA accumulates mutations about ten times faster than nuclear DNA. As a result, mtDNA is altered so quickly that it is easy to measure the difference between one human population and another, since separate groups have accumulated different sets of mutations. Two closely related populations should have only minor differences in their mtDNA. Conversely, two very distantly related populations should have large differences in their mtDNA.
MtDNA research into modern human origins has produced two major findings. First, the entire amount of variation in mtDNA across human populations is small in comparison with that of other animal species. This means that all human mtDNA originated from a single ancestral lineage -- specifically, a single mother -- fairly recently and has been mutating ever since, producing the small diversity that exists throughout the human species. Most estimates of the mutation rate indicate an origin of about 200,000 years ago. The second major finding is that mtDNA of African populations is more diverse than of peoples of other continents. This suggests that African mtDNA has been changing for a longer time than elsewhere. Thus Africa is the likely source of the original mtDNA mother (sometimes called "Mitochondrial Eve"). Some geneticists and anthropologists have concluded, then, that modern humans originated in a small population in Africa and spread from there.
The mtDNA studies have been criticized on several grounds. First, the mutation rate is not known exactly, and some estimates could mean an origin age closer to 850,000 years. If so, the original mtDNA line of all modern humans might have occurred in Homo erectus, which then spread and gradually evolved in Homo sapiens, an interpretation that favors the Multiregional idea. Second, mtDNA is a small part of the total genetic material that humans inherit. Although the diversity of mtDNA may owe its origin to a single African female 200,000 years ago, the rest of our genetic material (about 400,000 times the amount of mtDNA) was inherited from many individuals who lived at the same time as that female. Some scientists argue that these individuals may have been spread over a wide area. Third, the time at which modern mtDNA began to diversify does not necessarily coincide with the origin of modern biological and cultural traits. Finally, non-African populations may have been smaller or experienced large drops in numbers, which could explain the smaller amount of modern genetic diversity outside of Africa.
Despite these criticisms, many geneticists continue to favor the Out of Africa model. Studies of nuclear DNA also suggest an African origin for other genes besides mtDNA. Furthermore, in a remarkable study, ancient mtDNA has been recovered from the original Neanderthal fossil find in Germany, and it does not closely match modern human mtDNA. This finding suggests that at least the population of this one Neanderthal had diverged from the lineage to modern humans by about 600,000 years ago.
As with genetic research, fossil evidence also does not entirely support or refute either of the competing hypotheses of modern human origins. However, many scientists see the balance of evidence favoring an African origin of modern H. sapiens within the past 200,000 years. The oldest known modern-looking skulls come from Africa and date from perhaps 130,000 years ago. The next oldest come from the Near East, where they date from about 90,000 years ago. Fossils of modern humans in Europe are unknown prior to about 40,000 years ago. In addition, the first modern humans in Europe -- often referred to as Cro-Magnon people -- had elongated lower leg bones, as did African populations that were adapted to warm, tropical climates. This suggests that populations from warmer regions replaced those in colder European regions, such as Neanderthals.
On the other hand, fossils of archaic and modern humans in some regions show continuity in certain physical characteristics. These similarities may indicate multiregional evolution. For example, both archaic and modern skulls from eastern Asia have flatter cheek and nasal areas than do skulls from other regions. By contrast, the same parts of the face project forward in the skulls of both archaic and modern humans of Europe. Assuming that these traits were influenced primarily by genetic inheritance rather than environmental factors, archaic humans may have given rise to modern humans in some regions or at least interbred with migrant modern-looking humans.
Each of the competing major hypotheses of modern human origins has its strengths and weaknesses. Genetic evidence appears to support the Out of Africa hypothesis. In the western half of Eurasia and in Africa, this hypothesis also seems the better explanation, particularly for the apparent replacement of Neanderthals by modern populations. At the same time, the Multiregional hypothesis appears to explain some of the regional continuity found in East Asian populations.
Therefore, many paleoanthropologists advocate a view of modern human origins that combines elements of the Out of Africa and the Multiregional hypothesis. Humans with modern features may have first emerged in Africa or come together there as a result of gene flow with populations from other regions. These African populations may then have replaced archaic humans in certain regions, such as western Europe and the Near East. Yet elsewhere -- especially in East Asia -- gene flow may have occurred among local populations of archaic and modern humans, resulting in distinct and enduring regional characteristics.
All three of these views -- the two competing positions and the compromise -- acknowledge the strong biological unity of all people. In the Multiregional hypothesis, this unity results from hundreds of thousands of years of continued gene flow among all human populations. According to the Out of Africa hypothesis, similarities among all living human populations result from a recent common origin. The compromise position accepts both of these as reasonable and compatible explanations of modern human origins.
