Human Anatomy & Physiology (11th Edition)
Human Anatomy & Physiology (11th Edition)
11th Edition
ISBN: 9780134580999
Author: Elaine N. Marieb, Katja N. Hoehn
Publisher: PEARSON
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  1. Compare and contrast the multiregional and replacement models for human origins in the table below. 

 

If you are struggling with this, see the PowerPoint on the origins and dispersal of modern humans and review the tutorial: https://www2.palomar.edu/anthro/homo2/mod_homo_4.htm

 

 

 

Multiregional Model

Replacement

Main argument of this model

 

 

 

Evidence that supports this model

 

 

 

  1. Based on the evidence, which model of human origins is best supported?

 

Replacement Model Arguments
There are two sources of evidence supporting the replacement model--the fossil record and DNA. So far, the earliest finds of modern Homo sapiens skeletons come
from Africa. They date to nearly 200,000 years ago on that continent. They appear in Southwest Asia around 100,000 years ago and elsewhere in the Old World by
60,000-40,000 years ago. Unless modern human remains dating to 200,000 years ago or earlier are found in Europe or East Asia, it would seem that the
replacement model better explains the fossil data for those regions. However, the DNA data supporting a replacement are more problematical.
Beginning in the 1980's, Rebecca Cann, at the University of California, argued that the geographic region in which modern people have lived the longest should have
the greatest amount of genetic diversity today. Through comparisons of mitochondrial DNA sequences from living people throughout the world, she concluded that
Africa has the greatest genetic diversity and, therefore, must be the homeland of all modern humans. Assuming a specific, constant rate of mutation, she further
concluded that the common ancestor of modern people was a woman living about 200,000 years ago in Africa. This supposed predecessor was dubbed
"mitochondrial Eve". More recent genetic research at the University of Chicago and Yale University lends support to the replacement model. It has shown that
variations in the DNA of the Y chromosome and chromosome 12 also have the greatest diversity among Africans today. John Relethford and other critics of the
replacement model have pointed out that Africa could have had the greatest diversity in DNA simply because there were more people living there during the last
several hundred thousand years. This would leave open the possibility that Africa was not necessarily the only homeland of modern humans.
Critics of the genetic argument for the replacement model also point out that the rate of mutation used for the "molecular clock" is not necessarily constant, which
makes the 200,000 year date for "mitochondrial Eve" unreliable. The rate of inheritable mutations for a species or a population can vary due to a number of factors
including generation time, the efficiency of DNA repair within cells, ambient temperature, and varying amounts of natural environmental mutagens. In addition, some
kinds of DNA molecules are known to be more subject to mutation than others, resulting in faster mutation rates. This seems to be the case with the Y chromosome
in human males.
Further criticism of the genetic argument for the replacement model has come from geneticists at Oxford University. They found that the human betaglobin gene is
widely distributed in Asia but not in Africa. Since this gene is thought to have originated more than 200,000 years ago, it undercuts the claim that an African
population of modern Homo sapiens replaced East Asian archaic humans less than 60,000 years ago.
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Transcribed Image Text:Replacement Model Arguments There are two sources of evidence supporting the replacement model--the fossil record and DNA. So far, the earliest finds of modern Homo sapiens skeletons come from Africa. They date to nearly 200,000 years ago on that continent. They appear in Southwest Asia around 100,000 years ago and elsewhere in the Old World by 60,000-40,000 years ago. Unless modern human remains dating to 200,000 years ago or earlier are found in Europe or East Asia, it would seem that the replacement model better explains the fossil data for those regions. However, the DNA data supporting a replacement are more problematical. Beginning in the 1980's, Rebecca Cann, at the University of California, argued that the geographic region in which modern people have lived the longest should have the greatest amount of genetic diversity today. Through comparisons of mitochondrial DNA sequences from living people throughout the world, she concluded that Africa has the greatest genetic diversity and, therefore, must be the homeland of all modern humans. Assuming a specific, constant rate of mutation, she further concluded that the common ancestor of modern people was a woman living about 200,000 years ago in Africa. This supposed predecessor was dubbed "mitochondrial Eve". More recent genetic research at the University of Chicago and Yale University lends support to the replacement model. It has shown that variations in the DNA of the Y chromosome and chromosome 12 also have the greatest diversity among Africans today. John Relethford and other critics of the replacement model have pointed out that Africa could have had the greatest diversity in DNA simply because there were more people living there during the last several hundred thousand years. This would leave open the possibility that Africa was not necessarily the only homeland of modern humans. Critics of the genetic argument for the replacement model also point out that the rate of mutation used for the "molecular clock" is not necessarily constant, which makes the 200,000 year date for "mitochondrial Eve" unreliable. The rate of inheritable mutations for a species or a population can vary due to a number of factors including generation time, the efficiency of DNA repair within cells, ambient temperature, and varying amounts of natural environmental mutagens. In addition, some kinds of DNA molecules are known to be more subject to mutation than others, resulting in faster mutation rates. This seems to be the case with the Y chromosome in human males. Further criticism of the genetic argument for the replacement model has come from geneticists at Oxford University. They found that the human betaglobin gene is widely distributed in Asia but not in Africa. Since this gene is thought to have originated more than 200,000 years ago, it undercuts the claim that an African population of modern Homo sapiens replaced East Asian archaic humans less than 60,000 years ago.
Regional Continuity Model Arguments
Fossil evidence also is used to support the regional continuity model. Its advocates claim that there has been a continuity of some anatomical traits from archaic
humans to modern humans in Europe and Asia. In other words, the Asian and European physical characteristics have antiquity in these regions going back over
100,000 years. They point to the fact that many Europeans have relatively heavy brow ridges and a high angle of their noses reminiscent of Neandertals. Similarly,
it is claimed that some Chinese facial characteristics can be seen in an Asian archaic human fossil from Jinniushan dating to 200,000 years ago. Like Homo erectus,
East Asians today commonly have shovel-shaped incisors while Africans and Europeans rarely do. This supports the contention of direct genetic links between
Asian Homo erectus and modern Asians. Alan Thorne of the Australian National University believes that Australian aborigines share key skeletal and dental traits
with pre-modern people who inhabited Indonesia at least 100,000 years ago. The implication is that there was no replacement by modern humans from Africa
60,000-40,000 years ago. However, the evidence does not rule out gene flow from African populations to Europe and Asia at that time and before. David Frayer, of
the University of Kansas, believes that a number of European fossils from the last 50,000 years have characteristics that are the result of archaic and modern
humans interbreeding.
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Transcribed Image Text:Regional Continuity Model Arguments Fossil evidence also is used to support the regional continuity model. Its advocates claim that there has been a continuity of some anatomical traits from archaic humans to modern humans in Europe and Asia. In other words, the Asian and European physical characteristics have antiquity in these regions going back over 100,000 years. They point to the fact that many Europeans have relatively heavy brow ridges and a high angle of their noses reminiscent of Neandertals. Similarly, it is claimed that some Chinese facial characteristics can be seen in an Asian archaic human fossil from Jinniushan dating to 200,000 years ago. Like Homo erectus, East Asians today commonly have shovel-shaped incisors while Africans and Europeans rarely do. This supports the contention of direct genetic links between Asian Homo erectus and modern Asians. Alan Thorne of the Australian National University believes that Australian aborigines share key skeletal and dental traits with pre-modern people who inhabited Indonesia at least 100,000 years ago. The implication is that there was no replacement by modern humans from Africa 60,000-40,000 years ago. However, the evidence does not rule out gene flow from African populations to Europe and Asia at that time and before. David Frayer, of the University of Kansas, believes that a number of European fossils from the last 50,000 years have characteristics that are the result of archaic and modern humans interbreeding.
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