Molecular biologists compare DNA to find differences, called genetic markers, which they can then use to calculate the genetic distance between two populations Cavalli-Sforza, In some cases, they can use the genetic markers to estimate how long it has been since the populations split into two groups. For an example of how comparison of DNA can be used, take the Neanderthals.
There have been many questions about how Neanderthals and Homo sapiens were related Cavalli-Sforza, ; Sykes, The team that analyzed the first Neanderthal genomes found that humans and Neanderthals share about The results showed clearly that the Neanderthals were not our direct ancestors Cavalli-Sforza, The findings of this team suggested that the ancestors of Homo sapiens including Neanderthals and chimpanzees diverged about 6.
A decade ago, three teams, led by Cavalli-Sforza, by Sykes and by Wells, reported on years of work looking at relationships among human populations. In , Cavalli-Sforza summarized research conducted over 40 years. His team analyzed gene frequencies on blood system groups and proteins from samples of members of about 2, aboriginal populations throughout the world. He concluded that all modern humans are members of one species which originated in Africa.
He calculated that some modern humans left Africa between 80, and 50, years ago and reached Australia by 50, years ago. His model of human migration from Africa suggests that after Australia, humans settled Asia, then Europe, and finally, via East Asia, the Americas Cavalli-Sforza, In , Sykes summarized 25 years of work on mitochondrial DNA mtDNA , which has the advantage of being passed intact, directly from mother to child. A mutation occurs in mtDNA about once every 10, years, which enables scientists to estimate how long ago two lines shared a common maternal ancestor Sykes, After analyzing thousands of samples from all over Europe, Sykes found that nearly all native Europeans could trace their maternal ancestry to one of just seven different women, who lived between 45, and 10, years ago.
Sykes explained that each of these women was certainly not the only woman living in her hunting and gathering band at the time. But if a woman has no children or only boys, her mtDNA line dies out.
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Using mtDNA sequences from all over the world, Sykes identified 33 women who are the mothers of all people living today, and 13 of them originated in Africa. His findings suggest that a small group from one of the African mtDNA clans left Africa to populate the rest of the world. The 20 women whose mtDNA clans are found in the rest of the world are all descendants of that single woman whose descendants left Africa. Sykes writes, Incredibly, even though the African clans are easily the most ancient in the world, we are still able to reconstruct the genetic relationships among them.
One by one the clans converge until there is only one ancestor, the mother of all of Africa and of the rest of the world. The Y-chromosome is passed down from fathers to sons just as mtDNA is passed down from mothers to daughters. As with maternal lineages, if a man has no children or no sons, his Y-chromosome lineage is lost. Wells concluded that all modern humans originated in Africa. While there were other men alive then, their lineages have died out. As a result, scientists could trace the maternal line much further back into prehistory , years than they could trace the paternal line of all people living today.
Wells explained that the greater the diversity, as shown by the accumulation of additional genetic markers, the older the lineage. He emphasized that the real surprise is that as all modern humans lived in Africa until at least 60, years ago, our species has only had 60, years to populate the globe Wells, Wells divided the descendants of men who left Africa into a genealogical tree with 11 lineages.
Each genetic marker represents a single-point mutation SNP at a specific place in the genome. First, genetic evidence suggests that a small band with the marker M migrated out of Africa along the coasts of the Arabian Peninsula and India, through Indonesia, and reached Australia very early, between 60, and 50, years ago. Second, a group bearing the marker M89 moved out of northeastern Africa into the Middle East 45, years ago.
From there, the M89 group split into two groups. One group that developed the marker M9 went into Asia about 40, years ago. While the earlier studies compared frequencies of gene alleles for blood proteins, or genetic markers in parts of the mtDNA or the Y-chromosome, second generation sequencing now enables teams to compare whole genomes.
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The genomes of members of 51 populations were compared with each other and with chimpanzees. Australian aboriginal populations were not studied. Their study fully supports the recent African origin model. Recent work with second generation sequencing and new statistical modeling methods adds much finer detail on migration and admixture of populations. For example, in Europe, Stewart and Stringer show a cyclical pattern of interglacial expansion of Homo sapiens out of Africa and retreat back to Africa or warmer refuges during ice ages.
They showed that populations of Homo sapiens moved out of Africa during warm interglacial periods, then retreated from Europe and Asia into southern areas like Spain, Italy, the Balkans, and the Caspian Sea during the last ice age about 27,, years ago , and then expanded again to repopulate Europe after the climate warmed. The hunter gatherers then expanded to resettle Central and Western Europe after the end of the last ice age. The Yamnaya culture had horses, wheeled vehicles, and carried the gene for lactase persistence.
Much of northern India also speaks Indo-European languages. In the first major work on population history in India, Reich, Thangaraj, Patterson, Price, and Singh used second generation sequencing technology and statistical modeling to compare the genomes of 25 diverse groups from India to the genomes of people from Western Asia, Europe, and the Andaman Islands who are thought to be closest to the ancestral populations of southern India.
Using admixture modeling, they found that different Indian groups inherited different proportions of ancestry from the ANI who are related to western Eurasians, and the ASI, who are related to the Andaman Islanders p. This parallels the language data—Northern Indian groups speak Indo-European languages, while Southern Indian groups speak Dravidian languages.
Thus, the work of teams examining blood group allele frequencies, mtDNA, Y-chromosome DNA, and whole genomes, all shows the same pattern. By comparing the DNA of people from around the world, scientists can determine where their ancestors came from and approximately when migrations took place.
We are all Africans, no matter the color of our skin. The sixth argument against race as biology is that all humans living today are one biological species. In other words, one criterion that defines populations as part of the same species is that they can mate and reproduce successfully. By that criterion, it is clear that all humans today are part of the same species.
Different species, for example, fox and gray squirrels east of the Mississippi River in the U. All humans, from any population group, can and do mate and reproduce successfully unlike, for example, horses and donkeys, which can mate, but produce mules, which are infertile. This suggests that the idea of race had a social genesis tied to the justification of slavery.
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However, even during the time of Linnaeus, there were some authors, e. Since then, despite the fact that humans may look very different from each other on the surface, scientists have found that under the skin, we are all alike. In summary, there are a number of evidence-based reasons why race is not biological. First, there are no objective, identifiable traits by which people can be reliably divided into racial groups.
Second, skin color, which is the primary basis of folk conceptions of race in the United States, has been shown to be the result of natural selection; it is a result of the interaction between geographical environment and random mutations in genes that affect the biosynthesis of melanin in the skin.
Third, visible traits like skin color do not correlate with traits like blood groups, much less with complex traits like intelligence, athletic ability, or musical ability. Fourth, another way to show that skin color does not correlate with other human traits is to statistically compare within-population variation versus between-region variation.
Fifth, by comparing the genetic markers mutations or SNPs of people in populations all over the world, scientists can determine population expansion patterns of the past. Furthermore, all human populations outside of Africa are descended from one hunting and gathering band that migrated from Eastern Africa, probably about 60, years ago. Finally, because members of all human populations living today can successfully interbreed, by definition, humans constitute one species.
Although some populations lived long enough in certain geographic environments for natural selection to alter characteristics like skin color, these populations were not isolated long enough to become new species.
There is a great deal of both prehistoric and historic evidence for extensive global trading routes, which led to people from different groups meeting and mating over thousands of years of human history Fuentes, Many students in the United States equate race with skin color. The idea that race is not skin color, or a set of discrete biological types, but instead is socially constructed, is one of the most important ideas of our time.
Omi and Winant conclude as follows: Race is not something rooted in nature, something that reflects clear and discrete variations in human identity. But race is also not an illusion. It requires a paradigm shift. While this paradigm shift will not be sufficient to eliminate racism in our time, it is surely a necessary step in the right direction. When I used these six scientific arguments as to why race is not biological to teach my students, I found that more of them accepted the idea that race is socially constructed.
I suspect that I am not the only social scientist teaching diversity classes whose teaching may benefit from increased biological science literacy. Understanding how 21st century science supports arguments against race-as-biology will help educators assist students to make the necessary paradigm shift. Thanks to David Stoesz and Jim Bonacum who thoughtfully reviewed the manuscript prior to submission. Kay Young McChesney has an interdisciplinary background in psychology, sociology and social work.
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Race, genetics, and the lure of forbidden knowledge – Work in Progress
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