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Neandertals Were Not Close Relations, DNA Testing Finds

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Neandertals Were Not Close Relations, DNA Testing Finds

Bài gửi by Hasuongkch on Tue Nov 17, 2009 12:46 am

Neandertals Were Not Close Relations, Say DNA Tests
Recent groundbreaking genetic testing on the bone of a Neandertal indicated that these ancient humans were probably not close relations of modern humans. The following report on this investigation is from a July 1997 article in the Encarta Yearbook.


Neandertals Were Not Close Relations, DNA Testing Finds


In a milestone achievement for the study of ancient genetic material, German and American scientists extracted enough hereditary information from Neandertal bone to allow a comparison to the modern human genetic code.

The scientists' results, reported in the July 11, 1997, issue of the journal Cell, suggested that Neandertals are rather distant relations of modern humans and probably diverged from the lineage leading to modern humans about 550,000 to 690,000 years ago.

Neandertals were larger and more muscular than modern humans and are believed to have lived in Europe and western Asia from 300,000 years ago to as recently as 30,000 years ago. Archaic human beings (Homo sapiens) and modern human beings (Homo sapiens sapiens), who emerged at least 90,000 years ago, are believed to have coexisted with Neandertals (also called Homo sapiens neanderthalensis) in Europe. Whether and to what extent these two lineages intermingled has been a subject of debate.

The new genetic evidence supported the idea that Neandertals became extinct without interbreeding with modern humans. Researchers at the University of Munich in Germany and at Pennsylvania State University in State College were able to extract deoxyribonucleic acid (DNA), the chemical that encodes genetic information, from a Neandertal arm bone. This was the first time that DNA was successfully retrieved from a Neandertal fossil.

The bone sample was from the first Neandertal specimen ever found, which was uncovered in 1856 in the Neander Valley near Düsseldorf, Germany. Based on bone structure, this Neandertal fossil is believed to be between 30,000 and 100,000 years old.

The German and American scientists pulverized a small amount of the rare and valuable bone and were able to extract several small fragments of mitochondrial DNA. This type of DNA is located in the mitochondria of cells, structures that help generate energy for the cell. While one-half of the DNA stored in the nucleus of a cell is inherited from the mother and the other half from the father, in general mitochondrial DNA among mammals is inherited only from the mother.

Because mitochondrial DNA has a high mutation rate, it is very useful for comparing genetic similarities and differences over generations. However, experts cautioned that because this DNA is inherited only from the mother, it is possible that matings between Neandertal men and human women occurred. Such interbreedings would be undetectable in an examination of mitochondrial DNA.

By overlapping the small fragments of Neandertal DNA and using a technique known as polymerase chain reaction to make many copies of the molecules, the scientists were able to identify a sequence of 378 base pairs (chemicals that form the fundamental units of the genetic code) in a specific region of the Neandertal DNA. This area, called hypervariable region 1, is known to show changes over many generations. In general, the greater the dissimilarity in this region between two species, the more remote the relation is thought to be.

The researchers compared the Neandertal DNA sequence to sequences in the same region of DNA for 994 modern human lineages, which included Australians, Pacific Islanders, Africans, Asians, Native Americans, and Europeans. The Neandertal DNA sequence differed from all the modern human DNA by either 27 or 28 base pairs. In comparison, modern human sequences in this region of DNA differ from each other on average by 8 base pairs. As a result, the researchers concluded that Neandertals and modern humans are distant relations. Extrapolating from their findings, the study authors estimated that Neandertals split from early modern humans 550,000 to 690,000 years ago. The work indicates that although human beings and Neandertals coexisted for many thousands of years, they probably did not interbreed and Neandertals are not in our direct line of descent.

Source: Encarta Yearbook, July 1997.
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