Unlocking the secrets of human evolution

Published in The Hindu on February 19, 2009

Neanderthal
The mitochondrial DNA revealed that Neanderthals were our closest relatives and that the two shared a common ancestor.– Photo: Wikimedia Commons

Preliminary study of the Neanderthal genome reveals many thousands of differences between the two closest relatives

The genetic makeup of chimpanzees and modern humans differ by only 1 per cent. And in the case of Neanderthals, the difference is as small as 0.5 per cent.

The rough draft of the Neanderthal genome sequence completed by Svante Paabo and his team from the Max Plank Institute for Evolutionary Anthropology in Leipzig, Germany, would allow us to find out what the 0.5 per cent difference means to the crucial evolutionary changes that make us unique.

Not our ancestors

Studying the DNA found in the mitochondria of Neanderthals, Dr. Paabo in 1997 found no evidence of Neanderthals being our ancestors. On the contrary, the mitochondrial DNA revealed that Neanderthals were our closest relatives and that the two shared a common ancestor.

They found that the two split some 6,00,000 years ago and evolved separately.

Mitochondrial DNA lies outside the nucleus of a cell and it tends to be preserved longer than nuclear DNA. Yet mitochondrial DNA can provide only limited information.

A vast majority of the genome is comprised of nuclear DNA. And nuclear DNA contains all the genes. A wealth of information, and in this case the differences between the two species, can be unravelled only by studying the nuclear DNA.

Good way to start

Whith very little DNA that they could extract, Dr. Paabo and his team sequenced 60 per cent of the Neanderthal’s genome. Though it is not a full genome of Neanderthals, it is a good way to start understanding the differences.

Earlier studies have shown that Neanderthals and modern humans have the same gene (FOXP2) that is linked to language ability.

They also found that a gene that makes human adults intolerant to lactose found in milk is present in Neanderthals as well.

Since we already have the human genome sequence, it would be easy to see if the genes linked to cognition are found in our closest relatives.

Preliminary results

Dr. Paabo’s preliminary study of the genome of the 38,000-year-old samples has found that there are indeed many thousands of differences between the two closest relatives (Science, February 13, 2009).

In the absence of a Neanderthal genome, scientists had so far resorted to identifying a human gene of interest and tried to see if it was present in Neanderthals as well. They did find some differences and similarities.

But with the genome now sequenced, scientists would be interested in scanning the “entire genetic landscape for species-defining variants” (Science) In other words, the whole-genome approach does not concentrate on specific sequences.

Contentious issue

The Neanderthal genome would help answer one of the most contentious issue — did the Neanderthals and modern humans interbreed as the two coexisted in Europe for many thousands of years.

If they did, then there should have been more gene flow between Europeans and Neanderthals, and the genomes of Africans and Europeans should be different. Neanderthals were restricted to Europe and parts of Asia. No fossil has been found in Africa

Results of the mitochondrial DNA revealed that the two closest relatives did not interbreed. The whole genome would be able to give a more definite answer.

But before we start comparing the genomes of Neanderthal and humans, it would be essential to sequence the Neanderthal genome from more samples and to make sure that the first draft of Dr. Paabo is indeed accurate.

Contamination

That is easier said than done. If getting hold of Neanderthal samples is difficult, making sure that scientists are able to extract the genetic material that can be studied is all the more challenging.

The samples that Dr. Paabo studied were taken from bones that are 38,000 years old. Chemical changes, degradation and contamination by microbes make most of the genetic material nearly useless. Even in this case Dr. Paabo could extract just about 4 per cent of Neanderthal genetic material.

Innovative ways

Despite the limitations, scientists are finding some innovative ways of sequencing ancient DNA.

In 2005, Edward Rubin and others from the U.S. Department of Energy Joint Genome Institute and Dr. Paabo used an approach originally developed to sequence all microbial DNA found in the atmosphere. They succeeded in sequencing 27,000 bases using this approach.

According to a paper in Science (February 13, 2009), scientists from McMaster University, Canada used a next-generation technique to sequence 28 million Neanderthals bases.

Dr. Paabo has now used a still better technique to sequence the samples recovered from a cave in Croatia.

The samples studied are one of the recent ones. Older samples would require much more sophisticated or improvised sequencing techniques.

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