New species in human’s family tree found

Published in The Hindu on March 25, 2010

A team of scientists has successfully sequenced mitochondrial DNA (mtDNA) of a hominin who had lived in the Altai Mountain region of Siberia, Russia. This hominin had shared a common ancestor with anatomically modern humans and Neanderthals about 1 million years ago. Further investigation reveals that the hominin lived close in time and space with both Neanderthals and modern humans.

The findings were based on a study of mitochondrial DNA that was extracted and sequenced from the fifth digit of the hominin that was found in 2008 in a Denisova Cave in the Altai Mountains.

The study is published today (March 25) in Nature journal.

Double the difference

mtDNA results show that the Denisova Cave individual differed from modern humans much more than what is seen between Neanderthals and modern humans. To be precise, the Denisova individual has about twice the number of mtDNA differences with modern humans compared with that of the Neanderthal and modern human. For the record, Neanderthal mtDNA differs from modern humans at an average 202 necleotide positions; the Denisova individual differs at about 385 positions.

The study is of great significance to fill the gaps in the evolutionary tree of humans. mtDNA data clearly shows that the Denisova hominin lineage branched off well before modern human could branch off from Neanderthal.

Branching off

How does one arrive at this conclusion? The difference in mtDNA positions between Denisova individual and modern humans is far greater compared with Neanderthal and modern human.

On an average, humans diverged from chimpanzee mtDNA about 6 million years ago. Using this data, the most recent date when the Denisova hominin, Neanderthal and modern human shared a common ancestor is about 1 million years ago. Hence, the divergence of Denisova hominin should have happened less than 1 million years ago.

Based on stratigraphy and other indirect evidences, the authors have indicated that the Denisova individual lived between 30,000 and 50,000 years ago. Individuals carrying Neanderthal mtDNA lived at the same time, some 100 km away.

Evidence of Homo floresiensis who lived in Indonesia about 17,000 years ago was shown by an earlier study. The Denisova individual from Siberia appears to have lived about 40,000 years ago. This indicates that several hominin lineages had coexisted for long periods of time.

It is a fact that hominin migration out of Africa happened not in one but several steps. The period when the first hominin — Homo erectus — left Africa was about 1.9 million years ago. Genetic and archaeological data strongly indicate that two other groups of hominin had left Africa after H. erectus.

These groups are the ancestors of Neanderthal that left between 5,00,000 and 3,00,000 years ago, and the second group being the anatomically modern humans that left about 50,000 years ago.

The group that left Africa between 5,00,000 and 3,00,000 years ago is presumed to be the Homo heidelbergensis or Homo rhodesiensis.

Hostile conditions

Several fossil remains of Neanderthal have been recovered and extensive mtDNA sequencing done. However, remains of other hominins have been scarce. That is because the environment in the equatorial and tropical regions is not conducive for long-term preservation of DNA in bones, teeth and other remains.

The recent extraction of both mtDNA and nuclear DNA from hair samples of a male palaeo-Eskimo found in permafrost deposits in Greenland raises the possibility of hair becoming a more resilient source to preserve DNA.

Whether they will be capable of preserving the DNA in hostile environments remains to be seen.