Genome sequence of 50,000 year old finger bone connect to present-day human

The genome of the bone found in Denisova cave in 2008 in Southern Siberia is now sequenced. The Denisova bone belonged to a female and is considered to be 30000-50,000 years old. The results published in the scholarly journal Nature show that the individual was part of a population long diverged from humans and Neanderthals.

The scientists at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany sequenced the genetic material extracted from the finger bone found in Denisova. The sequencing of the nuclear genome from the ancient finger bone shows that the cave dwellers were neither Neandertals nor modern humans, but the individual is from a group that shares a common origin with Neanderthals. The data also suggest that it contributed 4-6% of its genetic material to the genomes of present-day Melanesians.

The authors designated this homin population 'Denisovans' after the cave where the bone was found and further suggest that this population may have been widespread in Asia during the Late Pleistocene epoch.

The mitochondrial DNA sequence of a tooth obtained from the Denisova cave in 2000 was also sequenced and compared with the mitochondrial DNA of the sample from which the genomic DNA was sequenced. The report says that this mitochondrial "sequence differs at two positions from the mitochondrial DNA of the phalanx whereas it differs at about 380 positions from both Neanderthal and present-day humans. The time since the most recent common ancestor of the two mitochondrial DNAs from Denisova Cave is estimated to be 7,500 years, with a 95% upper bound of 16,000 years."

Mitochondrial vs Nuclear DNA: The main genetic information in an individual is embedded in the genetic material within the nucleus of the cells; this complex material made up of four different nucleotides (represented by the letters A,C,G,T) is the nuclear DNA. This is also referred as the genome of the individual. However outside of the nucleus, in the organelle called mitochondria, there exists a short circular DNA called mitochondrial DNA. Nuclear DNA encodes thousands of proteins, but mitochondrial DNA encodes only 13 proteins and 16 other transcripts. Mitochondrial DNA is transferred from mother to the offspring, where as there is a shared transfer of nuclear DNA from parents to offspring.

According to the authors, the results show that on the Eurasian mainland there existed at least two forms of archaic hominins in the Upper Pleistocene: a western Eurasian form with morphological features that are commonly used to define them as Neanderthals, and an eastern form to which the Denisova individuals belong.

In an associated write-up in Nature, Carlos Bustamante and Brenna Henn of Stanford University School of Medicine write "what is particularly fascinating, however, is that the Denisovan sample seems to share an extra genetic affinity (beyond that for European and Asian genomes) with present-day island Melanesians. This is rather unexpected, as the earliest occupation of Papua New Guinea, an island in Oceania, by modern humans occurred only about 45,000 years ago, and suggests quite a complicated picture for the ancestry of the Denisovan finger fragment."

David Reich, an Associate Professor at Harvard Medical School who led the population genetic analysis, says: "The fact that Denisovans were discovered in Southern Siberia but contributed genetic material to modern human populations from New Guinea suggests that Denisovans may have been widespread in Asia during the Late Pleistocene." Svante Paabo of the Max Planck Institute of evolutionary Anthropology remarks: "In combination with the Neandertal genome sequence, the Denisovan genome suggests a complex picture of genetic interactions between our ancestors and different ancient hominin groups".

The results are a testament to the fact that our origins and early migrations still remain as ever-evolving unconnected dots.

Ref: 1. Reich et al. Genetic history of an archaic hominin group from Denisova Cave in Siberia Nature December 22, 2010 doi:10.1038/nature09710
2. Bustamante CD and Henn BM. Human origins: Shadows of early migrations. Nature 468, 1044-1045 23 December 2010, doi:10.1038/4681044a