August 31, 2012
Researchers Sequence Genome Of Ancient Denisovan Using Tiny Finger Bone Fragment
Watch the Video: Mysterious Hominids Discovered In Denisova Cave
April Flowers for redOrbit.com - Your Universe Online
A tiny bit of a finger bone, found in a Siberian cave, is shedding light on a rather enigmatic group of early humans called the Denisovans.
The 80,000 year-old finger bone, along with two molars, were found in 2010 at the Denisova Cave in southern Siberia's Altai Mountains. Other than the fact that they were contemporaries of Neanderthals, another extinct human "cousin", nothing was known about the Denisovans; not what they looked like, or even what happened to them.
A new study by a team of international scientists, published online in this week's edition of Science, sheds light on the Denisovans and their relationship to modern humans.
The genetic sample was so small that Svante Paabo of the Max Planck Institute for Evolutionary Anthropology and his research teach had to develop a new way of teasing information from it. They created a treatment that unzipped the DNA so that each of its two strands can be used to generate molecules for sequencing. This method allowed the team to generate an extremely thorough genome sequence (30X), similar in quality to what researchers can obtain for the modern human genome.
They found that the finger belonged to a young girl and that her genetic makeup did not contain much genetic spread. In other words, the Denisovans were a very tightly knit group without much outside genetic material.
"This is an extinct genome sequence of unprecedented accuracy", says Matthias Meyer, the lead author of the study. "For most of the genome we can even determine the differences between the two sets of chromosomes that the Denisovan girl inherited from her mother and father".
From this the researchers can tell that genetic variation of the Denisovans was lower than in present-day humans. This is likely due to an initially small Denisovan population that grew quickly while spreading over a wide geographic range.
"If future research of the Neanderthal genome shows that their population size changed over time in similar ways, it may well be that a single population expanding out of Africa gave rise to both the Denisovans and the Neanderthals", says Svante Paabo, who led the study.
According to the AFP news agency, the team then compared the girl's genome with that of Neanderthals and 11 modern humans from around the world. The results suggest that the Denisovans may have spread quite widely to parts of Asia, most especially the islands of Southeast Asia like Papua New Guinea. There were also genetic connections to the Melanesians and Australian Aborigines.
"We find no trace of Denisovan genetic material in mainland Eurasia, including in mainland Southeast Asia, to the limits of our resolution," says study co-author David Reich of Harvard Medical School. "However, it's clear that Denisovan genetic material has contributed 3 percent to 5 percent of the genomes of people in Australia and New Guinea and aboriginal people from the Philippines and some of the island nearby."
Certain genetic markers that the Denisovans and modern humans shared shed light on how they probably looked as well. The Denisovans most likely had dark skin, brown hair and brown eyes like the modern day native populations of those islands.
The research team did not focus solely on the similarities, however. They generated a list of approximately 100,000 changes in the Homo Sapiens genome since the time of the Denisovans. Most of these changes are associated with brain function and the development of the nervous system, while some lesser changes involve the skin, the eye and the shape of teeth.
"This research will help in determining how it was that modern human populations came to expand dramatically in size as well as cultural complexity, while archaic humans eventually dwindled in numbers and became physically extinct", says Svante Paabo.
Chris Stringer of London's Natural History Museum, author of "Lone Survivors: How We Came To Be the Only Humans on Earth," was asked by Dan Vergano of USA TODAY to comment on the findings. He said that the low genetic diversity was “surprising for two reasons: the mitochondrial DNA previously obtained from 3 Denisovan fossils (the finger bone and two molar teeth) had suggested relatively high diversity, and the implication that the Denisovans were widespread in Asia led to the expectation of a large and diverse population."
"Another surprise comes from using the higher quality Denisovan genome to take a sideways look at Neanderthal as well Denisovan interbreeding with modern humans. Previous research suggested that all recent populations originating from outside of Africa had received about the same amount of Neanderthal input, implying a single early hybridization event somewhere like the Middle East," Stringer continued. "But the new study allows separate estimates of Neanderthal and Denisovan input to the 23 human chromosomes in populations outside of Africa, showing considerable variation between them, but suggesting that overall Europeans have less Neanderthal DNA than populations to the east (averaging about 1% vs. 1.7%). This might imply changes in the proportions after interbreeding took place, or that there was more than one interbreeding event."