A few weeks ago an international group of scientists described a ground-breaking new method of sequencing DNA that makes it possible to gain as complete a genetic mapping of beings that lived tens of thousands of years ago as from saliva sampled from a modern human. The report, published in Science, outlines how the technique can be used to determine the small genetic mutations that set us apart from our closest ancestors or as team leader Svante Pääbo puts it “the genetic recipe for being a modern human”.
In 2010 the group presented results from DNA sequencing of two teeth and a finger bone uncovered in the Denisova cave in the Altai Mountains in Siberia.
The results showed that the bones had belonged to a girl of a previously unknown human species that was named Denisovans but the method of genetic mapping
was too crude to answer further questions.
Last week, the results of a three-year project to develop a new method of preparing DNA from fossils and applying that to the Denisovan girl’s remains were presented. German postdoc Matthias Meyer provided the breakthrough which entailed performing the sequencing process with single strands of DNA, instead of double strands as has previously been the norm, and thus doubling the amount of sequencing possible. Using the new approach the researchers were able to sequence every nucleotide position in the Denisovan genome about 30 times over, a 22-fold increase to previous attempts.
The high detail of the mapped Denisovan genome allowed the researchers to determine that the girl lived around 80,000 years ago and was brown-eyed with dark hair and skin, her species living in an area covering almost the whole of eastern Asia. This contrasts with their contemporary cousins the Neanderthals who were spread out over large parts of Europe and red-haired and light of skin. The results also show that modern humans interbred with the Denisovans at least once, the DNA evidence implicates a male Denisovan and a female modern human.
Most interesting is what the high quality of the genome mapping of Denisovans can tell us about what changed in modern humans. Our species started to diverge from the Denisovans in genetic terms around 800,000 years ago and among the genetic differences lies the answer to the very rapid development of modern human technology and culture. Among the roughly 100,000 single nucleotides that mutated in the modern human during the past 100,000 years or so, there are eight that specifically code for the synaptic wiring of the human brain. This is something we know since these genes have been previously implicated in mental disorders like autism.
That genes coding connectivity in the brain could be the answer ”makes sense” says Pääbo since, for example, Neanderthals had brains that were the same size as ours, and an even higher brain-to-body massratio.
Meyer M, Kircher M, Gansauge MT, Li H, Racimo F, Mallick S, Schraiber JG, Jay F, Prüfer K, de Filippo C, Sudmant PH, Alkan C, Fu Q, Do R, Rohland N, Tandon A, Siebauer M, Green RE, Bryc K, Briggs AW, Stenzel U, Dabney J, Shendure J, Kitzman J, Hammer MF, Shunkov MV, Derevianko AP, Patterson N, Andrés AM, Eichler EE, Slatkin M, Reich D, Kelso J, & Pääbo S (2012). A High-Coverage Genome Sequence from an Archaic Denisovan Individual. Science (New York, N.Y.) PMID: 22936568