Jan. 4, 2013
Medical Discovery News
The words screamed off the page: “Genomic Sequencing Brings Ancient Girl to Life!” While such headlines bring to mind reanimated mummies, they actually referred to the successful extraction and complete genomic sequencing of a young girl who lived 80,000 years ago. This is the first time the quality of the genomic sequence of any ancient species has rivaled that of the living people today.
This study pioneered a new approach in DNA sequencing. DNA, while known to be hardy, cannot usually be extracted in the double stranded form from fossils like it can be from living organisms today. Researchers used a small piece of the knuckle of her pinky finger to extract the girl’s ancient DNA.
Researchers developed an effective strategy utilizing the single-stranded DNA fragments they extracted. This new approach generated up to 20 times more readable DNA sequences than previous methods could have. In the end, researchers put together a high-resolution genomic sequence.
The quality of the sequence amazed scientists because it matches the resolution of genome sequencing from living organisms today. For example, they determined that the girl had brown skin, hair, and eyes.
The girl’s sequence holds important hints of human evolution. She belonged to a lesser-known sister species to Neanderthals called Denisovans, which have only been found in Southern Siberia. It was obvious that she had 23 pairs of chromosomes, the same as modern humans. Chimpanzees have 24 chromosome pairs, so Denisovans were definitely more human than ape-like.
By aligning differences in the DNA sequences, scientists estimated that the Denisovans split from humans between 170,000 and 700,000 years ago. When compared to chimpanzee sequences, there were fewer differences than those between modern humans and chimpanzees. Due to the new sequencing technique, scientists can more accurately date when a fossilized person or animal lived and died. Such advances may alter the time frame of human and animal evolution.
Other recent studies indicate 1 to 4 percent of European and Asian DNA came from Neanderthals. Since Africans have no Neanderthal DNA, interbreeding between Neanderthals and humans occurred after humans left Africa and migrated to Eurasia. Only humans from Papua, New Guinea, shared DNA with Denisovans, but a trace amount of similarity was seen in the genomes of the Han and Dai peoples in mainland China. Scientists don’t yet know the connection between these groups.
Overall, researchers documented more than 100,000 DNA sequence changes between Denisovans and modern humans. Of high interest were eight changes in genes that play a role in the “wiring” of the human nervous system, including one linked to autism and one associated with speech defects, raising questions about the speech and mental capabilities of these early ancestors. Scientists will continue to learn more about early humans and their predecessors using this new approach to DNA sequencing as a “molecular time machine.”
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Pamela K. Bond 