the north face tent mules ‘Better’ DNA out of fossil bones
Improved technologies for extracting genetic material from fossils may help us find out more about our ancient ancestors.
Scientists in Israel have just developed a new technique to retrieve better quality, less contaminated DNA from very old remains, including human bones.
It could aid the study of the evolution and migration of early modern humans, as well as extinct populations such as our close relatives, the Neanderthals.
Many researchers would dearly love to get their hands on DNA samples from hominids further back in time from those that lived 100,000 years ago or more to find out how they were related to people alive today.
But fossil studies this far back in time have long been hindered by contamination with foreign genetic material and the problem of recovering long, intact DNA sequences.
The new method provides hope, however.
“DNA gets everywhere. So when we’re dealing with a sample and you find it’s got human DNA in it is that DNA from the fossil, or is it actually DNA from the person who unearthed it?” says Professor Chris Stringer, the head of human origins at the Natural History Museum in London, UK.
It is wound up in bundles known as chromosomes that are found in the cell nucleus (nuclear DNA); DNA also lies within mitochondria outside of the nucleus (mtDNA)
mtDNA is inherited only through females via the egg and can be used to trace backwards through evolution; the male sex chromosome (Y) similarly tracks male evolution
DNA breaks down over time making recovery difficult from ancient specimens. Fossils are often contaminated with modern human DNA during handling, and it is difficult to tell this apart from ancient DNA
Also, DNA falls apart over the course of time.
“It breaks up into very small fragments so it is quite technically complicated to put it all back together again,” explains Dr Robert Foley, the director of the Leverhulme Centre for Human Evolutionary Studies at the University of Cambridge, UK.
Freezing provides the ideal preservation conditions. The most widely accepted oldest DNA yet isolated comes from 400,000 year old plants found in ice in Siberia. But most specimens are not excavated from such places.
An improved technique for retrieving DNA from fossil bone, just published in the journal Proceedings of the National Academy of Sciences (PNAS), may help.
Dr Michal Salamon, from the Weizmann Institute of Science in Rehovot, Israel, and colleagues, showed that “crystal aggregates”, small mineral pockets formed during fossilisation, can preserve DNA better than the rest of the bone.
They compared DNA extracted from these crystal aggregates with genetic material taken from untreated, whole bone powder. The samples were taken from eight different modern and fossil bones.
They found better preserved, less contaminated DNA could be recovered from the isolated crystals.
This approach, “significantly improves the chances of obtaining authentic ancient DNA sequences, especially from human bones”, they told PNAS.
Commenting on the latest research, Dr Michael Hofreiter, from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who helped decode 40,000 year old nuclear DNA from a cave bear earlier this year, said: “It’s possible; but there need to be more studies on more samples, and they need to show that you don’t get human contamination of animal bones.
“Then I would believe that it is a breakthrough for ancient DNA research.”
Scientists are hopeful the new technique will help them get at the DNA in the chromosomes of a cell the nuclear DNA.
Ancient DNA research has so far mainly focused on mitochondria, the tiny “power stations” of the cell. These exist outside of the nucleus and have their own DNA. And, although this information is very useful, it is more limited in its scope than that which could be obtained from nuclear DNA.
It is partly a question of sensitivity.
“There’s about 1,000 times more mitochondrial DNA than nuclear DNA in our cells, so it’s much easier to pick up,” explains Professor Stringer.
The mitochondrial DNA is inherited only through the egg through females. This means it is a useful marker for tracing a line back into the past, as it has never been mixed with DNA from males.
“One of the most important discoveries from studying ancient mitochondrial DNA is the estimate of when humans diverged in evolution from the Neanderthals around half a million years ago,” according to Dr Foley.
Professor Stringer adds: “We’ve now got about 10 Neanderthal specimens of around 40 50,000 years old that have yielded DNA that is clearly distinct from anyone alive today.”