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New ‘molecular clock’ aids dating of human migration history

Researchers at the University of Leeds have devised a more accurate method of dating ancient human migration – even when no corroborating archaeological evidence exists.

Estimating the chronology of population migrations throughout mankind's early history has always been problematic. The most widely used genetic method works back to find the last common ancestor of any particular set of lineages using samples of mitochondrial DNA (mtDNA), but this method has recently been shown to be unreliable, throwing 20 years of research into doubt.

The new method refines the mtDNA calculation by taking into account the process of natural selection - which researchers realised was skewing their results - and  has been tested successfully against known colonisation dates confirmed by archaeological evidence, such as in Polynesia in the Pacific (approximately 3,000 years ago), and the Canary Islands (approximately 2,500 years ago).

Says PhD student Pedro Soares who devised the new method: "Natural selection's very gradual removal of harmful gene mutations in the mtDNA produces a time-dependent effect on how many mutations you see in the family tree.  What we've done is work out a formula that corrects this effect so that we now have a reliable way of dating genetic lineages.

"This means that we can put a timescale on any part of the particular family tree, right back to humanity's last common maternal ancestor, known as 'Mitochondrial Eve', who lived some 200,000 years ago. In fact we can date any migration for which we have available data," he says.

Moreover, working with a published database of more than 2,000 fully sequenced mtDNA samples, Soares' calculation, for the first time, uses data from the whole of the mtDNA molecule. This means that the results are not only more accurate, but also more precise, giving narrower date ranges.

The new method has already yielded some surprising findings. Says archaogeneticist Professor Martin Richards, who supervised Soares: "We can settle the debate regarding mankind's expansion through the Americas. Researchers have been estimating dates from mtDNA that are too old for the archaeological evidence, but our calculations confirm the date to be some 15,000 years ago, around the time of the first unequivocal archaeological remains. 

"Furthermore, we can say with some confidence that the estimate of humanity's 'out of Africa' migration was around 60-70,000 years ago - some 10-20,000 years earlier than previously thought."

The team has devised a simple calculator into which researchers can feed their data and this is being made freely available on the University of Leeds website.

The paper is published in the current edition of The American Journal of Human Genetics.

Further information:

Jo Kelly, campuspr Ltd, tel 0113 258 9880, mob 07980 267756, email jokelly@campuspr.co.uk

Guy Dixon, University of Leeds press office, tel 0113 343 8299, email g.dixon@leeds.ac.uk

Notes to editors:

1.     The paper, titled "Correcting for purifying selection: An improved human mitochondrial molecular clock", is published in the current edition of The American Journal of Human Genetics. A copy of the paper is available on request - please contact Cathleen Genova at cgenova@cell.com or tel 1-617-397-2802.

2.        The research was funded by the European Union Marie Curie Early Stage Training program and the Bradshaw Foundation.

3.     Martin Richards is the UK's only Professor of Archaeogenetics and is based at the Institute of Integrative and Comparative Biology at the University of Leeds. Archaeogenetics is the use of genetic data to address questions concerning archaeology and human evolution. Professor Richards' research has focused on studies of mitochondrial DNA and the Y chromosome, and how results of these studies can inform our knowledge of prehistoric patterns of migration.

4.     The Faculty of Biological Sciences at the University of Leeds is one of the largest in the UK, with over 150 academic staff and over 400 postdoctoral fellows and postgraduate students. The Faculty is ranked 4th in the UK (Nature Journal, 457 (2009) doi:10.1038/457013a) based on results of the 2008 Research Assessment Exercise (RAE).  The RAE feedback noted that "virtually all outputs were assessed as being recognized internationally, with many (60%) being internationally excellent or world-leading" in quality.    The Faculty's research grant portfolio totals some £60M and funders include charities, research councils, the European Union and industry.  www.fbs.leeds.ac.uk

The University of Leeds is one of the largest higher education institutions in the UK with more than 30,000 students from 130 countries and a turnover of £450m. The University is a member of the Russell Group of research-intensive universities and the 2008 Research Assessment Exercise showed it to be among the top UK research powerhouses. The University's vision is to secure a place among the world's top 50 by 2015. www.leeds.ac.uk