Modern Britons are a cosmopolitan bunch. Peoples from across the globe now make the island home, bringing with them, theoretically, a diverse array of genes. But comparing the genetic material of more than 1,000 contemporary Englishmen with that of 48 of their ancient peers reveals that the ancients had even more diverse genetic codes.

Molecular ecologist Rus Hoelzel of Durham University in England and his European colleagues compared the genetic make up of six English ancestors from the Roman period, 25 from early in the Saxon conquest and 17 from the late Saxon period with the mitochondrial DNA sequences of more than 6,000 modern Europeans and Middle Easterners.

"We found higher mitochondrial DNA diversity in ancient England (Roman to Saxon times) than in either modern England or in a combination of northern European countries," Hoelzel says. "Modern human populations are highly diverse, just less so in northern Europe, at least, than the ancient populations in England."

Even when present-day Europeans were broken down into 10 smaller samples of 48 individuals each, they still were less diverse than their ancestors. The 48 ancients bore 36 different haplotypes—a set of variations in the genetic code. But their descendants more commonly carried one particular haplotype, known as Cambridge reference sequence (CRS); 6.3 percent of ancients carried it compared with nearly 22 percent of modern Britons and an average of nearly 19 percent of all Europeans.

The CRS haplotype imparts no known special traits in the humans who bear it, but the "black death" may have played a role in increasing its abundance in the modern population. This bubonic plague swept Britain (and all of Europe) in the 14th century, killing as much as half of the population, before recurring again in London in the 17th century. The dread disease could either have increased the proportion of certain haplotypes somehow associated with increased survival or simply led to the extinction of rare haplotypes in families or villages who had a particular susceptibility, Hoelzel says.

Only three small groups—modern peoples from Belarus, Palestine and Turkey—showed similar levels of diversity to these early ancestors, though studies by others have shown that southern Europeans, such as Italians, seem to retain a broader pool of genetic material. It is unclear exactly why contemporary Europeans are less genetically diverse despite a continuing influx of new populations but, according to researchers, the ancients possessed a more robust array of differing genetic stocks.