A long-standing mystery over the way men's skulls changed from long to round in medieval Europe has been deepened by discoveries at an English village.
Huge volumes of data collected at Wharram Percy in Yorkshire, Northern England, cast doubt on all current theories about the unexplained blip between the 11th and 13th centuries which has been recorded by archaeologists across the continent.
Immigration and climate change have been the two main hypotheses but neither makes sense of the 700 Yorkshire skeletons. They are expected to cause widespread revision of the period's history, as the first large-scale find from a single, accurately-dated indigenous community.
The leading theory, that Scandinavian incomers brought new racial characteristics to the rest of Europe, does not make sense at Wharram, a lonely valley in the Wolds near Malton. Its natural isolation was reinforced during the 200 years of the skull change by plague and sheep blight which soon after led to its abandonment.
The skulls also show that only men were affected, which would not have applied if its cause was new genetic stock.
Simon Mays, the human skeletal biologist in charge of the English Heritage study, said: "If immigration was responsible we would expect both sexes to be affected. There's also the puzzle of why male skull shapes eventually reverted back, becoming similar to those we have today."
This has been put down to climate change, with male bones more sensitive to environmental change during their development than those of women. Round skulls, rather than the proverbial Yorkshire big head, are found in colder parts of the world because they have less surface area and retain heat better.
"But the climate at Wharram during the critical period rose by 0.5oC and was actually warmer than it is today," Mays said. "Further, as the weather got much colder in the later medieval period skulls started to become longer and narrower again."
Attention is now turning to possible effects of the plague, which repeatedly devastated Wharram along with many other parts of western Europe. Villagers finally called it a day around 1500, leaving only mounds and the ruined St Martin's church today.
The study, the latest of 14 volumes on 40 years of excavation at Wharram, also shows that left-handedness was much more common in medieval times, at 15 percent compared with 8 percent today. Infant mortality was also unexpectedly low, possibly because illness and poor diet set in only after weaning off breast milk.