There are almost no written sources from which to re-construct the demography of early medieval Scotland. Estimates have been made of a population of 10,000 inhabitants in Dál Riata and 80-100,000 for Pictland, which was probably the largest region. This does not include figures for parts of Northumbria now in Scotland. It is likely that the 5th and 6th centuries saw higher mortality rates due to the appearance of bubonic plague, which may have reduced net population. The examination of burial sites for this period like that at Hallowhill, St Andrews indicate a life expectancy of only 26–29. The known conditions have been taken to suggest it was a high fertility, high mortality society, similar to many developing countries in the modern world, with a relatively young demographic profile, and perhaps early childbearing, and large numbers of children for women. This would have meant that there were a relatively small proportion of available workers to the number of mouths to feed. This would have made it difficult to produce a surplus that would allow demographic growth and more complex societies to develop. From the formation of the kingdom of Alba in the tenth century, to before the Black Death reached the country in 1349, estimates based on the amount of farmable land, suggest that population may have grown from half a million to a million. Although there is no reliable documentation on the impact of the plague, there are many anecdotal references to abandoned land in the following decades. If the pattern followed that in England, then the population may have fallen to as low as half a million by the end of the 15th century. Compared with the situation after the redistribution of population in the later clearances and the industrial revolution, these numbers would have been relatively evenly spread over the kingdom, with roughly half living north of the Tay. Perhaps ten per cent of the population lived in one of many burghs that grew up in the later medieval period, mainly in the east and south. It has been suggested that they would have had a mean population of about 2,000, but many would be much smaller than 1,000 and the largest, Edinburgh, probably had a population of over 10,000 by the end of the era.