the origins of the British Iron Age and the making of the modern nations. Linguistic evidence inferred from the surviving Celtic languages in Northern and Western Great Britain at first appeared to support the idea, and the changes in material culture that archaeologists observed during later prehistory were routinely ascribed to a new wave of invaders. From the early 20th century, the "invasionist" scenario was juxtaposed to a diffusionist view. By the 1960s, the latter model seemed to have gained mainstream support, but it in turn came under attack in the 1970s. There was certainly a large migration of people from Central Europe westwards during the early Iron Age. The question whether these movements should be described as "invasions", as "migrations" or as mostly "diffusion" is largely a semantic one. Examples of events that could be labelled "invasions" include the arrival in Southern Britain of the Belgae from the end of the 2nd century BC, as described in Caesar's Commentaries on the Gallic War. Such sudden events may be invisible in the archaeological record. In that case, it depends on the interpretation of Aylesford-Swarling pottery. Regardless of the "invasionist" vs. "diffusionist" debate, it is beyond dispute that exchanges with the Continent were a defining aspect of the British Iron Age. According to Caesar, the Britons further inland than the Belgae believed that they were indigenous. Demography Population estimates vary but the number of people in Iron Age Great Britain could have been three or four million by the 1st century BC, with most concentrated densely in the agricultural lands of the South. Settlement density and a land shortage may have contributed to rising tensions during the period. The average life expectancy at birth would have been around 25, but at the age of five, it would have been around 30. Those figures would be slightly lower for women, and slightly higher for men throughout the Middle Iron Age in most areas, on account of the high mortality rate of young women during childbirth; however, the average age for the two sexes would be roughly equal for the Late Iron Age. That interpretation depends on the view that warfare and social strife increased in the Late Iron Age, which seems to be fairly well attested in the archaeological record for Southern Britain at least. Early in the Iron Age, the widespread Wessex pottery of Southern Britain, such as the type style