... it is not obvious that getting fat is a natural response to plenty. Animals rarely get fat, even when food is abundant, unless they are old and domesticated. Young animals almost never get fat. Young people do all too frequently.
Humans have around ten times as many fat cells in relation to their body mass as most other animals. Polar bears are similarly off the curve, but then they have good reason to be fat: they need insulation and go for long periods without food. Pigs are well-padded, but they were bred for it.
Perhaps humans were bred for it too. That is what the thrifty-gene theory suggests. It was thought up by James Neel, a geneticist, in 1962, as he was seeking an explanation for the extreme porkiness of the Pima Indians. He postulated that they were fat because of the bad times their tribe had been through. They were minding their own business in Mexico and Arizona in the 19th century when incoming farmers disrupted their water supply. Some Pima starved and many were malnourished. Those who survived, suggested Neel, did so because they had some innate advantage%u2014a thrifty gene that meant they were particularly good at storing energy. Their descendants inherited a trait that became a burden, not a benefit.
Andrew Prentice, professor of international nutrition at the London School of Hygiene and Tropical Medicine, reckons that the thrifty gene is widespread among the human race. He puts it down to agriculture. Hunter-gatherers, he argues, did not much need a thrifty gene. They may not have had the plenty that agriculture could provide, but they were few and their food supply flexible. However, after man settled down to farming around 12,000 years ago, the population grew, and soon lots of people depended on a small range of crops in a limited area. When the crops failed, disaster struck. And it did so often, and relatively recently, in Europe.
In 1321, some 20% of English people are reckoned to have died of famine. There was cannibalism in 1563 and 151 famines were recorded in England before 1620, the year the Mayflower sailed to America. American genes may be especially thrifty. Some immigrants were survivors of disasters such as the Irish potato famine. They also had to survive the crossing and the business of making a new life, which wiped out plenty of weaker stock. From 104 passengers on the Mayflower, only 23 left descendants. That was mostly because of crop failures and starvation, relief from which Americans still celebrate at Thanksgiving.
It is weird that humans should be so different from all other mammals. Surely an ability to tolerate famine is important in rats? I'm not sure the thrifty-gene theory alone makes sense. I wonder if it's just a gene flaw, a side-effect of the very small human gene pool and high level of semi-clonal breeding.