This long period of breast-feeding severely reduced female fertility and made any demographic revival precarious

For whatever unexplainable reason, I get an odd jolt of satisfaction when an author recounts facts of which I was aware but then points out a ramification of those facts which I had never considered. I have just started reading Fernand Braudel's The Structures of Everyday Life Civilization & Capitalism 15th-18th Century. On page 36 he is discussing the difficulties of estimating past population numbers.

These huge numbers might tempt us to assume a total of some 80 to 100 million people for the whole of America in about 1500. No one is prepared to accept this blindly, despite the evidence of archaeologists and of so many of the chroniclers of the Conquest, including Father Bartolome de Las Casas. What is quite certain is that the European Conquest brought a colossal biological slump to America, perhaps not in the ratio of ten to one but certainly enormous and quite incommensurate with the Black Death and its concomitant catastrophes in Europe in the disastrous fourteenth century. This was partly due to the hardships of a ruthless war and to the unparalleled burden of colonial labour. But the Indian population at the end of the fifteenth century suffered from a demographic weakness, particularly because of the absence of any substitute animal milk. Mothers had to nurse their children until they were three or four years old. This long period of breast-feeding severely reduced female fertility and made any demographic revival precarious. Furthermore the Amerindian population, already barely holding its own, was overtaken by a series of terrible bacterial attacks similar to those dramatically spread by white men in the Pacific in the eighteenth and especially the nineteenth century.

I have a fairly intense interest in history, demographics, historical morbidity, etc. and have read dozens of fairly technical books on the topic. I am well aware of the various estimates of historical populations, of the suppressant nature of lactation on fertility, the history and consequences of recurrent plagues (east and west), and of the rarity and value of genetic change which permits lactose tolerance in adulthood. But I do not recall ever having seen anyone make the connection of lactose tolerance on a population's capacity to rebound from punishing plagues, as Braudel does, almost in a throw away line above.

It makes a lot of sense and explains some minor mysteries which have stumped me in the past. The genetic mutation which permits lactose tolerance in adults has arisen three times in three different populations; in northwestern Europe, in East Africa and in India. The advantage of lactose tolerance is obvious from three perspectives. It permits an additional energy and nutrition source (milk as food), it permits energy densification (milk converted to cheese has a high energy density allowing you to get by with less volume), and it permits energy duration (milk converted to cheese can be used over longer durations of time and is more easily transportable).

The gene mutation permitting adult lactose tolerance occurred about 7,000 years ago in northwestern Europe but has now spread through selection to most of Europe. It has always puzzled me how fast that spread occurred. Yes, there are all those advantages, but still. Braudel's comment makes this more comprehendible.

Europe was serially ravaged by plagues (bubonic, typhus, cholera, etc.) and war over a two thousand year stretch with plagues recurring every twenty five to seventy five years with mortality rates as low as 10% to as high as 50-60%. That is a population winnowing of fearsome proportions. As with everything demographic, there are always two faces to the coin - death and birth. It is natural to focus on the causes of death such as plagues. But Braudel's insight highlights the other side of the coin. It isn't just about how many people die, it is also about how quickly they can be replaced. If you can wean a child from breast milk to animal milk within a year, then your replacement rate is three or four times faster than for those populations tied to the natural breast feeding cycle.

So why did the genetic mutation for adult lactose tolerance spread so fast in Europe? Yes, all the advantages of energy density and preservation were important. But the population rebound effects owing to animal milk tolerance would have been a significant factor as well.

Another question I have always had is why the Amerindian populations collapsed so catastrophically. One example of such complete collapse are the differing accounts between the Spanish explorer Hernando de Soto in 1539, stumbling through the southeast from Florida to Texas, reporting large healthy Indian populations including many dense townships and those of later explorers following within just a couple of decades, reporting a landscape denuded of people.

One explanation has always been the greater mortality rates of virgin populations exposed to new pathogens. This explanation is supplemented with the fact that the Amerindians were exposed to serial diseases in a tight timeframe. Europeans had a couple of millennia to adapt to the varied plagues - Amerindians just a couple of centuries. A further explanation was that the social structures of Amerindian societies and cultures were so devastated by the mortality rates that it essentially constituted a reversion to nature. Those were adequate explanations but still did not feel complete.

Again, the fact that biological reproduction would be so dramatically slowed by maternal milk dependency suddenly makes the collapse much more explicable.