On the other side of the DT life was more fleeting. Most people lived directly off the land and were more reliant on the bounty of the earth and subject to the fluctuations in ecological conditions. This other world, in comparison to ours, had slower rates of growth, lower life expectancy and higher mortality rates, especially among children. It was a time of population vulnerability. The population lived on a knife's edge as even small deteriorations in living conditions could easily reduce population levels very quickly and sometimes almost to the point of annihilation. Rapid and large-scale population loss leads to the undermining of the basic functioning of society as lives were lost, keystone societal positions remained unfilled and cultural practices were wrecked. In this chapter I show how the consequences of this pre-transition demography shaped and continue to shape our present world.

THE GENETIC LEGACY OF POPULATION BOTTLENECKS

One legacy is that genetic characteristics of today's population are a result of previous bottlenecks. Population bottlenecks occur when there is a marked decline in the overall size of the population. They become more important the smaller and more isolated the population. Bottlenecks are caused by war and famine, disease, pandemics and the collapse of environmental support systems.

It is now generally agreed that the human population originated in Africa. From 100,000 years ago, small groups of migrants left Africa in successive waves to settle the world. It is estimated that fewer than 6,000 people were part of the first wave. They constituted just a small part of the genetic diversity of the total African population. The difference remains to this day so that the farther a traditional group lives away from Africa the less the genetic diversity. Ashraf and Galor (2013) propose an interesting idea: that there is an optimum level of diversity for long-term economic growth. Too much genetic diversity, as in the case of Africa, can lead to lack of cooperation. Too little diversity, as the case of the Americas, can lead to lower levels of innovation. They tested their hypothesis on conditions before colonialism, to concentrate on endogenous rather than exogenous factors, and used population density as a proxy for the level of development. They concluded that an increase in diversity in more homogeneous societies and a decrease in more heterogeneous societies led to an increase in population density.