Positive feedbacks in deep-time transitions of human populations

Abstract

Abrupt and rapid changes in human societies are among the most exciting population phenomena. Human populations tend to show rapid expansions from low to high population density along with increased social complexity in just a few generations. Such demographic transitions appear as a remarkable feature of Homo sapiens population dynamics, most likely fuelled by the ability to accumulate cultural/technological innovations that actively modify their environment. We are especially interested in establishing if the demographic transitions of pre-historic populations show the same dynamic signature of the Industrial Revolution transition (a positive relationship between population growth rates and size). Our results show that population growth patterns across different pre-historic societies were similar to those observed during the Industrial Revolution in developed western societies. These features, which appear to have been operating during most of our recent demographic history from hunter-gatherers to modern industrial societies, imply that the dynamics of cooperation underlay sudden population transitions in human societies. This article is part of the theme issue 'Evolution and sustainability: gathering the strands for an Anthropocene synthesis'.

Keywords: cooperation; human expansions; population; positive feedback.

Figure 1.

Regional SPD time series for different geographical regions (y-axis) and the calibrated years before the present (x-axis); the vertical dotted red line denotes the estimated breaking date of the major shifts in population expansions, and the vertical black dotted lines the periods of population growth used in the population transitions analyses. (a) Middle East; (b) Europe; (c) East Asia (China); (d) South America; (e) South Africa; (f) West Africa; (g) North America and (h) Australia. Periods of major demographic transitions in each SPD time series were estimated by using the least squares method, so pre-shift and post-shifts means SPD values were estimated concurrently with the change point (electronic supplementary material, figure S1).

Figure 2.

Time series of the logarithmic net reproductive rates (R_t) based on the selected periods of sustained growth from the SPD time series. (a) Near East, (b) Europe, (c) East Asia (China), (d) South America, (e) South Africa, (f) West Africa, (g) North America, and (h) Australia. The shaded orange areas denote each of the sequences of uninterrupted periods of population growth used for fitting the model of equation (2.4) (see Material and methods for details).

Figure 3.

Examples of the observed reproduction curves (R_t = (X_t+1 − X_t) against (X_t) of the SPD times sequenced time series when logarithmic reproduction rates are plotted against the natural logarithm of SPD (X_t) with a time lag of one generation (30 years). (a) Near East, (b) Europe, (c) South America, and (d) Australia. The fitted curves (model 5) parameters are shown in the electronic supplementary material, table S1.

Figure 4.

Frequency distribution of the periods (years) of uninterrupted population growth observed during the demographic transitions (mean = 365 years).