The ecology of social transitions in human evolution

Abstract

We know that there are fundamental differences between humans and living apes, and also between living humans and their extinct relatives. It is also probably the case that the most significant and divergent of these differences relate to our social behaviour and its underlying cognition, as much as to fundamental differences in physiology, biochemistry or anatomy. In this paper, we first attempt to demarcate what are the principal differences between human and other societies in terms of social structure, organization and relationships, so that we can identify what derived features require explanation. We then consider the evidence of the archaeological and fossil record, to determine the most probable context in time and taxonomy, of these evolutionary trends. Finally, we attempt to link five major transitional points in hominin evolution to the selective context in which they occurred, and to use the principles of behavioural ecology to understand their ecological basis. Critical changes in human social organization relate to the development of a larger scale of fission and fusion; the development of a greater degree of nested substructures within the human community; and the development of intercommunity networks. The underlying model that we develop is that the evolution of 'human society' is underpinned by ecological factors, but these are influenced as much by technological and behavioural innovations as external environmental change.

Figure 1.

The classical model of socioecology, in which owing to the different costs of male and female reproduction, females are more strongly influenced by resources, and males by the distribution of females (Wrangham 1980). During the course of human social evolution, the increased ability of males to control resources has led to a closure of the cascade model, with males exerting control over female distribution through the control over resources.

Figure 2.

The pattern of hominin evolution. First and last appearance ages of recognized hominin taxa provide chronological ranges. The colour coding indicates major groupings that represent adaptive shifts discussed in the text. Red bars, earliest hominins; blue bars, australopithecines and allies; orange bars, smaller-brained Homo; green bars, larger-brained Homo.

Figure 3.

(a) Pattern of evolution in brain size and (b) sexual dimorphism as indicated by the fossil record. Points shown are approximate, and ranges and potential errors can be considerable, especially for sexual dimorphism.

Figure 4.

The timing of the five key transitions discussed in the text, based on archaeological and palaeontological evidence (table 2). (a) The first four transitions occurring on a time scale of millions of years; (b) the fifth transition on a time scale of thousands of years.

Figure 5.

The community is the fundamental hominin and African hominoid social unit, and during the course of hominin evolution, it has been elaborated by increased fissioning and fusion (transition 1 and later transitions), greater infracommunity structure (transitions 2 and 3) and greater supracommunity networks and associations (transitions 4 and 5). See text for discussion.