The shaping of human diversity: filters, boundaries and transitions

Abstract

The evolution of modern humans was a complex process, involving major changes in levels of diversity through time. The fossils and stone tools that record the spatial distribution of our species in the past form the backbone of our evolutionary history, and one that allows us to explore the different processes-cultural and biological-that acted to shape the evolution of different populations in the face of major climate change. Those processes created a complex palimpsest of similarities and differences, with outcomes that were at times accelerated by sharp demographic and geographical fluctuations. The result is that the population ancestral to all modern humans did not look or behave like people alive today. This has generated questions regarding the evolution of human universal characters, as well as the nature and timing of major evolutionary events in the history of Homo sapiens The paucity of African fossils remains a serious stumbling block for exploring some of these issues. However, fossil and archaeological discoveries increasingly clarify important aspects of our past, while breakthroughs from genomics and palaeogenomics have revealed aspects of the demography of Late Quaternary Eurasian hominin groups and their interactions, as well as those between foragers and farmers. This paper explores the nature and timing of key moments in the evolution of human diversity, moments in which population collapse followed by differential expansion of groups set the conditions for transitional periods. Five transitions are identified (i) at the origins of the species, 240-200 ka; (ii) at the time of the first major expansions, 130-100 ka; (iii) during a period of dispersals, 70-50 ka; (iv) across a phase of local/regional structuring of diversity, 45-25 ka; and (v) during a phase of significant extinction of hunter-gatherer diversity and expansion of particular groups, such as farmers and later societies (the Holocene Filter), 15-0 ka.This article is part of the themed issue 'Major transitions in human evolution'.

Keywords: African prehistory; Holocene filter; dispersals; human evolution; human evolutionary genetics; modern human origins.

Figure 1.

Schematic drawing illustrating the role of extinction of some populations and differential expansion of others in creating population outliers, increasing the distinctiveness of groups and shaping population boundaries.

Figure 2.

(a) Cartoon of the complex phylogenetic pattern through time, shaped by repeated expansions, extinctions and assimilations; (b) geographical expression of a complex phylogenetic pattern in the fossil and archaeological records; (c) distribution of key fossil specimens and archaeological sites in Africa between 400 and 130 ka in the context of climate change. In (c), blue lines represent fossils of Homo heidelbergensis, purple lines and circles fossils of Homo helmei and red lines and circles fossils of Homo sapiens. Archaeological sites are represented by black squares, Acheulean industries by a hand-axe, and Middle Stone Age industries by a Levallois point.

Figure 3.

Schematic of the five transitions in the evolutionary history of Homo sapiens. Dark blue lines represent Denisovans, light blue lines Neanderthals and grey/black lines modern humans. Red arrows represent events of interspecific admixture. To the right, climate variation (x axis) based on oxygen isotope data through time (y axis).