The evolutionary process is characterized by diversification of populations leading to the establishment of different adaptive trajectories. Diversification itself is strongly associated with allopatry, which involves the initial geographical and, in most cases, demographic expansion of populations. However, expansion and diversification is one outcome of the evolutionary process. The other is population contraction and extinction. The evolution of diversity within any one lineage is shaped by both expansion and contraction/extinction.
Extinction has been traditionally considered a major mechanism in shaping late hominin evolution through the disappearance of all archaic groups in the last 100,000 years. However, significantly less emphasis has been given to the extinction of modern human populations and its role in shaping the pattern of diversity we observe today. Morphological and genetic evidence suggests that a significant number of modern populations became extinct in the course of the last 100,000 years. These extinctions have not only altered the pattern of relative diversity through time, but also provide insights into the competitive conditions that underlie the adaptive trajectories taken by different groups.
The evolution of human diversity underwent several expansion and contraction phases. The former led to the original diversification of the ancestral African population (> 100Kyr), the southern dispersals (+- 60 Kyr), the colonisation of Eurasia by modern humans (< 50Kyr), and subsequently to the establishment of food production strategies (<12 Kyr). The latter led to the structuring of the ancestral African population from which non-African diversity derives, and to the formation of population outliers through time. Together, subsequent expansions and contractions gave human diversity a multi-layered structure, creating the human palimpsest. Extinction is the unknown variable in this process – which past populations left living descendants is important in tracing the historical pattern, while the level of distinctiveness of any group, and consequently its phylogenetic proximity to others, is perceived through the existence or not of intermediate forms. This paper explores the role of extinction in shaping human diversity, highlighting both the very early and very late disappearance of modern populations and the implications for the interpretation of genetic patterns.
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