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The shaping of human diversity: filters, boundaries and transitions

View Article: PubMed Central - PubMed

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’.

No MeSH data available.


Related in: MedlinePlus

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).
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RSTB20150241F3: 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).

Mentions: The diversification of modern humans was, necessarily, a gradual and continuous process. However, as briefly outlined above, the rate at which population change took place was different across space and, especially, across time. Major changes in the biology and/or behaviour of human populations follow from periods of intense selection and/or drift, when pressure on survivorship was a major challenge, or from periods of rapid population growth, when biological and cultural novelties accompany demographic expansion. In evolutionary terms, transitions often conflate both mechanisms. The biological and/or behavioural changes that take place during population collapse are only revealed if the group survives, and particularly if it subsequently expands demographically, and thus magnifies the differences acquired during the period of increased drift, to which the novel variation due to population growth is added. However, not all populations survive severe bottlenecks, or recover from one at the same rate. In the first case, when small groups become extinct, the areas left vacant fuel the pace of expansion of neighbouring groups when conditions improve; in the second case, when demographic recovery is slow, the scope of success depends on the distance from the wave of expansion of others. The result is that periods of rapid population growth of some groups are also associated with the extinction of others, acting like a filter. Therefore, transitions result from the interaction between (i) the addition of variation during population collapse and rapid expansion and (ii) the loss of variation due to localized population extinction and/or their partial assimilation into expanding groups. These transitions are thus the moments when the level, distribution and structure of diversity in the history of modern humans changed significantly (figureĀ 3).Figure 3.


The shaping of human diversity: filters, boundaries and transitions
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).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4920297&req=5

RSTB20150241F3: 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).
Mentions: The diversification of modern humans was, necessarily, a gradual and continuous process. However, as briefly outlined above, the rate at which population change took place was different across space and, especially, across time. Major changes in the biology and/or behaviour of human populations follow from periods of intense selection and/or drift, when pressure on survivorship was a major challenge, or from periods of rapid population growth, when biological and cultural novelties accompany demographic expansion. In evolutionary terms, transitions often conflate both mechanisms. The biological and/or behavioural changes that take place during population collapse are only revealed if the group survives, and particularly if it subsequently expands demographically, and thus magnifies the differences acquired during the period of increased drift, to which the novel variation due to population growth is added. However, not all populations survive severe bottlenecks, or recover from one at the same rate. In the first case, when small groups become extinct, the areas left vacant fuel the pace of expansion of neighbouring groups when conditions improve; in the second case, when demographic recovery is slow, the scope of success depends on the distance from the wave of expansion of others. The result is that periods of rapid population growth of some groups are also associated with the extinction of others, acting like a filter. Therefore, transitions result from the interaction between (i) the addition of variation during population collapse and rapid expansion and (ii) the loss of variation due to localized population extinction and/or their partial assimilation into expanding groups. These transitions are thus the moments when the level, distribution and structure of diversity in the history of modern humans changed significantly (figureĀ 3).Figure 3.

View Article: PubMed Central - PubMed

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’.

No MeSH data available.


Related in: MedlinePlus