188
J. D. Wall
recently, anatomically modern humans first appear in the fossil record 150–200
thousand years ago (Kya) in Eastern and Southern Africa. Modern humans then
spread across the rest of the inhabitable world, mostly replacing (but with low
levels of hybridization and admixture) the indigenous hominin groups that they
encountered. Within this broad outline though, there are many specific details that
have yet to be worked out.
In this review, we will focus on what we have learned from genetic data over
the past 50 years. Roughly speaking, genetic similarity can be used as a proxy for
relatedness—the more similar two sequences are, the more likely they are to share
a recent common ancestor. By examining DNA sequences from many individuals
at once and by developing computational and statistical models for what these
sequences should look like under different scenarios, we can begin to piece together
aspects of our species’ history in ways that other approaches cannot. This review
highlights some of the insights that have come from the study of human genetic
data. It is not meant to be comprehensive, but rather a personal opinion on the most
important discoveries over the past several decades.
8.2
Human Population Structure
The first large-scale studies of human genetic variation involved classical markers
such as blood groups and allozyme polymorphisms (e.g., Brues 1954; Edwards
and Cavalli-Sforza 1964). These studies showed that the vast majority of genetic
variation (~85–95%) is shared among different populations, while a much smaller
fraction differentiates populations (reviewed in Cavalli-Sforza et al. 1994). Subsequent studies of microsatellite, genotype, and resequencing data (e.g., Rosenberg
et al. 2002; International HapMap Consortium 2005, 2007, 2010; Wall et al.
2008; 1000 Genomes Project Consortium 2010, 2012, 2015; GenomeAsia 100K
Consortium 2019) have found similar levels of population structure between human
populations (as measured by F ST ). These results have been interpreted as human
populations being much more (genetically) similar to each other than different from
each other, and we now know that the level of population structure in humans is
similar to what is found in many other species. At the time these results formed an
important counterweight to the “candelabra model” of Coon (1962), which posited
that different continental groups had been mostly isolated for hundreds of thousands
of years and which was used to justify segregationist policies in the United States
(Jackson 2001).
Despite the overall similarity of different human groups at specific genetic
markers, the small differences at many markers in aggregate can be used to
accurately distinguish between different human populations (e.g., Pritchard et al.
2000; Patterson et al. 2006). For example, Rosenberg et al. (2002) showed that
377 microsatellite markers were sufficient to distinguish individuals from the major
continental groups, such as sub-Saharan Africans, Europeans, Asians, Melanesians,
and Native Americans. In addition, for recently admixed individuals, these methods
can estimate the proportion of ancestry that comes from multiple ancestral groups
J. D. Wall
recently, anatomically modern humans first appear in the fossil record 150–200
thousand years ago (Kya) in Eastern and Southern Africa. Modern humans then
spread across the rest of the inhabitable world, mostly replacing (but with low
levels of hybridization and admixture) the indigenous hominin groups that they
encountered. Within this broad outline though, there are many specific details that
have yet to be worked out.
In this review, we will focus on what we have learned from genetic data over
the past 50 years. Roughly speaking, genetic similarity can be used as a proxy for
relatedness—the more similar two sequences are, the more likely they are to share
a recent common ancestor. By examining DNA sequences from many individuals
at once and by developing computational and statistical models for what these
sequences should look like under different scenarios, we can begin to piece together
aspects of our species’ history in ways that other approaches cannot. This review
highlights some of the insights that have come from the study of human genetic
data. It is not meant to be comprehensive, but rather a personal opinion on the most
important discoveries over the past several decades.
8.2
Human Population Structure
The first large-scale studies of human genetic variation involved classical markers
such as blood groups and allozyme polymorphisms (e.g., Brues 1954; Edwards
and Cavalli-Sforza 1964). These studies showed that the vast majority of genetic
variation (~85–95%) is shared among different populations, while a much smaller
fraction differentiates populations (reviewed in Cavalli-Sforza et al. 1994). Subsequent studies of microsatellite, genotype, and resequencing data (e.g., Rosenberg
et al. 2002; International HapMap Consortium 2005, 2007, 2010; Wall et al.
2008; 1000 Genomes Project Consortium 2010, 2012, 2015; GenomeAsia 100K
Consortium 2019) have found similar levels of population structure between human
populations (as measured by F ST ). These results have been interpreted as human
populations being much more (genetically) similar to each other than different from
each other, and we now know that the level of population structure in humans is
similar to what is found in many other species. At the time these results formed an
important counterweight to the “candelabra model” of Coon (1962), which posited
that different continental groups had been mostly isolated for hundreds of thousands
of years and which was used to justify segregationist policies in the United States
(Jackson 2001).
Despite the overall similarity of different human groups at specific genetic
markers, the small differences at many markers in aggregate can be used to
accurately distinguish between different human populations (e.g., Pritchard et al.
2000; Patterson et al. 2006). For example, Rosenberg et al. (2002) showed that
377 microsatellite markers were sufficient to distinguish individuals from the major
continental groups, such as sub-Saharan Africans, Europeans, Asians, Melanesians,
and Native Americans. In addition, for recently admixed individuals, these methods
can estimate the proportion of ancestry that comes from multiple ancestral groups