Lecture 11: Genetics and recent human evolution
Genetics and recent human evolution
One of the most controversial issues in the study of human origins focuses on the
origins of modern humans, Homo sapiens.
Roughly 50,000 years ago, the Old World was occupied by a morphologically
diverse group of hominids. In Africa and the Middle East there was Homo sapiens;
in Asia, Homo erectus, and in Europe, Homo neanderthalensis. But this taxonomic
diversity vanished and humans everywhere have evolved into the anatomically and
behaviorally modern form. Starting with a publication by Cann et al. in 1987,
genetics has played an increasingly important role in understanding the recent
human evolution.
Although most genetic data support the view that our species originated recently in
Africa, it becomes increasingly clear that some interbreeding occurred during its
range expansion. Several scenarios of modern human evolution have been proposed
on the basis of molecular and paleontological data but the focus is still on a debate
between two schools of thought: one that points out multi-regional continuity and
the other that suggests a single origin for modern humans.
Models of human evolution: The ‘Candelabra Model’
Europe
Africa
Asia
(Figure 1)

according to C.S. Coon, 1962

supposed that the predecessor of modern human
1 mio years
Figure 1
migrated out of Africa in the early Pleistocene

each lineage independently evolved into their modern form
This model requires an amazing parallel evolution of non-modern humans into their
modern form, so it was largely discredited just on the basis of theoretical
Lecture 11: Genetics and recent human evolution
implausibility. Moreover, less variation among the major populations of humans was
found than expected under this model.
The ‘Multi-regional Model’ (Figure 2)

according to F. Weidenreich, 1946 and
Europe
Africa
Asia
M. Wolpoff, 1994/2000

still the predecessor of modern human migrated
out of Africa and evolved locally

1 mio years
but population expansions were accompanied
Figure 2
by interbreeding and not total replacement
This model cannot be rejected by the molecular data using mtDNA because a broad
range of gene flow and selective conditions could explain the pattern of recent
human populations.
The ‘African Origin Model’ / ‘Replacement Model’
Europe
Africa
Asia
(Figure 3)

according to R. Cann and A. Wilson, 1987

first migration wave out of Africa in early Pleistocene 100.000 years

second migration event about 100,000 years ago
replacing former groups of hominids such as
Homo erectus and Homo neanderthalensis
1 mio years
Figure 3
This model was subjected to some revision in recent years as both Neanderthal and
Denisovan DNA was found in humans.
Genetic evidence for evolution models
Cann et al. (1987) presented a genetic survey based on restriction site
polymorphism in 147 human mtDNA samples (placenta) whose origins came from
Lecture 11: Genetics and recent human evolution
five different geographical regions. They estimated the tree of the resulting
133 mitochondrial haplotypes, using maximum parsimony. The mitochondrial tree
showed an African ancestral population and all branches were relatively short
implying a recent common mitochondrial ancestor (dubbed ‘Mitochondrial Eve’).
This publication was strongly criticized because of the following points:

only 2 of the human mitochondrial DNA samples were sub-Saharan Africans,
the other 18 ‘Africans’ were black Americans,

the method to generate the tree was not guaranteed to find the most
parsimonious tree,

the method used to root the tree while placing it at the midpoint of the
longest branch (midpoint rooting) could lead to false results if for example
the evolution rate is higher in Africa,

RFLP (Restriction Fragment Length Polymorphism) is not suitable for
estimating mutation rates which is essential in timing evolutionary events,

weak statistical analysis.
Therefore Vigilant et al. (1991) and then Ingman et al. (2000) repeated the study
while avoiding its major flaws. The latter

sampled mitochondrial DNA from 53 individuals, where 32 of them were
Africans from different regions of sub-Saharan Africa,

they sequenced the complete mtDNA (16.5 kb) but excluding the rapidly
evolving D-loop of mtDNA,

chimpanzee mtDNA was used to root the tree (outgroup rooting) which is a
much more reliable method than midpoint rooting,

and they used sequences to check evolutionary events and found the time
point for the split from Africa about 110,000 years ago.
In contrast to Ingman the study of Vigilant (1991) tried to distinguish human
populations while using the sequences of the rapidly mutating D-loop of mtDNA.
This study also used ‘real’ African mitochondrial DNA samples and phylogenetic
Lecture 11: Genetics and recent human evolution
reconstruction to analyze the data (this is possible because mtDNA do not
recombine). Although D-loop mtDNA does not evolve according to the molecular
clock, all sequences go back to an common ancestor, which reflects the results of
both other studies.
But the mitochondrial haplotype tree described by Cann and Ingmann with no long
branches and a recent common ancestor is compatible with the multi-regional
model if there was sufficient gene flow to prevent long-term isolation among
subpopulations of the former Homo genus.