THE EVOLUTIONARY STORY OF HOMO SAPIENS 557
mutation originated in the Denisovans, and introgressed into humans by hybrid-
ization with them [31]. Intriguingly, some parts of the human genome are sig-
nificantly more free of Neanderthal DNA than average. One is around the FOXP2
gene, which is associated with speech and language. These regions of the genome
may have been important in adapting to the new human lifestyle, and so resisted
introgression of Neanderthal genes.
After coexisting with humans for tens of thousands of years, Neanderthals
became extinct about 40 Kya [28]. While the reasons are not known, it seems very
likely that growing competition with our ancestors was a major cause of their
demise.The diversity of human populations
As our ancestors colonized the planet, populations began to diverge genetically.
The genetic differences persist today despite the greatly increased mobility of
people during the last 100 years. Differences in skin color, height, facial bones,
body fat, and innumerable other traits evolved. Those traits, which are so obvious
and distinctive to us, in fact give a misleading impression of genetic differences
among modern human populations. Comparing the genomes of East Asians, the
Yoruba of Nigeria, and Europeans shows they are genetically very similar. Recall
from Chapter 8 that FST measures the fraction of genetic variation that results from
differences between populations. FST among these three populations is 0.12 [73].
That is, the genetic differences among them account for only 12 percent of all the
genetic variation found in those populations combined. The vast bulk of genetic
variation, a full 88 percent, is found within each of those populations. The differ-
ences between human populations that are so striking to us are not representa-
tive of our genomes as a whole. The genetic similarity among populations today
is the result of the small number of generations since our species spread across
the planet. Many other species are much more genetically fragmented, even over
much smaller geographic ranges (see Chapter 8).
Many of the striking phenotypic differences among human populations are
adaptations to the different environments in which we live. Convergent evolution
of similar phenotypes in similar environments strengthens the case for adaptation
(see Chapter 16). For example, the light skin color of Europeans and East Asians
evolved by mutations at different loci as these populations adapted to the limited
sunlight at northern latitudes.Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_21.14.ai Date 01-10-2017Homo erectus?18006504506050DenisovanNeanderthalEast AsiansEuropeansMelanesiansAfricansFIGURE 21.14 Comparison of DNA of living humans from different populations
with DNA from Neanderthal and Denisovan fossils reveals there was hybridiza-
tion among these lineages at several times and in several places. Lineages of
modern humans are shown in blue. Africans have no DNA from Neanderthals
or Denisovans, indicating that the matings with Neanderthals happened outside
Africa. East Asians, Europeans, and Melanesians have DNA sequences from
Neanderthals that were inherited from a pulse of hybridization in their common
ancestor. Melanesians have DNA inherited from Denisovans that is not found in
other modern human populations. East Asians and Europeans share DNA from
a second pulse of Neanderthal hybridization that occurred in their common
ancestor. East Asians also have Neanderthal DNA not found in other populations
that is inherited from a third period of hybridization that happened after their an-
cestors diverged from European populations. Denisovan DNA shows evidence
of much older hybridization with another species of hominin, possibly Homo
erectus. The numbers show approximate dates of branch points in thousands of
years ago. The breaks in the phylogeny indicate that the deeper branches are
not shown to scale. The phylogeny is simplified, and the timing and number of
hybridization events are not yet certain. (After [50, 61, 77].)21_EVOL4E_CH21.indd 557 3/22/17 1:51 PM