THE EVOLUTIONARY STORY OF HOMO SAPIENS 555
The Arrival of Homo sapiens
About 200,000 years ago (200 Kya), the first hominins appeared
in Africa that were so similar to us that we recognize them as
members of our own species, Homo sapiens. The evolutionary
path to reach that point was not smooth: different physical fea-
tures evolved at different rates. Average brain size increased
throughout hominin history (FIGURE 21.11). Along the lineage
leading from afarensis to africanus to erectus and finally to sapi-
ens, there were many other changes in the teeth, face, pelvis,
hands, and feet. Although some details remain unresolved, one
key point has been proven: modern humans evolved from an
apelike ancestor.
Humans first spread out of Africa roughly 60 Kya (FIGURE
21.12) [23]. Remarkably, analyses of DNA from living humans
(using methods described in Chapter 7) suggest that only about
2000 individuals dispersed. They were the ancestors of almost
all of the 6 billion people now living outside Africa [21]. Humans
colonized Europe, East Asia, and Australia between 60 and 40
Kya. They walked from Siberia into Alaska about 20 Kya, when the sea level was
low and the Bering Strait was dry. It then took them only about 8 Ky to spread
throughout North and South America [63]. In less than 50 Ky, our species colo-
nized the entire planet. Perhaps no other species, except those that travel with
humans, has ever gone so far so fast.
The story of these adventures is written in our genes. Gene trees of mtDNA
sequences make several key points (FIGURE 21.13). The gene trees decisively con-
firm the fossil evidence that humans are closely related to the extinct Neander-
thals. Among living humans, the deepest branches in the tree (representing many
nucleotide changes) are from Africans, showing that our species originated there.
The data allow us to estimate the age of the most recent common ancestor of mito-
chondrial DNA in living humans. The woman who carried that mitochondrion
is thought to have lived about 125 Kya [5]. (As we discussed in Chapter 7, many
other humans were also living then, and they contributed other genes to modern
humans.) Figure 21.13 also shows that all mitochondria outside Africa descended
from just one branch of the African gene tree, corresponding to the expansionFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_21.11.ai Date 02-02-20173.0 2.0 1.0 Present1000150020000
Time (Mya)Brain volume (cc)AustralopithecusHomo habilisHomo erectusHomo sapiens500Modern
mean valueFIGURE 21.11 The brain volume of hominins has increased
through time. This figure assigns specimens to Homo habilis
and H. erectus, but some of those specimens are intermedi-
ate and cannot be classified with certainty. (After [34].)FIGURE 21.12 The colonization of Earth
by humans. The arrows show paths of
colonization, and the numbers show the
times of arrival in years before present.
Several of the paths and dates are not
known with great certainty.Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_21.12.ai Date 01-11-201750 Kya13 Kya12 Kya20 Kya55 Kya
60 Kya40 KyaMatings with
Neanderthals
in Middle EastOther lineages
remain in
AfricaEarly wave of
migrants reaches
AustraliaNorth and South
America peopled
in only 8000 yearsCrossing into the
Americas when
Bering Strait is dry21_EVOL4E_CH21.indd 555 3/22/17 1:51 PM