Evolution__3rd_Edition

..
Homo sapiens
c. 100,000
years ago
~1.5–1.8 Myr
(a) Multiregional (b) Out of Africa
African Asian European African Asian European
Homo erectus Homo erectus
Two hypotheses have been
proposed for the origin of
anatomically modern humans
CHAPTER 18 / The History of Life 549
Figure 18.13
Two hypotheses about the evolution of anatomically modern
humans. (a) The “multiregional” hypothesis suggests that
anatomically modern humans evolved independently, by parallel
evolution, in Africa, Europe, and Asia, from populations that
ultimately originated in Africa and emigrated from Africa perhaps
about 1.8 million years ago. (b) The “out of Africa” hypothesis
suggests that anatomically modern humans evolved uniquely
in Africa some time between 500,000 amd 100,000 years ago,
emigrated from there, and replaced the indigenous humans in
Asia and Europe. The first emigration from Africa around 1.8
million years ago is not controversial. The two hypotheses differ
over the second emigration. The timing of the first colonization
of Europe is uncertain, as is whether it was from Africa or Asia;
hence the dashed lines. Intermediates between hypotheses
(a) and (b) are possible.
specimens as H. ergaster and reserve H. erectus for the Asian and perhaps European
forms. Others use H. erectus for the African specimens as well.)
Paleoanthropologists refer to the kind of human beings that live today a Homo
sapiens a as “anatomically modern humans.” Anatomically modern humans differ
from the fossils discussed so far in a series of details in skull anatomy. Our brains are a
different shape from H. erectus, and our faces are flatter. When and where did anatomically
modern humans originate? For the past 15 years, this question has been the topic of
a debate between two hypotheses (Figure 18.13). By 500,000 years ago, human populations
descended from H. erectus were established in Asia (and Australia) and Europe
as well as Africa. The taxonomic names of these regional forms are not agreed on. Some
taxonomists refer to all the regional forms as “archaic Homo sapiens”; some classify different
regional forms as different species (H. neanderthalensis in Europe, for example);
and some call them subspecies of H. erectus. The classificatory differences partly reflect
the problem of squeezing continuous evolutionary change into discrete Linnaean
categories and partly reflect the different theories in Figure 18.13.
Anatomically modern humans were fully established in Africa, Europe, and Asia by
30,000–40,000 years ago. The earliest definite fossils of anatomically modern humans
are African, and over 100,000 years old. (Some earlier, also African, fossils may well be
anatomically modern humans too.) Some paleoanthropologists argue that anatomically
modern humans evolved independently in Asia, Europe, and Africa; this is the “multiregional”
hypothesis (Figure 18.13a). Others argue that anatomically modern humans
originated only in Africa, and then emigrated to Asia and Europe, replacing the indigenous
people, with little or no interbreeding. Such is the “out of Africa” hypothesis
(Figure 18.13b). (We can now see how the different names for regional forms make
sense. On the multiregional hypothesis, it is appropriate to classify the 500,000-yearold
populations as subspecies of H. sapiens, or to call them archaic H. sapiens. It is more
appropriate to classify them as subspecies of H. erectus on the out of Africa hypothesis.)
Genetic evidence has tended to favor the out of Africa hypothesis. Two examples are
discussed elsewhere. In Box 13.2 (p. 365) we saw that humans show little geographic
variation, at a genetic level, as compared with the geographic races of other species.