Evolution__3rd_Edition

550 PART 5 / Macroevolution
Genetic evidence counts against the
multiregional theory for Europe ...
. . . but the Asian picture is more
ambiguous
This suggests that modern humans share a recent common ancestor. Figure 15.18
(p. 454) showed mitochondrial DNA evidence for a recent, probably African, common
ancestor of modern humans. Other pieces of evidence have been added. Ancient DNA
has been extracted from Neanderthal fossil bones. Its sequence lies outside the range of
modern humans. On the multiregional hypothesis, Neanderthal DNA should fit in the
human phylogeny with modern European populations, inside the phylogeny of all
modern humans. In fact it does not, suggesting that Neanderthals were completely
replaced and made no genetic contribution to modern European populations.
The archeological and fossil evidence in Europe also fits with the out of Africa
hypothesis. Anatomically modern humans appear suddenly in Europe, in the form
called Cro-Magnon man, around 40,000 years ago. Neanderthals went extinct at that
time. The artistic and symbolic artifacts associated with Cro-Magnons were far more
elaborate than those connected with Neanderthals. The cave paintings of southern
Europe, for instance, were created by early Cro-Magnons.
In Asia, the evidence is less clear-cut. No ancient DNA has been successfully
obtained from Asian fossil humans. (Indeed none may ever be, because the fossils have
to be preserved at low temperatures to maintain their DNA, and none of the Asian fossil
sites have been cold for the past 100,000 years or more.) Some fossil evidence may fit
with a continuous evolution of H. erectus into anatomically modern humans in Asia.
However, it is controversial and many experts favor the out of Africa hypothesis for
Asian populations too.
In summary, we have a fairly continuous fossil record of human evolution from
about 4 million years ago to the present. It shows how at least most, and perhaps all,
human evolution took place in Africa. The first major changes were locomotory:
bipedality had evolved over 3 million years ago. Changes in brain size and prognathism
came later. Brain size probably spurted up in early Homo around 2 million years ago.
But our brains and jaws did not reach their final size and shape until anatomically
modern humans originated a perhaps a little over 100,000 years ago.
18.8 Macroevolution may or may not be an extrapolated
form of microevolution
The distinction between microevolution and macroevolution is the distinction
between evolution on the small scale and evolution on the large scale. Microevolution
refers to the topics we looked at in Part 2 of this book. It refers to changes in gene
frequencies within populations, under the influence of natural selection and random
drift. Macroevolution refers to the topics we are looking at in Part 5 of the book. It
refers to the origin of higher taxa, such as the evolution of mammal-like reptiles
into mammals, fish into tetrapods, and green algae into vascular plants. It also refers
to long-term evolutionary trends, which we look at in Chapters 21 and 22, and to
diversification, extinction, and replacements of higher taxa, which we look at in
Chapter 23.
Microevolution and macroevolution can be thought of as vague terms, like “small”
and “large,” and as the ends of a continuum from evolution on the smallest scale to the
..