Encyclopedia of Evolution.pdf - Online Reading Center

0 DNA (evidence for evolution)
second table). Humans and chimps shared all of them; gorillas
and orangutans shared five of them with humans; the
rhesus monkey, an Old World monkey, shared three of them
with humans; the capuchin monkey, a New World monkey,
shared one; the hamster had none of the six pseudogenes.
This pattern corresponded perfectly to the primate evolutionary
lineage.
Humans and chimpanzees. Two species of chimpanzees
are the living animals most closely related to Homo sapiens.
Humans and chimpanzees have a 99 percent similarity of nucleotide
base sequences. The genetic differences between humans
and chimpanzees have major effects but are few in number.
The molecular clock also suggests that the evolutionary lineages
leading to humans and to chimps may have diverged as recently
as five million years ago (see australopithecines).
Modern human diversity. Most anthropologists are now
convinced that African hominins (similar to Homo ergaster)
spread throughout Europe (as Homo HeiDelbergensis)
and Asia (evolving into Homo erectus). H. erectus
became extinct. H. heidelbergensis evolved into neandertals
and became extinct. The only survivors were the H. ergaster
individuals that remained in Africa and evolved into H. sapiens
in Africa between 100,000 and 200,000 years ago. This
is consistent with the fossil evidence, which shows H. sapiens
appearing only in Africa, and then spreading around the
world. This has been called the Out of Africa hypothesis.
If the Out of Africa hypothesis is correct, then there
should be very little DNA variability among modern
humans. University of California geneticists Allan Wilson,
Mark Stoneking, and Rebecca Cann analyzed mtDNA
from 150 women from Africa, Asia, Europe, Australia, and
New Guinea. By using computers that can make thousands
of comparisons among the nucleotide sequences of DNA,
the researchers were able to reconstruct a human family
tree. Their results showed that the evolutionary divergence
of human races was relatively recent, having occurred only
100,000 to 200,000 years ago. Because mtDNA is inherited
only through mothers, the conclusion of the Wilson group
has been popularized as the “mitochondrial Eve” or “African
Eve.” Their results also showed that DNA variability in the
human species is very low, compared to the variability found
in populations of most other species. The greatest degree of
variability was from the African samples, which implies that
the evolution of H. sapiens has been going on in Africa longer
than anywhere else.
More than a decade of study since the original report
of the Wilson group has failed to find human genetic variation
outside of Africa that cannot be found within African
populations. More recent studies on nuclear genes confirm
the pattern that was revealed by mtDNA. Studies on the Y
chromosome suggest that the pattern inherited through males
is the same as that inherited through females. Not surprisingly,
the Y chromosome studies have been popularized as
the “African Adam.” These results indicate that all modern
humans share a recent African origin.
These results also indicate that all non-African human
populations were originally emigrants from Africa. Ancestors of
aborigines left Africa and arrived in Australia over 60,000 years
ago. Their burial customs, and the fact that they would have to
Investigators compared nucleotide sequence differences between
different humans; between humans and Neandertals; and between
humans and chimpanzees. The comparisons were made between
homologous (corresponding) locations on chromosomes. The narrow
vertical lines represent approximate standard deviations of the data.
Comparisons among humans averaged eight nucleotide differences;
comparisons between humans and Neandertals averaged , over three
times as great; and comparisons between humans and chimpanzees
averaged about . Neandertals differed from humans about half as much
as chimpanzees differ from humans.
build boats to reach Australia, strongly suggest that the people
who colonized Australia were fully modern humans. Bones of
some of these people, found in Mungo by anthropologist Alan
Thorne, appear to be 62,000 years old. Mitochondrial DNA
extracted from the Mungo bones is outside the modern range
of human genetic variation, which to some researchers suggests
that they came from a population of humans that no longer
exists. However, the Mungo DNA is only slightly outside the
range of modern humans, and most researchers do not consider
it to discredit the Out of Africa hypothesis.
Humans and Neandertals. Another test of the hypothesis
that all modern humans are descended from African H. sapiens
would be to compare modern humans with Neandertals.
Comparisons of bone structure have suggested that Neandertals
were a separate lineage of evolution from H. ergaster.
However, a DNA comparison would clinch the case. It
would seem impossible to find Neandertal DNA—they have
been extinct for almost 30,000 years—but the laboratory of
Svante Pääbo was able to obtain a little bit of mtDNA from
some of the youngest Neandertal bones. Because mtDNA
is so much more abundant than nuclear DNA, it was more
likely to be preserved in these ancient bones. The Neandertal
DNA turned out to have a base sequence very different from
that of any modern human (see Neandertals). Since 1997,