Chimpanzee Genome
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On the basis of comparative molecular studies, chimpanzees are the closest living
relatives of humans. On the level of their genomes, this is reflected in a nearly
identical chromosomal organization and an average DNA sequence difference
only about 10 times higher than that between any two humans. This renders
the comparison of chimpanzees and humans, and the resulting catalog of their
genetic differences, suitable to gain relevant insights into human evolution and
human disease. The collection of genetic differences between both species as a
whole comprises an ideal data set to analyze how DNA sequences change over
time on a high-resolution scale. Of further interest are those differences that are
located in the functional regions of the genome. They form the genetic basis of
the distinct biological properties of humans and chimpanzees and thus might
bear an answer to the question ‘‘What makes us human?’’ More practically, these
differences provide access to the molecular factors that account for the different
disease spectra in humans and chimpanzees, and might, therefore, open up new
therapeutic approaches to Fght human disease.
The relevance of chimpanzees as a subject in evolutionary and biomedical research
is obvious. However, their close relationship to humans demands ethical guidelines
that clearly distinguish chimpanzees from common genetic model organisms in
laboratory research.
1
Human and Great Ape Phylogeny
±or more than a century, scientists have
been busy reconstructing the phylogenetic
relationship of humans and the great apes.
On the basis of the presence of shared
morphological or molecular characters,
individual species can be grouped to
clades of an evolutionary tree. At the end
of the nineteenth century, morphological
evidence led to the generally accepted view
that the great apes together with gibbons
(
Hylobates
sp. and
Symphalangus
sp.) and
humans share a common ancestor from
which no other living organism descended.
In phylogenetic terms, they represent the
monophyletic group of hominoids within
the primates (±ig. 1).
While this view has remained unchal-
lenged since then,
the relative order
in which the lineages to the individual
hominoid
species
emerged – often
re-
ferred to as the ‘‘branching order’’ – pro-
voked intense debate. ±rom the shared
consensus that the gibbon lineage di-
verged Frst, several competing hypotheses
were proposed concerning the branching
order of the remaining species. Accord-
ing to one favored scenario, the African
apes (chimpanzees, bonobos, and goril-
las) branched off second, allying humans
with orangutans (±ig. 2a). An alternative
view already taken by Darwin and Hux-
ley positions the African apes closer to
humans than the orangutans. Obviously,
this hypothesis requires a further decision
whether the closest relatives of chim-
panzees and bonobos are the gorillas – as
most morphologists believed – (±ig. 2b) or
whether either of the species is closer to
humans (±ig. 2c, d). The limited number
of phylogenetic informative morphologi-
cal characters as well as their sometimes
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