568
Chimpanzee Genome
A
comparison
of
this
gene
between
humans, chimpanzees, and other primates
revealed that it carries two human-speciFc
nucleotide substitutions, each of which
translates into an amino acid substitution
in the encoded protein. This becomes
especially interesting in the light that this
protein is evolutionarily highly conserved.
Between humans and mice, only three
amino acid changes are observed; two
of which occur on the human lineage.
±urthermore, there is indication that this
gene has been the target of selection
during recent human evolution. Both
observations in combination can be judged
as an indication that the respective gene
could have a certain relevance for the
evolution of spoken language in humans.
These examples demonstrate that, in-
deed, the comparison of humans and
chimpanzees is capable of detecting ge-
netic
changes
that
can
be
correlated
with phenotypic differences between both
changes. This justiFes the hope that in
a step-by-step process, even the genetic
frameworks of traits as complex as (spo-
ken) language can be identiFed.
3.3
Comparative Analysis of Gene Expression
The phenotypic appearance of a species
is not only dependent on the functional
integrity of its genes but also on their
correct activation with respect to time,
place, and extent. Genetic changes that
alter this construction plan of a species
provide a powerful tool to achieve sub-
stantial phenotypic changes over a very
short time period. In view of the low
genetic difference between humans and
chimpanzees, it was considered in early
comparative studies that changes in a
few ‘‘master genes’’ – genes that control
gene expression at certain key points of
development – account for the phenotypic
differences between both species.
To date, such master changes, if they
exist, remain to be identiFed. However,
as a Frst step, evidence was provided
that the pattern of gene expression in-
deed varies in a species-speciFc way. When
the expression pattern of nearly 18 000
genesisana
lyzedforsevera
lhumanand
chimpanzee individuals, it is found to be
more similar among individuals of the
same species than between species (±ig. 6).
Starting from this observation, follow-up
studies have to determine whether the evo-
lution of gene expression is mainly due to
regulatory changes in individual genes or
whether a cluster of functionally related
genes changes their expression in a con-
certed manner due to a switch in a shared
master gene. ±urthermore, those individ-
ual genes that have substantially changed
Chimp 1
Chimp 2
Chimp 3
Human 1
Human 2
Human 3
Orang
Fig. 6
Relative extent of the differences
in the expression pattern within and
between species. The expression pattern
jointly determined from nearly 18 000
genes is found to be more similar within
humans and chimpanzees respectively,
than between both species.
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