The consortium found that the chimp and human genomes are very similar and encode
very similar proteins. The DNA sequence that can be directly compared between
the two genomes is almost 99 percent identical. When DNA insertions and deletions
are taken into account, humans and chimps still share 96 percent of their sequence.
At the protein level, 29 percent of genes code for the same amino sequences in
chimps and humans. In fact, the typical human protein has accumulated just one
unique change since chimps and humans diverged from a common ancestor about 6
million years ago.
To put this into perspective, the number of genetic differences between humans
and chimps is approximately 60 times less than that seen between human and mouse
and about 10 times less than between the mouse and rat. On the other hand, the
number of genetic differences between a human and a chimp is about 10 times more
than between any two humans.
The researchers discovered that a few classes of genes are changing unusually
quickly in both humans and chimpanzees compared with other mammals. These classes
include genes involved in perception of sound, transmission of nerve signals,
production of sperm and cellular transport of electrically charged molecules
called ions. Researchers suspect the rapid evolution of these genes may have
contributed to the special characteristics of primates, but further studies are
needed to explore the possibilities.
The genomic analyses also showed that humans and chimps appear to have accumulated
more potentially deleterious mutations in their genomes over the course of evolution
than have mice, rats and other rodents. While such mutations can cause diseases
that may erode a species’ overall fitness, they may have also made primates more
adaptable to rapid environmental changes and enabled them to achieve unique evolutionary
adaptations, researchers said.
Despite the many similarities found between human and chimp genomes, the researchers
emphasized that important differences exist between the two species. About 35
million DNA base pairs differ between the shared portions of the two genomes,
each of which, like most mammalian genomes, contains about 3 billion base pairs.
In addition, there are another 5 million sites that differ because of an insertion
or deletion in one of the lineages, along with a much smaller number of chromosomal
rearrangements. Most of these differences lie in what is believed to be DNA of
little or no function. However, as many as 3 million of the differences may lie
in crucial protein-coding genes or other functional areas of the genome.
“As the sequences of other mammals and primates emerge in the next couple of
years, we will be able to determine what DNA sequence changes are specific to
the human lineage. The genetic changes that distinguish humans from chimps will
likely be a very small fraction of this set,” said the study’s lead author, Tarjei
S. Mikkelsen of the Broad Institute of MIT and Harvard. Among the genetic changes
that researchers will be looking for are those that may be related to the human-specific
features of walking upright on two feet, a greatly enlarged brain and complex