That elusive one per cent
There is only a 1.2 per cent difference between the human genome and
the chimpanzee genome, according to a landmark paper published in the
leading journal Nature earlier this month. Following this achievement
by an international research team, the Chimpanzee Sequencing and
Analysis Consortium, scientists are eager to complete the sequencing of
other great ape genomes in order to nail down the elusive difference
between homo sapiens and his animal cousins. Dr Julio Coll, a
geneticist at the Complutense University of Madrid, was interviewed by
the Spanish publication Aceprensa about the significance of the event.
A comparison of the human genome and the chimpanzee genome shows that
the difference between their DNA sequences is just a bit more
than one per cent and that the number of genes is almost the same.
However, the differences between the species are still enormous. Can
just one per cent be responsible for this?
Coll: In principle, you would think so. Since some genes control how other
genes are expressed, a change or a mutation in a gene can affect the
expression of many other genes which have not changed and can give rise
to profound differences. Nonetheless, it is surprising -- and the
authors of the study acknowledge this -- that such a small difference
is responsible for all the differences between humans and chimpanzees.
The Nature study shows that the genes expressed in the human brain and
in the chimp brain are nearly the same. If this is the case, can the
huge differences in intellectual capacity between the two species be
expressed only by the genes or are other factors at work?
Coll: It appears that there must be new functions or regulatory mechanisms
distinct from these genes in the brain. In fact, according to the
study, humans have inserted 7,000 repeated fragments, compared to the
2,300 fragments in the chimpanzee genome, since chimpanzees and hominids
diverged. These inserted fragments, which are not genes, may hold the
key to these hypothetical new regulatory mechanisms. But this remains
to be demonstrated.
If evolution is nothing more than a change of genomes, comparing
genomes would reveal the history of evolutionary changes. Are
scientists getting closer to convincing results about how evolution
Coll: Comparing protein sequences has been going on for many years now. Now
that complete genomes for species have been sequenced, more and more
comparisons have been carried out . This is sure to influence our
knowledge of the evolutionary process. Some of this data confirms the
evolutionary lines established by morphological studies, while other
data points in different directions. For example, the fragments to which I referred earlier, are independent of the line
of traditional evolution. The same types of fragments appear in some
unrelated species and not in other closely related species.
Much of the promise of biotech is based on the notion that repairing a
single defective gene will suffice to cure an illness. Now everything
seems more complex. What other mechanisms need to be investigated to
understand human diseases better?
Coll: Every genetic sickness is very complex and one cannot generalise.
Repairing a single gene to cure an illness is sometimes a
solution, but other diseases are caused by several genes. We have to
proceed on a case-by-case basis. Now that we know what is a gene and
what is not a gene, we will be able to advance more quickly. In this
aspect medicine is making great progress by using animal models like
the rat and the zebra fish, which have genomes which are quite similar
to humans. This allows us to do comparative studies and makes our
research much easier.
The fact that man and chimpanzees share 99 per cent of their DNA helps
us to feel closer to this species. On the other hand, it is humans who
are studying chimpanzees and not chimpanzees who are studying humans.
There is an enormous intellectual gap. Does this mean that the complexity
of our humanity cannot be explained simply as the emergence of another
Coll: The emergence of one species from another related species is still a
poorly understood process. In spite of the fact that we explain these
transformations by evolutionary changes, to date no one has succeeded
in changing one species into another in a laboratory. The complexity of
human beings does not seem to reside in new genes. But any other
explanation of the transition from chimpanzee to hominid -- as, for
example, through new regulatory mechanisms between similar genes --
will be very difficult to demonstrate. In any case, with all this new
information we can go deeper into our knowledge of homo sapiens. On the
other hand, the capacity of humans for self-reflection, for
self-consciousness, the relationship between the self and the brain,
and so on, will, as far as I can see, always be outside the realm of
Translated from the Spanish by Michael Cook.
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