That elusive one per cent

An international research team has finally sequenced the chimpanzee genome and found a mere one per cent difference with the human genome. Spanish geneticist Julio Coll explains the significance in an interview.

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 works?

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 species?

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 science.

Translated from the Spanish by Michael Cook.