Date: 31 January 2003
Subject: Biology

It has become almost a mantra amongst some biologists that we share a high percentage of genetic materials with our "closest relatives" the great apes. Figures of 98.5% or 99% genetic similarity have often been cited as "evidence" that we are really not all that different. Now, however, this cherished figure has been called into question by work by Roy Britten of the California Institute of Technology, himself involved for many years in this research.

The commonly claimed high figure of identity has been used by those who attack the "special" nature of being human. Professor Richard Dawkins, in his anti-Christian polemic The Blind Watchmaker bewails as "speciesism" the human tendency to value human zygotes above great apes, and adds: "Chimpanzees and we share more than 99 per cent of our genes." (p.262) Dawkins argues that had all the survivors intermediate between ourselves and chimps (from whom we split off "as recently as five million years ago"), then we would have to make an arbitrary line between who would have "rights" and who not. This apparent plea for "human" rights for chimps seems to ignore the difficulty that since (according to Dawkins' own anti-saltation views) the transition has been smooth from the first organisms onwards, either he will have ultimately to make an "arbitrary line" somewhere or grant rights to mushrooms.

It is not, of course, that Christians would wish to deny animals some levels of rights - or to deny that "higher" animals may rightly be given more of them. It is not that we would wish to deny a degree of self-awareness to chimps and gorillas that is not present in our pet cats or dogs. It is just that an argument for moral worth based on a genetic similarity seems a bit spurious.

Now, it seems, even the degree of similarity has been exaggerated. In an article in the October 15th Proceedings of the National Academy of Sciences Dr Roy Britten (a distinguished physicist and geneticist) describes how he used a computer program to compare nearly 780,000 base pairs of the human genome with a similar number from that of the chimp.

The DNA is a double helix formed of two "strands" of base pairs adenine-thiamine, and guanine-cytosine. In reproduction it splits, each strand finding appropriate base pairings to recombine with. The old "hybridisation" method of DNA comparison involved splitting the helix strands of two species, combining one from each, and then finding the temperatures at which similar DNA strands from the two species break apart from each other. The lower the melting temperature, the less compatibility between the two species because lower energy is required to break the bonds. This method estimates numbers of single base substitutions, ie where a single base of the genetic code differs between the species. It ignores, however, "indels" (where a whole section of DNA may be in one species and not the other) and their various lengths. Britten used a computer program to compare known sequences of chimp-human directly and found around 1.2-1.69% mismatch, but also some 3.4% attributable to indels (occurring equally in repeated and nonrepeated sequences). This gives a comparison figure around 95% - though admittedly it is still based on a comparatively small section of the total genome.

If, then, the figure is actually nearer 95%, does this imply that chimps are 95% human? This would be a foolish inference to make. We are all too aware of the dramatic difference in terms of functionality to the human organism that a small genetic change can make. In terms of personhood the point is surely not the number of changes in the code, but what effect any given change makes. Whether, for example, chimps or gorillas have numerically greater DNA divergences from humans will not necessarily tell us which are more like us in psychological terms - let alone whether there are any "spiritual" aspects.

Britten himself is more interested in the relevance of his work for evolutionary conclusions: the usual presumption being that greater genetic similarity implies a more recent common ancestor. In these terms his paper concludes that "comparison of the DNA sequences of closely related species reflects many events of insertion and deletion" and so "It is the result of a major evolutionary process". We are, in other words, more distantly related than previously thought - major processes would be necessary to produce this much divergence. Christians differ as to how far God created through natural processes or apart from them, but even Richard Dawkins (as above) sees chimp-human divergence as five million years ago whereas few date back modern homo-sapiens-sapiens as more than a couple of hundred thousand and many now see it as less. We should in any event, therefore, resist the temptation to anthropomorphise modern great apes - even though we should be rightly concerned for their welfare and protection.
NB: Dr Roy Britten's paper is on http://www.pnas.org/cgi/reprint/99/21/13633.pdf?ijkey=QtVhMDilon7HE