It is often claimed that similarities between organisms in their genes and biochemistry are evidence that they have evolved from a common ancestor. For example, here is an extract from Advanced Biology by Michael Kent (Oxford University Press, 2000):The most persuasive evidence that all organisms have evolved from a common ancestor comes from studies comparing the cell biology and biochemistry of different organisms, which reveal that:
- the genetic code contained within nucleic acids is almost universal...
- physiological processes vital to all organisms, such as respiration, follow very similar metabolic pathways
- ATP is the universal energy currency
In Advanced Biology by Roberts, Reiss, and Monger (Nelson, 2000), a section on human evolution claims that: a combination of the fossil record and evolutionary trees derived from DNA hybridisation studies and analysis of the amino acids in certain proteins suggests we shared a common ancestor with chimpanzees from which we split about five to eight million years ago.(p. 772-774)
Longman GCSE Biology by Bradfield and Potter (Longman, 2002) claims that 98% of our DNA is the same as that of the chimpanzee and that this is evidence for evolution. Since the sequencing of the chimpanzee and human genomes, this figure has been revised down to 96%.
The EdExcel A Level Human Biology course says that data from immunological studies of blood sera; amino acid sequences; DNA hybridisation and base sequences gives evidence for human evolution.
There are two major problems to this kind of evidence. First, the similarities can be explained just as well by intelligent design, pointing to a single designer. Second, if similarity is evidence for evolution, then dissimilarity should be an argument against, and there are many dissimilarities between organisms. I discuss these in turn below.
Design
Similarities between living organisms could be because they have been designed by the same intelligence, just as we can recognize a Norman Foster building by his characteristic style (Figure 1), or a painting by Van Gogh. We expect to see repeated motifs and re-used techniques in different works by the same artist/designer.
In addition, similarities can be interpreted as evidence for design economy. Why should a designer come up with, for example, a new system of genetic code for each type of organism? Why should basic metabolic pathways or the universal energy currency differ between organisms? They do not need to because an intelligent designer can build diversity around common features.
Part of the reason for similarity in design, is that organisms have similar demands placed upon them, which can only be met in a limited number of ways. All cars have wheels, not because they have evolved from each other, but because the car designing community recognizes wheels as an efficient way of moving over a flat surface. In the same way there are limited ways of respiring, photosynthesizing or transporting energy.
Dissimilarities
If similarity is evidence for common ancestry, then does its opposite – dissimilarity – provide evidence against common ancestry?
There is, of course, much dissimilarity between living organisms, some of these at a very fundamental level. For example, the standard system of genetic code used by humans is not universal. Eighteen different genetic codes have been found in various species. Many scientists see this as evidence that all life does not come from a single common ancestor.
Similar genes and proteins in organisms are taken as evidence for common ancestry. But as we sequence more and more genomes, we repeatedly find genes which are unique to organisms. These are known as ORFans, and provide a real conundrum for evolutionists.
The DNA sequences of humans and chimpanzees are 96% similar, but the 4% difference represents 40 million individual differences at the nucleotide level.
When genes and proteins are used to try to reconstruct the ancestry of different organisms, and how they are linked in a tree-like pattern, different sources of evidence give different results. Different genes and proteins have conflicting patterns of similarity and difference between organisms. Evolutionists can only get round this problem by working out the most efficient way in which evolution could have worked. When they do this, they have to come up with scenarios where some similarities between organisms are not due to common ancestry, but to convergent evolution. This raises another problem: if similarities are not always due to common ancestry, how can they be evidence for common ancestry?
If the living world is designed, the patterns of similarity and difference we see in the living world could be due to selective use of designed modules to produce different combinations of features.
Comparative biochemistry and cell biology does not give clear evidence for macro-evolution. In fact, recent discoveries such as the non-universality of the genetic code are strong arguments against common ancestry. The patterns of similarity and difference in living organisms are fully consistent with design.