Simulating Human Origins and Evolution by Ken Wessen (2005) p 3:
Sarich and Wilson employed an immunological technique, measuring the cross-reaction of antigens and antibodies from different hominoid species, as a method of comparing amino acid sequences, the degree of cross-reaction being a measure of similarity. The immune system is obviously highly important in natural selection, and therefore the results obtained by using this method are strongly correlated with the evolution of the species being studied. Results from this new research revealed the fact that humans, chimpanzees and gorillas are in fact more closely related to each other than any of them is to orangutans, so a more accurate phylogeny groups humans, gorillas and chimpanzees (the African apes) together, with orangutans as a sister taxon (see Figure 1.1). The ground-breaking aspect of this work was the imposition of a time scale, leading to an estimate of the time of the human–chimpanzee common ancestor of around 5 million years ago, far more recent than was being indicated by other work at the time.
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