The two major issues in investigating human/ape relationships, are the nature of the relationships between humans and great apes, and the timing of their divergence. Various molecular techniques can be used in order to try and establish molecular clocks, which rely on the regularity of change over time. Allthough the accuracy of such molecular clocks is questionable, the culmination of evidence from many different techniques and areas of research, enable us to piece together probable relationships and timescales, as well as enabling us to rule out previous suggestions which have been made in error.
One form of molecular evidence of our ancestry is provided by immunological studies. An organism’s immune system protects its body against invasion by foreign material, such as protein, as it may cause harm to the organism. The body responds to these foreign substances (antigens) by producing anti-bodies. These anti-bodies react against the antigen to either destroy or neutralise it. Each anti-body is specific to the invading antigen.
The serum of blood contains proteins, such as albumin, but no clotting factor, therefore proteins from one animal are foreign to an animal of a different species. Such proteins therefore act as antigens in the other animal’s body, and that animal produces antibodies to try to attack the antigen. If the sensitised serum containing the antibodies is then extracted from the animal, and recombined with the original protein, the antibodies react against the protein forming a precipitate. This experiment can be performed with rabbits in the following way.
Blood sample taken from human Blood cells separated from plasma Serum from human is injected into the rabbit, and rabbit subsequently develops antibodies to human serum A blood sample is the taken from the rabbit and separated to produce sensitised rabbit serum This sensitised rabbit serum is then reacted with serum from other animals The level of precipitation is recorded. A large amount of precipitate indicates that the two species are closely related, whereas a small amount suggests greater divergence in both time and relatedness.
This is due to the fact that anti-bodies are highly specific. For example, a reaction with baboon albumin will produce less precipitate than a reaction with human albumin, as a less tightly bound and smaller albumin-antibody complex will be produced with the baboon albumin, as the albumin genes in baboons are genetically different from the albumin genes in humans. Such genetic variation means that the amino acid sequences of the albumin protein varies from species to species. The following table shows the comparison of the antigen/antibody reaction in a number of mammal species.
This technique is useful as it enables comparisons to be carried out between different species. However, the technique is also criticised on these grounds as it is indirect. The similarity between protein molecules is assessed using a third species (the animal used to make the anitbodies), which makes the value of such results disputable and controversial. However, it is now clear that these results are representative of the genome as a whole, as protein and DNA studies show equally strong relatedness as that found in albumin studies.
It is therefore clear that there is remarkable similarity between ourselves and the living apes. There are two possible explanations for this. The first being that the common ancestor of humans and apes lived recently, and there has been little time for genetic divergence. The second explanation is that divergence has occurred slowly along the lineage that link our common ancestor with that of living apes. By calibrating the immunological molecular clock, it is possible to estimate approximately when humans separated from gorillas and chipanzees. In order to do this it is necessary to know the relationship between the number of amino acid sequence differences and the immunological distance, and the timescale on which we are operating. Each 1% decrease in the precipitate indicates about 2 sequence differences.
Modern primates first appear in the fossil record about 56 million years ago, and no primate at all is present before 60 million years ago. Therefore the maximum divergence times amongst living primates therefore cannot be much greater than 60 million years. There is approximately 35% change along the albumin lineages of the simian primates since their separation from those leading to living prosimians. This translates to approximately 0.6% immunological difference per 1 million years of lineage. From this figure, it is predicted that gorillas chimpanzees and humans separated from each other a minimum of 4 million years ago. This is a minimum estimation as it is possible that the radiation of primates began more than 60 million years ago.
Allthough this molecular clock has many flaws which may cause inaccuracies, these findings have ruled out previous misconceptions about primate evolution. We can now be sure that Ramapithecus, which had been dated at around 14 million years ago, could not have been a hominid, as was previously thought. We can also rule out the idea that humans, gorillas and chimpanzees shared a common ancestor 20 million years or more ago. As 20 million years is approximately a third of the time for which primates have existed, we would expect an albumin difference between humans, chimpanzees and gorillas of 20% to 25%, but only a 5% divergence was found.