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.
The amino acid sequences in many other proteins are equally as informative technique for studying human/ape relationships. Biochemical techniques can be used to determine the amino acid sequence in proteins or specific parts of them. Comparisons have been made of the α and β chains of haemoglobin from different species using this technique. The α chain is composed of 141 amino acid residues and the β chain of 146 residues. Humans and chimpanzees are identical in these two chains, orang-utans differ from humans in two amino acids and gorillas differ in four, suggesting that humans are more closely related to chimpanzees than to orang-utans or gorillas.
Another form of molecular evidence which can be used is the extent to which the DNA sequence of modern species have diverged from that of its ancestor. This can be done using DNA hybridisation. This technique relies on the complementary nature of DNA, where the base cytosine always pairs with guanine, and adenine with thymine. Random mutations cause changes in the base sequence of the DNA, and the accumulation of such mutations can be used as a molecular clock. The greater the number of differences which have accumulated, the longer the time of divergence.
The first step in DNA hybridisation is to extract a length of a DNA molecule, and then to denature it by gentle heating. The heat causes the hydrogen bonds connecting adenine with thymine and guanine with cytosine to break. The two complementary strands therefore separate. When the DNA is cooled, it reattaches to the complementary strand, in a process called re-annealing. A single strand of DNA can be allowed to re-anneal with a complementary strand from another species, forming hybrid DNA. The stability of this hybrid DNA is dependant on how closely the two strands were able to match up their complementary bases, and therefore how closely the two species are related. The greater the number of bases which pair up, the more hydrogen bonds there are bonding the two strands together, and therefore the higher the temperature needed to denature the hybrid DNA. DNA hybridisation indicates that the closest relationship is between humans and chimpanzees, then humans and gorillas and orang-utans. Gibbons and old world monkeys are more distant.
Fragments of DNA can occasionally be extracted from fossil material, and these small amounts of DNA can be amplified using the polymerase chain reaction. This DNA can then be subjected to analytical techniques such as DNA profiling, and therefore can be used to provide molecular evidence for phylogenic relationhips of extinct species.
Before DNA sequencing was possible, techniques such as those outlined above, showed that chimps, gorillas and humans are all very similar in terms of their genetic make up. The advent of DNA sequencing strengthened this belief. However, although humans and apes have been shown to be very genetically similar, they are both anatomically and behaviourally distinct. This is possibly because the molecules and genes used by scientist to approximate genetic similarity, are not those molecules involved in coding for anatomical differences. Generally, it is the structural components of tissues that have been studied, and not the control of development.
DNA sequencing has many advantages over the techniques listed above. The technique is direct and objective, and is therefore very accurate and useful as a form of evidence. It is superior over protein sequencing as, instead of determining which amino acid was used as part of the protein, it is now possible to know which particular codon was used to code for that particular amino acid, as different codons can code for the same amino acid. DNA sequencing therefore makes it possible to observe changes at silent sites, which previously could not have been detected. This allows patterns of codon usage to be compared between species, as it seems that different species tend to prefer the use of particular codons. DNA sequencing also allows the identification and location of pseudogenes, which are not expressed.
There are more mutations in the 97% of non-coding DNA than in the 3% of coding DNA. This is due to the fact that mutations in coding DNA are far more likely to have a deleterious effect than mutations in non-coding DNA, so such mutations are lost from the gene pool. Non-coding DNA is therefore more useful in revealing close patterns of relatedness, whereas amino acid sequences evolve more slowly, and coding DNA is therefore more useful for observing more distant evolution
Mitochondrial DNA (mtDNA) can also be used to trace phylogenetic relationships. Mitochondrial DNA contains relatively few genes, and in 1981, the human mitochondrial genome was sequenced. As mtDNA is inherited through females, any changes which occur in the DNA are due solely to mutation. Therefore mtDNA is a very useful way of tracing the inheritance of particular sequences contained in DNA. Scientists have even attempted to trace the origin of all humans back to a single mother, known as ‘Eve’. It is postulated that this woman lived 200,000 years ago, possibly in Africa, although this idea is now seen to be very optimistic.
Molecular evidence is powerful in studying human/ape relationships in its own right, but it is even more powerful when linked with fossil evidence. Many studies have linked the two in order to establish a timescale for the divergence of different hominoids, as molecular evidence can only produce a relative timescale, not an absolute one. The following figures for divergence have been suggested:
- The gibbon line diverged between 13 and 22 million years ago
- The orang-utan line diverged between 12 and 19 million years ago
- Humans, chimpanzees and gorilla diverged from each other between 4 and 10 million years ago.
Above is an evolutionary tree for primates based on globin and mitochondrial gene sequences. The bars indicate ranges of estimated times for the branch points.
Despite the large number of studies that have been done, and the many different ways of obtaining evidence about human/ape relationships, there is still uncertainty about the relationships between humans, chimpanzees and gorillas. For example, a recent study of the skin protein involucrin has suggested that there is a closer relationship between chimpanzees and gorillas than either have to humans. It is hoped that future fossil discoveries and molecular studies will help us to discover more about our origins, although we are unlikely to ever find conclusive evidence which enables us to know without doubt the nature and timescale of the relationship between humans and apes.
