Essay:Human evolution and the great apes

''This is a practice essay for uni. I used information from RationalWiki's common descent article, which ended up being very helpful. Please be brutal in your criticism, it's the only way I'll learn.''

Introduction
Modern humans are part of the taxonomic family Hominidae, known as the great apes. This family consists of four extant genera; namely humans, chimpanzees, gorillas and orangutans. The phylogenetic relationships between these species has been analysed in detail, with current thinking suggesting that the chimpanzee is our closest living relative.

In this essay, the key evidence identifying chimpanzees as the closest living relatives to humans will be described and explained, using appropriate references. This will include a brief history detailing the discovery of human evolutionary relationships, as well as considering the conflicting evidence and drawing an appropriate conclusion.

History
One of the first scientists to review the various differences between the great apes was Thomas Henry Huxley, a nineteenth century biologist. In his book ‘Evidence as to Man’s Place in Nature’, he determined that the anatomical differences between humans and the African apes, the chimpanzee and gorilla, were less distinct than the differences between the African apes and the orangutan, found in Asia. He concluded that ‘the structural differences which separate Man from the Gorilla and the Chimpanzee are not so great as those which separate the Gorilla from the lower apes’. Charles Darwin used Huxley’s evidence in his work ‘The Descent of Man’ to suggest that because humans are morphologically closer to the African apes than the sole Asian great ape, the ancestors of modern humans are more likely to be discovered in Africa.

The development of biochemistry and immunology in the early twentieth century moved investigation from anatomy to the morphology of molecules. This area of research was described as ‘molecular anthropology’ by the biochemist Linus Pauling. A report by Emile Zuckerkandl, published as part of a collection in 1963, described the process of isolating haemoglobin from the red blood cells of various species, splitting the protein into peptide fragments, and then comparing using electrophoresis. The patterns produced by humans, chimpanzees and gorillas were indistinguishable (Washburn, 1963). Another report by Morris Goodman published in the same collection also compared the protein albumin from blood serum using similar techniques. The results for humans and chimpanzees were almost identical (Washburn, 1963).

The discovery of DNA in 1953 potentially removed the need to rely on both anatomy and the morphology of proteins, instead comparing specific DNA sequences. Upon comparison of both nuclear DNA and mitochondrial DNA, it was found that human and chimpanzee DNA is more closely related to each other than to gorilla DNA. This finding is echoed in numerous investigations (Chimpanzee Sequencing and Analysis Consortium, 2005). The current phylogenetic tree for the family Hominidae splits humans, chimpanzees and gorillas from orangutans, followed by a split from gorillas, then a final split between chimpanzees and humans.

Evidence
A key piece of genetic evidence linking the evolution of modern humans and chimpanzees is the human chromosome 2, a product of fusion. A chromosome fusion event occurs when two chromosomes combine and decrease the chromosome number in descendent species. Often, the karyotype patterns created can be used to generate an accurate phylogenetic history.

All great apes have 24 chromosome pairs, apart from humans, who have 23 pairs. In the case of chromosome 2, there are a number of features that suggest a strong evolutionary link with chimpanzees (Ijdo, et al., 1991). Chromosome 2 has an almost identical karyotype banding pattern to that of two chimpanzee chromosomes stacked on top of each other. Also, the telomere regions found on the ends of chromosomes to protect against deterioration are found in the middle of chromosome 2, further suggesting a fusion event. Finally, a non-functional second centromere has been found on the chromosome, genetically identical to that found on a chimpanzee chromosome.

Other similar chromosome fusion events have been identified, supporting the chimpanzee as the closest living relative to humans (Yunis & Prakash, 1982).

Pseudogenes, genes that no longer function, also help to provide evidence of common ancestry through the identification of shared errors. Whilst convergent evolution can cause two similar genes to be produced in different lineages, errors shared between two genes indicates a common ancestor, as errors are independent of desired traits and are very unlikely to be replicated exactly in two separate lineages throughout evolution. Since all pseudogenes are descended from a parent functional gene, it is possible to compare two pseudogene sequences and determine if the same errors are present in both.

A well known example of a pseudogene establishing a common ancestor uses the gene coding for gulonolactone oxidase, an enzyme that synthesises ascorbic acid, Vitamin C (Nishikimi & Yagi, 1991). While this gene is present in most animals, in some it has been inactivated due to mutation, including the great apes, guinea pigs and some fruit bats. In each group, the specific mutation that inactivates the gene is different, so the mutation in guinea pigs is in a different location to that of the great apes. Many other pseudogenes have been found that are only shared by humans and chimpanzees (Hughes, et al., 2005). Intriguingly, a truncated pseudogene has been discovered that is only shared by gorillas and humans, although the report suggests that chimpanzees may have lost that gene altogether (Ueda, et al., 1985).

Finally endogenous retroviral insertions, or ERVs, can also provide strong evidence of common ancestry, especially when distinguishing the phylogenetic relationship between humans and the other great apes. When retroviruses use reverse transcriptase to code RNA into DNA, errors often occur, some of which can inactivate the retrovirus. However, the randomly inserted DNA remains in the host organism genome, and if these insertions are made in a germ line cell, they will be passed on to any offspring generated by that cell. Due to their random nature, if any two species share the same ERV in the same location with the same inactivating mutations, they almost certainly share a common ancestor in which the ERV initially occurred.

Using this information in relation to the great apes, the number of shared ERVs can be compared and used to determine phylogenetic relationships. Whilst there are common ERVs amongst all the species concerned, humans share more ERVs with chimpanzees than with any other great ape (Johnson & Coffin, 1999). This evidence again supports the chimpanzee as the closest living relative to humans.

Conclusions
This essay has described the scientific discovery of human evolutionary relationships and has used appropriate evidence to show the particularly close relationship between modern humans and chimpanzees. While this is the general consensus, there are alternate viewpoints. The orangutan has been suggested as a replacement, supported by anatomical similarities between orangutans and humans, such as our dental structure, shoulder blades, and the presence of a distinct beard and moustache (Grehan, 2006). Opposition to this viewpoint suggests that these physical characteristics are misleading, relying instead on genetic evidence. However supporters say that orangutans may have undergone more genetic change since their split from a common ancestor, explaining the apparent differences.

Whilst it is true that orangutans resemble humans more closely in physical appearance, and early hominids share characteristics unique to orangutans, there is still not adequate reason to dismiss the chimpanzee. Unless the powerful genetic similarities detailed in this essay can be accounted for, without suggesting convergent evolution which is not applicable to the evidence described, the conclusion at this time must be that the chimpanzee is indeed our closest living relative.