The genomes of humans and orangutans differ by 2.4%, humans and gorillas by 1.4%, and humans and chimpanzees by only 1.2%. There is more difference between chimps and orangutans (1.8%) and gorillas and orangutans (2.4%) than for humans and gorillas or chimps. A group of New Zealand biologists studied five different proteins in eleven different animals, including humans, to see if a tree of relationships based on one protein would hold true for five proteins and for as many of the eleven widely different animals as possible. Computer studies produced a parsimonious tree of relationships, five trees, in fact. The probability of that all five trees would be identical by sheer chance or “luck” is infinitesimally small. The five trees from five different proteins were in substantial agreement but were not identical. However, all five proteins studied placed monkey, chimpanzee, and human molecular agreement as the closest of all of the organisms studied. There was some disagreement in the results about which of the remaining organisms fell in which order of relationship to that cluster, but not about humans.
The differences between our closest relatives, chimpanzees, and us are multifactoral. The most telling difference is genetic: Chimps have 24 chromosomes, and humans have 23. The genomes of all of the primate groups have been established. Comparison of human chromosomes to those of the great apes—chimpanzees, gorillas, and orangutans—reveals only that one glaring difference: the human genome contains one fewer chromosome than the genomes of the great apes. They all have 24, whereas the human has only 23. The exception is human chromosome 2, which matches two different chromosomes in chimpanzees and the other great apes.
In a land mark article, researchers confirmed the findings of several earlier studies in exquisite detail. They demonstrated that chromosomes from humans, chimpanzees, gorillas, and orangutans are highly similar and can be aligned with one another. (Jorge Ynis and Om Prakash, The Origin of Man: A Chromosomal Pictorial Legacy, Science 215:1525-1530, 1982). More recent studies have confirmed that, not only are the chromosomes of primates remarkably similar, but even the DNA sequences and configurations match to a heretofore undreamt of degree. In chimpanzees and humans the match of sequence and organization is strikingly more than 98%. All researchers arrived at the same conclusion that all human and chimpanzee chromosomes are very nearly identical with the exception of the human chromosome 2 which matches two different chimpanzee chromosomes.
This occurred either by a duplication of a chromosome in chimps–which turned to be the correct hypothesis–or a merger in a human chromosome after we separated from the common ancestor, a matter of some debate among scientists until fairly recently. The DNA sequence in humans contains around 3 billion base pairs, and in 98.8% of chimp DNA, the bases are identical–a difference of only 1.2%. That 1.2% is 36 million different base pairs–half of which are chimp specific–which leaves 18 million pairs. About 5% of our DNA is used in coding or regulatory functions leaving only a few thousand for changing human form—apparently enough for considerable Hox gene, tool-kit, and switch gene–probably the most important ones for changing form–space in the protein base for differing functions to arise and persist. In humans as in most other animals, evolutionary differences are largely due to changes in gene regulation.
The process which separated humans from chimpanzees from a common ancestor and each other has been very clearly explained by Daniel J. Fairbanks, PhD., Relics of Eden, The Powerful Evidence of Evolution in Human DNA, Prometheus Books, Amherst, New York, 2007. In 1991, Yale University scientists sequenced the DNA from the site in the middle of human chromosome 2 and found it to match the telomeres (ends) of chimpanzee chromosomes 2A and 2B. Their findings clearly revealed that fusion of two chimpanzee chromosomes was the process that produced human chromosome 2, perhaps the clearest evidence yet of the evolution of humans from a common ancestor with chimpanzees. In the region of the human chromosome studied, there is a DNA sequence with 158 copies of the tandem repeat found in telomeres of the chimps. It is inarguable that at some point in human ancestry, the telomere of one chromosome of the common ancestor fused head-to-head with the telomere of a different chromosome, and that exact fusion site is preserved in the DNA, indicative of one of the crucial elements of becoming human.
At the fusion site, the sequence in the upper strand abruptly changes from repeats resembling TTAGGG–as it does in 50-100 repeats of the same 6 pairs of amino acids–to repeats resembling CCCTAA. This abrupt switch is convincing evidence that the DNA in a telomere of one chimpanzee chromosome and the DNA in the telomere of the other chromosome broke, and then the two chromosomes fused at the broken ends, and they did so rotated 180 degrees which inverted the DNA sequence. This abrupt switch is convincing evidence that the DNA in a telomere of one chimpanzee chromosome and the DNA in the telomere of the other chromosome broke, and then the two chromosomes fused at the broken ends. The fusion event was survived and presumably had at least neutral or even selective survival value. Further study of the fusion site indicates, in fact, that the ancient telomere at the fusion site is now a nonfunctioning relic of evolution embedded in the middle of human chromosome 2. Furthermore, in humans, there is only one centromere where there should be two.
The findings at this site in humans compared to chimpanzees is a vivid example of the cause of a genetic change that produced a great leap forward by producing the human branch separate from a chimpanzee branch of the common ancestor stem. The more generations humans are separated from the fusion event in the common ancestor with chimpanzees, the more mutations we should expect to accumulate in the sequence. Because the majority of the repeated segments now have mutations in them, the chromosomes
A small piece of human DNA (Photo credit: Wikipedia)
must have fused a very long time ago, probably tens of thousands of generations deep into our ancestry. The work of R. Avarello, et al, Evidence for an Ancestral Alphoid Domain on the Long Arm of Human Chromosome 2, Human Genetics 89: 247-249, 1992, and A. Baldini, et al, An Alphoid Sequence Conserved in All Human and Great Ape Chromosomes: Evidence for Ancient Centromeric Sequences at Human Chromosomal Regions 2q21 and 9 q13, Human Genetics 90: 577-583, 1993 is an eloquent proof of what happened very long ago to produce the last surviving member of the hominin line.
There are myriads of remnants of ancient gene errors in the human genome that attest to the frequency of such errors. Most of our genetic makeup consists of such relics which demonstrate the history of our genome as the tens of thousands of generations followed upon each other. These so-called retroelements—the relics–provide significant evidence of human evolution when human and other primate retroelement genomic positions are compared. The statistical chance of two retroelements independently inserting themselves into two different individuals, let alone two individuals of different species is infinitesimally small. Through the course of many genomic studies of different primate species, molecular geneticists have found an unmistakable pattern. In a remarkable number of instances, they found transposable elements in human DNA that were present in exactly the same positions in chimp DNA and to a lesser degree in gorillas and an even lesser degree in orangutans. Chimp and humans were 98% similar. These findings can only be explained by the existence of a common ancestor of the two species with subsequent variations by mutations after the species diverged.
There is one more evidence, not only of the evolution of man, but of the closer approximation of descendancy between humans and chimpanzees. The evidence of human evolution comes from an unlikely and negative source—the genetics of CMT [Charcot-Marie-Tooth disease] which is one of the most common inherited neurological disorders, affecting approximately 1 in 2,500 people in the United States. CMT–also known as HMSN [hereditary motor and sensory neuropathy] or peroneal muscular atrophy–comprises a group of disorders that negatively affect peripheral nerves. Typical features include weakness of the foot and lower leg muscles.
The evolutionary changes in both species were directed after the chromosomal duplication error because the retroelements located close to each other target the DNA between them for duplication. The target in question for the serious neurological disease is a retroelement—ancient relic gene, or pseudogene—known as CMT1A. Duplication of CMT1A in the two species predisposes the area to even further duplications. In the case of CMT disease, duplication of the sequence between the two CMT1A segments adds another copy of a gene that has come to be known as PMP22. Two copies of that gene result in CMT disease. This is an unfortunate legacy of our imperfect evolutionary heritage and militates against the Creationist argument of a designer driven process; it would be a violation of such a concept to suppose that the perfect Creator would create or even allow such an erroneous and unfortunate inheritable condition.
Another example—another evidence—of primate (and human) evolution is the presence of a unitary pseudogene called the GULO (or ψ GULO) pseudogene. There is no functioning GULO gene in humans, but a functional version is present in many other animals, which allows them to make vitamin C. Animals with a functioning GULO gene–such as dogs and cats–do not need to consume vitamin C in their diet. Their cells can make it from other food substances. All primates lack the gene and must consume vitamin C to survive. As it turns out, the difference between function and nonfunction in GULO is the result of a change in only a single nucleotide.
From an evolutionary point of view, it is reasonable to question why we lack such an essential gene. In fact, a piece of the gene is still in our DNA, but it is highly mutated as a unitary pseudogene with much of the original gene missing and is therefore nonfunctioning. About 20% of the DNA sequence is mutated in comparison to other mammals. The presence of so many mutations requires that the original gene had to have been disabled long ago. The evidence of our evolution from a common ancestor of the primates is found in the fact that all primates have the same highly mutated fragment; so, the gene had to have been disabled before the different primate lineages diverged from the common ancestor. Chimpanzees and humans have nearly identical (98%) copies of the useless pseudogene in the same place in the DNA. Remarkably, the same can be said for almost every other pseudogene in the human genome which is incontrovertible evidence of our evolutionarily close relationship to chimpanzees. The reason primates ended up with only a relic of such a beneficial gene is quintessential Darwinian natural selection—which allows for nonfatal errors. The scientific molecular genetics data accumulated by decades of patient work has identified literally millions of such examples.
The laws of probability govern inheritance of DNA in part, and mutations occur as random variations. When very large amounts of DNA sequences are compared, the random variations—the ‘noise’—are averaged out, and the signal becomes much more apparent. Satta, et.al., compared 45 chromosomal genes covering 46,855 base pairs and determined that the human-chimpanzee relationship is closest in the trichotomy—which includes gorillas. (Y.Satta, et.al., DNA Archives and Our Nearest Relative: The Trichotomy Problem Revisited. Molecular Phylogenetics and Evolution 14:258-275, 2000). Wildman, et. al., evaluated 97 genes encompassing nearly 90,000 base pairs in 2003 and came to the same conclusion. Shi, et.al., studied 127 genes on chromosome 21 in 2003 and concluded unequivocally that their research, “unambiguously comes to the conclusion that chimpanzees were our closest relatives to the exclusion of other primates”.
Man is related to all flora and fauna of the earth, past and present, through a relatively few ancient collections of DNA proteins that remain remarkably very little altered to the present. Man evolved. Man is still evolving. Evolution is not a matter of man becoming a more prestigious species or a “higher animal”, but rather evolution through natural selection led to a truly remarkable species, one that has dominated the earth for tens if not hundreds of thousands of years. Barring cosmic, climatic, or self-inflicted catastrophes, it is likely that gradually evolving man will continue to be dominant in the foreseeable future. Paleoanthropology, the scientific discipline that studies hominids, will undoubtedly have sufficient new material to keep the study of man a vibrant exercise well into the future.