Sunday, March 29, 2009
The relation between primates and humans
A common misconception of people who gloss over the fundamentals of evolution is that humans are descendants of primates. Although we are not direct descendants of primates, according to the evolutionary tree of hominoids (figure 4.4 pg. 101), which solidifies what should be common knowledge, both primates and humans are descendants of a shared ancestor. Using figure 4.4 and the statistics provided regarding “DNA typing and hominoid evolution” (figure 4.3 pg. 100), analyze the ultimate causes of divergence presented by the tree. What is the correlation between the divergent structures of, for example, a human and chimp, and how do these disparities serve their function of improving fitness (in addition to the example provided regarding trichromatic/dichromatic vision). Is there a particular region of DNA that has been continually modified to give rise to these adaptations, or is it dispersed? Is there some sort of pattern to proportionally how far away the genetic information is from the centromere that organizes the divisions of the tree? If not, what seems like a proper estimate of how far the genetic code should be from the centromere on a chromosome to facilitate the changes seen? What characteristics would make-up the genetic code of the shared ancestor of both the primate and humans? Is there a key redundancy in the genetic code that unifies the branches of the tree while still providing the “wiggle room” for evolution of structure and function?
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In their article “Genetic evidence for complex speciation of humans and chimpanzees” which was published in Nature magazine in 2006, Patterson and Richter, through intensive studies of the various genomes, found that the chimpanzees and the humans diverged from a common ancestor anywhere between 6.5 and 7.4 million years ago. However, their studies proved that this divergence was not a clear split in speciation. Their was about a one million year period of interspecies mating called hybridization that would interlink the varying genomes, thus leading to some of the more nominal similarities found in chimps and humans today.
ReplyDeleteHowever, Sofia was asking what some of the physiological differences are between the humans and chimps that better adapt them to their respective niches on this planet. In a vast study of the human and chimp genome done by Celera genomics, it was concluded that, in terms of genetic make-up, we have only a 1% difference with our long lost, hairy, relatives. And most of the coding that makes up this difference is in junk that does not code for any genes with physiological connections. However, as it is clear, some physical differences did emerge from this 1% divergence between our genomes. The main differences are sense of smell, digestion, long-bone growth, hairiness and hearing. These differences were a result of changes in the feeding and mating habits of the humans. The feeding changes could have been a result of the global changes of about 2 million years ago, which inclined humans to change their diets to a meat based one. Another major difference between humans and chimps is our complex use of speech. One of the main physiological developments that aids human speech is the gene that codes for a unique protein in the tectorial membrane of the inner ear. With out this protein, humans would be deaf, and verbal language would not be possible. Human speech is a key advancement that allowed humans to group and fight in packs, communicate mating interests, and even distinguish between clans. These are but a few of the physiological differences that set apart us from the chimps. *
*(http://www.medicalnewstoday.com/articles/4945.php)
As Jack stated above, the divergence of humans and their closest relatives, the chimpanzees, happened an estimated 6 million years ago. With only a few percent difference between humans and chimpanzees, whether taking into account insertions, deletions, and other specific genetic differences, humans and chimps are around 96 to 99% similar. There are an estimated 40 million differences in the 3 billion nucleotides comparing the two species. Of these 1.2% are single nucleotide changes to the genetic code while 2.7% are duplications. According to evolutionary time, 6 million years is an extremely short amount of time to account for the vast physiological differences between humans and chimpanzees. Because of this, the differences seen between humans and chimps must be due to mutations and evolutionary development of a few very important genes. Some scientists believe that duplication of already existing genes are the main mechanisms that brought about the great difference between the two species. This can be seen with the development of trichromatic vision in humans and other higher primates. The duplication of the MWS/LWS opsin gene and the differential accumulation of fine-tuning mutations led to trichromatic vision and a conversion to a very different lifestyle based on visuals than on smell perceptions. The simple mutation of a couple important sites on the duplicated gene cause the turn from dependence on other senses to visual sense. This is found through the extensive fossilization of olfactory genes found in most humans and higher primates with trichromatic vision. it’s the idea that mutations are likely to accumulate if multiple copies of a gene exist, selecting for a variation for better or enhanced function. Many scientists are concentrating on copy number variation sites, the sites where genes are redundantly duplicated, to find the evolutionary history that led to the division of chimps and humans.
ReplyDeleteThe relationship between humans and chimps has become a popular one with many different fractions and groups of scientists researching the possible explanations for it. So other groups of scientists are pointing to another possible explanation between the vast amount of physiological differences. Especially with the statistic stating the extreme genetic similarities between the two relative species, scientists believe there must be some other mechanism that accounts for the vast differences. Some scientists point to the possible differences in protein expression under the regulation of transcriptional and protein splicing. At the protein level, only 29% of genes code for the same amino acids in humans and chimps, with only a 1-4% difference in genetic codes. One possibility that understands the little difference in the genetic code between the two species is that only a few and very important mutations account for the vast differences. Generally the non-coding section of the genetic code usually differs between the two species, which supports the notion and these non-coding regions are the main contributors of protein regulation in the process of splicing. As well it was found that the class of genes that most differed between the species was the genes that code for transcription factors and other molecules that regulate the activity of other genes and play important roles in embryonic development. In addition, scientists report that six to eight alternative splicing mechanisms are different between humans and chimps. The idea of alternative protein splicing and regulation of gene expression, originating from the small genetic differences of humans and chimps and accounting for the vast physiological differences, makes the statistic of 96-99% genetic seemingly irrelevant.
But this idea of protein splicing and gene regulation originating from the non-coding regions of the genetic code does not directly explain other vast differences found between the species. It is hard to imagine that in only 6 million years, humans and chimps could have become so different through the mechanisms and processes described above For example, the reduction of jaw-muscle strength, which is hypothesized to have allowed the brain case of humans to grow larger, must have been accompanied with the reduction of many other features including teeth size. Combining together the simultaneous reduction of many important features that would probably have been preserved for its importance in survival could not have developed from such a short amount of evolutionary time. While more than 50 genes that are present in the human genome, but absent or partially deleted in chimp genome exists, could these in combination with other mutated genes have accounted for the division of the two species? Some other huge differences that separate humans and chimps is the development of the notion of walking on two feet, an enlarged brain, and complex language skills.
But comparing the two species, it was found that both classes of genes in chimps and humans are changing unusually quickly compared to other models of evolution and natural selection. Generally these included the genes that code for processes in the perception of sound, transmission of nerve signals, production of sperm, and processes in cellular transport of ions. As well there are significant differences in genes coding for cell differentiation and the immune response. When comparing the genome of the chimp and the human, one significant difference was the discovery of three key genes that partially explain the differences in the immune response, specifically the inflammatory response. Three key genes involved in the inflammatory response was absent in the chimp genome. This was found by searching for signs of selective sweeps or signs extensive selective processing that chooses such an advantageous variation, that it is quickly spread to the rest of the population. Selective sweeps are extremely possible in the human species because over the past couple of million years, there is evidence supporting the notion that human populations have gone extinct in certain areas. Then through recolonization, the extinct humans were replaced. This pattern of development makes for a smaller effective population size that contributes to the traits of the following generations. The developmental process of chimpanzees does not follow a similar path, giving the idea that humans could have developed such extensive physiological changes through this pattern. But to continue, among these sites were the FOXP2 and CFTR gene, which has implications with the acquisition of speech and ion transport processes in humans. Other regions if selective sweeps include regions with one genes, one including elements regulating the expression of a nearby protocadherin gene, which has been implicated in the patterning of the nervous system. The discovery of other sites could enhance the hypothesis of non-coding regions of the genetic code accounting for the physiological differences through protein splicing processes and regulatory genes.
New approaches are emerging to answer the questions regarding the relationship between humans and chimps. One such promising method is the examination of the human genome and chimp genome to that of the recently sequenced genome of macaque monkey, by Jianzhi Zhang, a scientist already introduced to us by Sean Carroll. This process concretes the idea that by comparing the genomes of humans and chimps to another related species will demonstrate similarities between the macaque monkey and the chimp, demonstrating that there was some event that led to a genetic change in humans. Zhang has already expelled myths as well as overturn prevalent hypotheses concerning the relationship between humans and chimps. For example, he states that the evidence shows that it is not likely that humans underwent more positive natural selection than compared to the chimpanzee species. As well, evidence points to the notion that changes in a small amount of genes constitutes the major differences between humans and chimps. Another hypothesis that the evidence supports is the “thrifty gene hypothesis” proposed by James Neel. The hypothesis states that certain genes in humans were advantageous in the evolutionary past when food was scarce for humans, but now it has become disadvantageous in times of an adequate amount of food, leading the carries of these mutated genes to disease such as diabetes and obesity. More intensive research and comparisons will lead to new findings in the developmental history of humans and the mechanisms that greatly separated the humans from their relatives, the chimps.
As for the question regarding the position of genes in relation to the centromere, I think that it was a small series of mutations on a small group of genes that led to the subsequent division of humans and chimps rather than the extensive variation in which chromosomes were received in successive generations. There are only 23 pairs of chromosomes in humans, but 24 pairs of chromosomes in chimps so this discrepancy must be included in the analysis of where the centromere might have been placed. But further exploration into this question could include research regarding chromosomes 4, 9, and 12 that express extensive remodeling and differentiation between the two species. As well chromosome Y is incongruous between humans and chimps and chromosome 21 contains many large, non-random insertions of genetic code. But overall, despite these difference and the role that the centromere could have played in the creation of intermediate species, it was the group of mutations most responsible for the division of the species.
http://blog.wired.com/wiredscience/2008/11/exploring-the-g.html
http://www.ns.umich.edu/htdocs/releases/story.php?id=5808
http://news.nationalgeographic.com/news/2005/08/0831_050831_chimp_genes_2.html
http://www.infoniac.com/science/main-genetic-difference-between-humans-chimpanzees-revealed.html
http://www.evolutionnews.org/2006/10/time_makes_berras_blunder_in_e.html
http://www.genome.gov/15515096
http://www.answersingenesis.org/tj/v17/i1/DNA.asp