Saturday, March 21, 2009
Fossilization of Human Olfactory Genes
On page 128, Carroll talks about how "about half of all our olfactory receptor genes are fossilized and incapable of making functional receptors." Also, Carroll states that there was "a striking correlation between the proportion of fossilized olfactory receptor genes and the evolution of full color vision." Which do you think came first in our ancestors: the fossilization of olfactory genes or the evolution of full color vision? In other words, do you believe that the evolution of full color vision reduced the necessity for many of our olfactory genes (relaxed evolution), or did the fossilization of these genes promote natural selection for full color vision in order for many of our ancestors to survive? Also, does full color vision give animals a better relative fitness than having more functional olfactory genes?
Subscribe to:
Post Comments (Atom)
Linda Buck and Richard Axel were awarded the 2004 Nobel Peace Prize in Physiology and Medicine for “cracking the mystery of the genetics of smell” by discovering “a family of genes that encoded odorant receptors” (127). The olfactory receptor genes are the largest family of genes in mammals; for example, out of 25,000 genes, mice possess approximately 1400 olfactory genes.
ReplyDeleteIn contrast, humans, whose olfactory genes have been studied in detail, have been found out to have half of their olfactory genes fossilized. To be specific, approximately half of a human’s olfactory receptor genes are “fossilized and incapable of making functional receptors (128)”; mice have 160 functioning V1r receptors, which play a critical role in pheromone detection, and humans have 5 functioning V1r genes our of more than 200. This insinuates that humans no longer rely on their sense of smell to the extent that their ancestors did and that they also are less reliant on pheromones that other mammals because their ancestors relied more on visual signals in certain behaviors.
Carroll states that there is “a striking correlation between the proportion of fossilized olfactory receptor genes and the evolution of full color vision”. Scientists that studied mice, lemurs, and New World monkeys that lack full color vision saw that 18% of olfactory receptor genes were fossilized while in colobuses and Old World monkeys, about 29% of olfactory receptor genes were fossilized and in orangutans, chimps, and gorillas, it is 33% and in humans, it is 50% of olfactory receptor genes that are fossilized. In other words, species with full color vision have a higher fraction of fossil olfactory receptor genes. The evolution of tri-chromatic vision reduced full-color species’ reliance on their sense of smell; tri-chromatic vision allows species to detect food, mates, and danger. Selection of olfactory receptor genes lapsed and thus, tri-chromatic species have allowed the genes to decay. Therefore, the evolution of full color vision developed and olfactory genes began to fossilize; then, “the evolution of full color vision reduced the necessity for many of our olfactory genes” (Lewis), also known as relaxed evolution, because color vision allowed the color-seeing mammal to more efficiently to detect food, mates, and danger.
Additionally, there are physical, behavioral, and genetic signs of a reduced reliance on olfactory genes. The vomeronasal organ, which detects pheromones, is greatly reduced in humans and “higher primates” compared to other mammals. Full color vision does not give animals a better relative fitness than having more functional olfactory genes but full color vision animals do not depend on the presence of pheromones but more on visual cues. When “an entire organ or process falls into disuse, different genes responsible for different steps in a process may experiences relaxed selection and then undergo fossilization” (129). Again, evolution comes into play as unnecessary genes were fossilized in certain primates and humans, which also exhibited change in certain species to encourage proliferation and survival in a species.
Red and green vision in primates allows them to select young red leaves, which were the most nutritious, among the green hue of the trees (source: http://news.bbc.co.uk/1/hi/sci/tech/1220210.stm). This finding invalidated the “long-held theory that the ability to see red, green and yellow/blue light (trichromatic colour vision) arose to help primates pick ripe fruits to eat”.
I believe that the evolution of full color vision came before the fossilization of our olfactory receptor genes. This is because we needed at least one thing that gave us some sort of “advantage” in our development as human beings. According to an article (link below), “the evolutionary history of a critical eye protein has revealed a previously unrecognized relationship between certain components of vertebrate eyes and those of the more primitive light-sensing systems of invertebrates.” This means that our vision did begin to develop before our sensory receptors began to fossilize because our vision began to develop in prior species, before the evolution of humans. Likewise, “Fish, frogs, birds and mammals all experience image-forming vision, thanks to the fact that their eyes all express crystallins and form a lens.” Because we developed after these species, it is obvious that we had vision before we lost much of our olfactory sensory receptors. The way smell works is that it is mediated by specialized sensory cells of the nasal cavity of vertebrates, and by analogy, sensory cells of the antennae of invertebrates. Mammals have about a thousand genes expressing for odor reception. In humans, not very many are actually functional, meaning they have been fossilized. Humans actually have less active odor receptors than other mammals and primates. This is obviously because of relaxed evolution. Because we do not need such strong and developed olfactory sensory receptors, we do not select for them as much, meaning they have been fossilized. As Jennifer mentioned, Linda B. Buck and Richard Axel, scientists who were awarded the Nobel Prize in 2004, studied the olfactory sensory system. They were attempting to clone olfactory receptor proteins in order to understand exactly how they work and how to “develop” new ones. Full color vision was and remains more necessary for humans right now because humans do not depend completely on their sense of smell for survival. We have developed a sort of intuition that we use our vision with. If we see a situation, we can react quickly based on what we are seeing. We don’t have to depend on smelling the situation to react. As Carroll explained with the monkeys, they developed full color vision and lost some of their olfactory sensory receptor genes. This was not a bad thing because with full color vision, they were able to more accurately pick their food. They saw the red leaves as opposed to the green leaves and were able to pick them out and eat them. This was a good adaptation because the red leaves had more useful nutrients than the green leaves did. Therefore, the development of full color vision was more important then the upkeep of the sensory olfactory receptors. For some animals, full color vision does give them a better relative fitness than having more functional olfactory genes because the environment that they live in necessitates developed vision and not olfactory sensory receptors. For example, the fish that live in dark caves depend more on their olfactory sensory receptors than they do on their eyes. They are completely blind and have lost the ability to see. This relates to the biological theme of structure to function. The structure of specific organs is directly related to its function. Dogs, for example, have a big nose; much bigger than humans, and this is because they depend more on their olfactory sensory receptors than their visions. Dogs have not developed full colored vision because their lives do not necessitate this development. They do not need to have keen vision to be able to survive and reproduce; unlike humans, who don’t necessarily need their sense of smell as much as vision. It can even relate to being in a car. You need your vision MUCH more than you need your sense of smell. Therefore, in the evolution of the human race, the olfactory sensory receptors have not been selected for and have thus degraded over many hundreds of thousands of years.
ReplyDeletehttp://www.sciencedaily.com/releases/2005/09/050926074038.htm
http://en.wikipedia.org/wiki/Olfaction