Jerry Coyne states on pg. 75-79 that the development of the human embryo is powerful evidence for evolution. All vertebrate embryos begin as the same thing, with evolutionary changes occurring as the embryo develops. As the embryo changes, it mirrors the pattern of evolutionary history: first it looks like a fish embryo, then a reptile embryo, and so on. Eventually, all of the previous characteristics have disappeared, and the result is a human.
How does the development of the human embryo relate to gene regulation and expression? Explain the connection between the obsolete DNA in humans and the various stages through which an embryo passes. What examples are there of organs from other types of animals that disappear during embryonic development?
Subscribe to:
Post Comments (Atom)
The development of the human embryo relates to gene regulation because according to this theory that Coyne discusses, called the theory of recapitulation, embryonal development of an individual organism followed the same path as the evolutionary history of its species. For example, this would mean that humans have essentially the same genes as fish because human embryos start out looking like fish embryos, but then as the human embryos develop, specific genes are activated and inactivated that change the appearance of the embryo to that of a reptile embryo and finally to that of a human embryo. This gene regulation would also result in the change in functions of the human organs forming inside the womb compared to the functions of the fish organs inside its egg. Obsolete DNA in humans, then, could have possibly been genes that were inactivated during the embryo development process that resulted in the human embryo becoming more of a human embryo and resembling a fish embryo less and less. Also in humans, we possess lanugo, a fine coat of hair, while we're in the womb, but then that coat of hair sheds because it's not evolutionary advantageous to us. It was shown, however, that primate embryos also have lanugo and don't shed their coat of hair before they're born because their hair is advantageous to them. This connects to the theme of evolution in that the activation and inactivation of certain genes causes certain traits to appear that can either be classified as a selective advantage or a disadvantage. Over time, those organisms with the selective advantages survive and reproduce more, passing on their genes so that over many generations, the species will have evolved so that at least most of the members of that species possess the desirable trait and that the genes that express that trait are the ones that are activated.
ReplyDeletehttp://en.wikipedia.org/wiki/Recapitulation_theory
http://homepage.uibk.ac.at/~c720126/humanethologie/ws/medicus/block6/HumanDevelopment.pdf
http://www.absoluteastronomy.com/topics/Lanugo
http://creation.com/blind-fish-island-immigrants-and-hairy-babies
Generally, there are two patterns in the relationship between the human embryo and the genes that are regulated and expressed within it. The first pattern is the differentiation and maturation of the cells, which in turn causes genes to begin expressing unique qualities. For instance, at the beginning of embryonic development, “the first embryonic cells are totipotent, this status spans only a few cell cycles” (Leandri et. al.). But by the time the embryo has shaped into a blastocyst, cells have already begun being expressed differently. Therefore, the development of the human embryo is linked to the increased expression of genes. The second pattern is, as Sara described, the “evolutionary walkthrough” of the organisms that lie on the evolutionary tree linking waters organisms to humans. As the resemblance between the human embryo passes through its various stages, it appears to mirror those stages of the organisms from water to amphibians to reptiles, in that order. Thus, various genes are turned on and off to help imitate these stages. The genes that are not needed in the long run of development are rendered obsolete because they don’t provide an evolutionary advantage. But this connection is not universal. For instance, Coyne discusses the ability of some humans to retain the ability to wiggle their ears (Coyne), so while humans are developing through the “mammalian embryonic stage”, at one point or another, the genes that are needed for cats, dogs, and so on are retained in some humans, even though this ability is virtually useless for humans. However interpreting obsolete as vestigial shows a similar case involving branchial arches, which for fish and sharks develop into gill arches; meanwhile for humans, they develop into middle ear bones, the Eustachian tube, the carotid artery, the hyoid bone, the larynx, and neck muscles (Coyne). While the genes that guided the evolution of the branchial arches are obsolete of their original purpose, the development of the human embryo has utilized it differently when it passed through the early water organism stage, possibly turning it on at a different developmental time to create parts of the neck and head.
ReplyDelete(Already graded)
In addition the example of the branchial arches, Sara brought up a good example of lanugo. In terms of disappearing in other animals, she forgot to mention the modern whale, which also has lanugo that disappears during development (Coyne). This is probably because whales have a layer of blubber and lanugo is an evolutionary leftover during development. Another example for humans could be the caudal vertebrae, or the bones that developed into a tail, which has stopped developing in humans (Thinkquest).
ReplyDeleteTo quickly touch on humans once again, I stumbled upon an interesting article while researching embryonic developments in humans. Unlike many evolution-based articles, discussed the future of evolution. Basically, “‘At first, bits of DNA within this additional region were readily swapped between the X and Y chromosomes, but some time between 80 and 130 million years ago, the region became two completely separate entities that no longer swapped DNA. One of the regions became specifically associated with the X chromosome and the other became specifically associated with the Y chromosome’” (Physorg.com). It continued to say that “‘Our research revealed that the Y-specific DNA began to evolve rapidly at the time that the DNA region split into two entities, while the X-specific DNA maintained the same evolutionary rate as the non-sex chromosomes,’ said Makova”. Prior to embryonic evolution, the passage of chromosomes is completed through meiosis and fertilization. But because the faster-evolving Y chromosome is being passed on, it has become a selective disadvantage, potentially wiping out the male sex as the article discussed because “there is a chance that the Y chromosome eventually could disappear.” Because all human embryos start of as females (Campbell, Reece), this could have very severe impacts on the evolution of development because it would put an evolutionary pressure on embryos during development.
http://physiolgenomics.physiology.org/cgi/content/abstract/36/2/98 http://www.physorg.com/news167026463.html
http://library.thinkquest.org/26070/data/eng/3/index.html
Jerry Coyne: Why Evolution is True
Neil Campbell, Jane Reece: Biology: AP Edition, 8e
(Already graded)