Sunday, March 28, 2010

Palimpests in Embryos

On page 55 Coyne describes how manuscripts were made in medieval Europe. They would write on thin sheets of dried animal skin. "Because these were hard to produce, many medieval writers simply reused earlier texts by scraping off the old words and writing on the newly cleaned pages" (55). These were called palimpsests. How do these palimpsests relate to the theory of evolution and natural selection? More specifically how do palimpsests relate to embryos? See page 73, and explain how embryos are similar and different, and at what times of development these similarities and differences occur (use this to answer the question as to how palimpsests relate to embryos).


  1. The palimpsests represent natural selection in the idea that the remnants of the past remain with the present. In this case, the traces of ink on the dried animals skin were still identifiable by modern means despite the scraping off. This analogizes with the embryo because the developmental stages of the embryo undergoes similar changes as its ancestors it possibly could have evolved from. An example of this would be the resemblance to a fish embryo the human embryo takes place during development such as the branchial arches, which turn into gills for fish but head structures for humans, and the development of our circulatory system, which resemble reptilian embryonic circulatory development.

  2. Jerry Coyne uses the analogy of the palimpsests to explain how the genetic code and traits of modern organisms today could, in fact, give a glimpse into the genetic code of the past. Just like the faded words of past manuscript writers may be somewhat deciphered by modern technology today, so too can some of the remnant DNA and traits inherent in today's organisms. Vestigial traits, like the useless eyes of cave fish, may provide one glimpse into the evolutionary timeline of a species' past.
    An even more striking example brought up by Coyne concerns the stages of embryo development in humans. As he explains, "each vertebrate undergoes development in a series of stages, and the sequence of those stages happens to follow the evolutionary sequence of its ancestors" (77) For example, a human embryo at the four week stage shows much evidence of its predecessors; it has the branchial arches of its fishy ancestors (which develop into structures like the ear bones or the larynx rather than the gill arches of our ancestors), the yolk sac (albeit empty) similar to the first egg laying terrestrials, and a distinct tail-like structure as well. The evidence of the past doesnt simply end there, however. According to Chapeville and Fromageot in an article published in ScienceDirect, there is also chemical evidence as well; 'vestigial' enzymes of no real use for human embryos today.
    All of these structures, although far less readily visible than in past ancestors, do provide a glimpse into the evolutionary path of modern organisms.

  3. In a way, the palimpests are a piece of history. In the medieval times, the palimpests were a piece of writing that was reused by scraping off, like it said in the question. In relation to the organisms, and to the embryos, there are many features that explain the palimpest like features. Initially, in the nucleus of a human cell, there are many genes that exist inactively. Even though the genes do not function as an active recipe, it can be seen that this is one of the palimpest of natural selection. Through countless numbers of mutations, the ancestors of humans survived through the selective advantage that these mutations offered. Throughout the existence of many species, such genetic prints can be seen. Although not much of a use, human tail bone is in a place similar to the most recently branched off species, chimpanzees. Such branching off shows that even though humans do not fully use the tail bone, there were similar genes in play during the developmental stage of humans and chimpanzees. Because of the recent branching off between humans and chimpanzees, the developmental features of the two species is similar. Both embryos grow internally in the womb of the maternal specie. Yet even in other species, such as amphibians, who have external development, there are few similarities. One similarity is that there are usually 4 process of development: cleavage, patterning, differentiation and growth. Also there are stages such as blastula, gastula and etc.Some species have extracellular membranes such as amion and others. These striking features show that the palimpests of natural selections are at work with these species. This also does not contradict the theory of evolution, stated in the book Why evolution is true.


  4. Palimpsests act as a very good analogy to natural selection and the process of evolution. These thin sheets of dried animal skin served as paper. Since these were difficult to produce, it made more sense to reuse them by scraping off old letters to form new ones. However, traces of the earlier writing remained. Some of these traces proved to be incredibly important to scientific history. These traces represent underlying traits of our ancestors that are still detectable today despite the change. Even though we do not look like our ancestors (like the new writing does not resemble the previous writing), one can still detect what was there before. In reference to embryos, the parchment with the original writing is like the embryo when it has the traits that are similar to other species (vertebrates all very similar). When those traits begin to disappear is when the old letter are scratched off, and when certain traits form once again, that is when the new writing is written over the old writing. Embryonic structure in general looks very similar among all vertebrates: like fish. All start out with "some blood vessels, nerves, and organs" (Coyne). The pictures Jerry Coyne shows allow us to see that the embryos appear to have similar shapes and objects, yet those objects become very different things on the grander scale: branchial arches on a shark become the adult gill structures, where as in humans the branchial arches develop into structures in the head and upper body. As the human embryo develops, it basically goes through the evolutionary process so that our systems form resembling the sequence of our ancestors. The full change in our embryos so they no longer resemble embryos of other species takes a little longer than 7 weeks total.

    Why Evolution is True