Starting in Chapter Six on page 97, Neil Shubin talks about the differences of body plans in humans and other organisms, as most mammals exhibit bilateral symmetry, with front/back, top/bottom and left/right sides. Why do the major sensory organs and nervous systems always seem to collect at the top or anterior ends of mammals' bodies? Is this merely a convenient way of categorizing anatomical sides, or is there an underlying selective advantage to placing brains/nervous organs at the end where cephalization occurs? Also, compare and contrast the spinal structure and placement of these organs in bilaterally and radially symmetrical organisms.
Christine Lin
choco_cat11@comcast.net
Cephalization in fact evolved with the move toward bilateral symmetry, and the reason it did so was to allow the sensory organs to be oriented toward the organisms’ direction of movement (http://en.wikipedia.org/wiki/Cephalization). Once sensory organs began moving (over evolutionary time) toward the anterior end of the organism, a larger head needed to form to accommodate the newly aggregated organs (http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/nervous%20system/nervous1.htm).
ReplyDeleteOne selective advantage to placing brains and nervous organs where cephalization occurs is that the sensory organs can be most efficiently and directly linked to the brain, which is the control center of the central nervous system (Campbell 1048). Another is that, as the organism moves through its environment (by definition anterior-first) it will be able to detect and respond to new stimuli more rapidly with cephalization than without its sensory organs grouped at its anterior end.
In radially symmetrical organisms, sensory organs are not grouped in the head of the organism like they are in bilaterally symmetrical organisms. Instead, the sensory organs – which are often less developed in radially symmetrical organisms than in bilaterally symmetrical organisms – are spread evenly around the organism, like the primitive eyes of box jellyfish (http://www.msnbc.msn.com/id/17913669/ns/technology_and_science-science/).
- Vincent Fiorentini
(vincent@panatechcomputer.com)
I agree with Vicent's statement about how animals developed heads to orient their sensory organs toward the direction of their movement, and this idea seems to hold true across members of the Phylum Mollusca. Bivalves are bilaterally symmetrical organisms, but they have no cephalization. This is probably because they live most of their lives in the same place, and hardly need to move unless they need to escape from potential predators (Gregory, 2006). Also, "slow-moving animals have some cephalization, enabling sensory reception as the animal moves through the environment." (Gregory, 2006). These slow-moving animals, such as snail and slugs, did not need major cephalization because movement was not the key factor in orienting its sensory organs in one place. Lastly, "the active predatory lifestyle of cephalopods require complex sense organs; they are highly cephalized." (Gregory, 2006). This pattern seems to make a lot of sense given these examples. As Vincent stated before, organisms can more rapidly react to stimuli when the sensory organs are in a location closest to where they would experience the stimuli first.
ReplyDeleteHowever, I noticed a significant exception to this idea. Our ancestoral relatives, apes, generally walk hunched over and their head is always in front of the rest of their body. Strangely enough, The human head is not the most anterior part of our body because we walk upright. The sensory organs located in the head are not in the leading part of the body, or else they would need to be located on our abdomen. Why do you think that is?
http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/nervous%20system/nervous1.htm
Troy Glickstern
cleverstar8@comcast.net
Cephalization has provided many advantages in the development of complex body plans. It allows for the advent of sensory organs, increased communication among different body parts, and directional movement. But behind the benefits it provides, cephalization exhibits a complex evolutionary history.
ReplyDeleteCephalization was first seen in phylum Platyhelmenthes, the invertebrates typically referred to as flatworms (Campbell 659, 674). The beginnings of brain, sensory organ, and nerve development in these worms allowed them to take a more aggressive stance towards their environments. In platyhelminthes we see increases in mobility and responsiveness to the environment (http://cas.bellarmine.edu/tietjen/images/platyhelminthes.htm). This is most likely due to the concentration of sensory organs. A selective advantage to the concentration of sensory organs is stronger organization and increased efficiency between the organs and the brain (http://www2.fiu.edu/~pitzert/flat.htm). Also, because of their proximity to the brain – the regulator of the nervous system - neurons don’t have to send their messages as far and thus they arrive to the organs faster, a principle crucial to their function. These sensory neurons detect external stimuli and work with the numerous interneurons of the brain – or neurons that transmit only local messages, increasing efficiency (Campbell 1048).
Another important example of heads in evolutionary history is Tiktaalik who exhibited a flat head with eyes on top. This structure probably gave Tiktaalik a selective advantage in shallow waters by enabling him to better scrounge for food along the floor (Shubin 40). In contrast, humans have round heads which allow for a larger brain cavity and thus greater brain development and more powerful jaws (http://www.onelife.com/evolve/manev.html). Therefore, different head structures allowed for greater development of brain function, and ultimately aided in human intellect. Both heads, however, are located at the anterior end of the body. This is a selective advantage because it allowed for the concentration of sensory organs to deal with multiple related functions: finding prey, avoiding predators, and maintaining or preventing other key relationships with the outside environment (http://www.novelguide.com/a/discover/ansc_01/ansc_01_00062.html). The concentration of these organs allows scientists to distinguish between the anterior and posterior ends of animals.
Although some radially symmetrical organisms do exhibit cephalization, Vincent was correct in that it is minimal and not as efficient as in bilaterally symmetrical organisms. Cephalization in radially symmetrical organisms is a loose concentration of sensory organs that does not provide many of the benefits of more extensive cephalization, such as directional movement (http://www2.fiu.edu/~pitzert/flat.htm). Cephalization has been more successful in bilaterally symmetrical organisms because separation and concentration of different organs is easier to accomplish in a bilateral body plan.
(...see next post!)
As for Troy’s question, I think that the reason our sensory organ are not technically at our most anterior point, the abdomen, and are rather located in the head, is due to the evolution of man from apes. Although in apes the head was the most anterior part of the body, the evolution of man to stand upright, discredited the head as most anterior. However, by evolution, the sensory organs can just relocate to the abdomen; in fact, that would be more harmful than beneficial to an animal because those sensory organs require proximity to the brain (Campbell 1048). Instead the straightening of the spinal cord evolved independently from the sensory organs, leaving these organs at their previous central location, even if they were no longer anterior.
ReplyDeleteUltimately, cephalization is more complex than simply a concentration of sensory organs at the anterior end of bilaterally symmetrical organisms. There are so many exceptions such as radially symmetrical cephalized organisms and concentration of neurons in the brain not necessarily located in the anterior end of the body. Understanding cephalization is crucial to understanding the inner workings of complex body systems, especially relative to interactions with the nervous system.
Sami Kopinsky - sami_kopinsky@yahoo.com