Ch. 4 of Neil Shubin's book explains the importance of the head and how it is formed from the embryo. He gives details on how all organisms' heads are similar to each other, and how evolution changed head development. What similarities does the human head share with the heads of previous organisms? How are our heads different from them? How did these differences come about and what advantages do these differences give to humans in comparison to others? Explain in detail using Shubin, Campbell, or any credible outside sources.
-Austin Lee, 2B/3
austinklee7@gmail.com
A human head and a shark head share many similarities as well as differences. In chapter 5, Shubin states that "Every head on every animal from a shark to a human shares those four arches [arches of the head] in development" (90). Yet it is not that all animals with heads have these arches within them, but that what happens in each of these arches may be different in every animal. In the first arch of a human head and a shark head, there are similarities - that being the jaw. The difference between the first arch of the human and the shark is that the first arch of a human can also form ear bones that does not happen with the first arch of the shark. In humans and sharks, the second arch "divides, changes, and gives rise to a bar of cartilage and muscle" (91). The difference between shark and human is that in shark, the second arch breaks to form 2 bones that support the lower jaw (compared to a human hyoid - functions in swallowing) and the upper jaw. In humans, the second arch breaks in order to form "one out of the three bones of our middle ear" (91). The third and fourth arch is used by humans to talk and swallow while it is part of the tissue that supports the gills in sharks. While the similarity between a shark head and a human head lies in the structure of the four arches that both of these organisms have, the function of these four arches is different. Although the structure of the head and the arches may be similar, the function of these arches in the two different animals can be very contrasting.
ReplyDeleteAnother important matter of the head - the brain - are very different in humans and sharks. Although the brain has similar function in all animals, the structure of the brain is not necessarily similar at all. The human brain is known to have three times as large a brain as a typical mammal. The brain of the shark, on the other hand, is known to be tiny (the size of a walnut, to be exact) but this may be a common misconception, as new studies show that this walnut sized structures is not the full brain of certain shark, but a small part of it - the cerebrum. The real size of the brain of a white shark may be up to 2 ft long. Although the brain of a shark may be as big as, or maybe even bigger than a human brain, the brain mass per lb (or kg) of a human is massively larger than the brain mass of a shark. Not only are the brain masses completely different, the brain of a white shark is structured, organized, and shaped differently than a human's brain. The brain of a white shark is a relatively compact structure while the human brain consists of two hemispheres. One example of a difference in the structure of a brain between humans and sharks is that the top of a white shark's midbrain contain optic lobes that are responsible for visual input. Unlike humans, sharks vision is directly connected to the brain which means the shark may have extraordinary vision.
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ReplyDeleteAs evolution occurred over millions of years, the head of a shark - which is not nearly as elaborately organized and detailed as the head of a human - has changed into that of a human head, which contains a brain that is the center of a very complex nervous system - not nearly as elaborate or complex and fully functional as that of a shark. The advantages of having this more elaborate and detailed head (as well as a very complex nervous system) are overwhelming - as humans have very strict and specific control over most functions of the body, something that sharks and other organisms aren't able to do. The function of a human brain, also being much more complex than a shark's (due to evolution over millions of years) gives humans the ability for form fully developed thoughts and opinions that sharks and other animals are not able to form.
Although sharks and humans may have similar structures (in terms of the four arches), a human head is very different from a shark head functionally, and evolution has given humans the advantage of having a complex and well developed brain that is the center of a very complex and well developed nervous system.
Sujin Ko (sujinko93@gmail.com)
Sources: Shubin - Chapter 5 (pages 90-92)
http://www.elasmo-research.org/education/white_shark/structure_brain.htm
http://en.wikipedia.org/wiki/Human_brain
The development of a head in the animal body plan serves many advantages that allow us to perform everyday functions. These include functions of the sensory organs such as seeing, tasting, hearing, and smelling, as well as functions of the brain and nervous system. Cephalization, or the concentration of these functions at the anterior end of the animal body (Campbell 659), can be traced through evolutionary history.
ReplyDeleteWhile the human head has many similarities to the shark head, as Sujin pointed out, cephalization can be found in almost all animals that exhibit bilateral symmetry, as it developed from the bilateral body plan. Like Sujin said, sharks and humans both exhibit a nervous system managed by the brain, but they also mimic a similar skull structure shared with most other animals with heads: plates, blocks and rods. The skull structure has slight variation in each animal, but the theme of plates, blocks, and rods is preserved. In humans, the plates are originally separate but they fuse as we get older. The blocks hold up our brains and the rods help to form our jaws, ears, and throats (Shubin 83). This is similar to other continuous body plans seen throughout evolutionary history such as limbs: one bone, two bones, blobs, then fingers or toes (Shubin 31).
Also, there are other animals that Sujin did not mention which also provide key background information to the development of heads. First off, cephalization 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). 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 allows 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.
One thing Sujin said that I disagree with though is that development of the shark head is directly linked evolutionarily to the development of the human head. This is not entirely true because aquatic mammals such as whales and sharks are of terrestrial animal decent and they later returned to water, where through genetic variation they developed into separate species (http://open.jorum.ac.uk/xmlui/handle/123456789/925). This indicates that sharks and humans head development are not directly linked. Instead there are numerous divergences in the developments of their heads, skulls, and brains. While they are undoubtedly similar, the shark head did not directly evolve into the human head, but rather there is a complicated web of evolutionary connections between the two.
All in all, the head, in any animal which exhibits one, allows for centralization of sensory organs, the nervous system, and brain. While some heads are designed for deep ocean diving or scrounging, others, such as the human head, are designed for large brains and powerful jaws for chewing. Our heads are essential to our ability to function and communicate and with out them, the animal kingdom and human race would be as advanced as we are today.
Sami Kopinsky (sami_kopinsky@yahoo.com)