Neil Shubin discusses the deep connection that we hold between ourselves and sharks in regards to our development. When compared, human embryos begin with four-arched gills, the same as in shark embryos. These gill arches in humans later develop into our brain and head (91). Each arch of our original gills develop into specific structures. For example, the second arch of the gills in sharks develops into two bones of the jaws and in humans, the hyoid that we use for swallowing and our middle ear bone (89-90). Evolution such as this always has a reason because of natural selection. The selective advantage of having a middle ear bone over a retractable jaw bone must have been greater. What may have been the reason for this advantage? How does a shark's ear compare to a human ear, and why have we evolved to have a significant ear bone or swallowing ability over having a retractable jaw- what is the selective advantage of this?
Sonia Doshi soniadoshi7@gmail.com
Like human ears, sharks also use their ears to maintain balance and equilibrium (1). However, unlike human, sharks' ears are completely internal. Sharks have a cartinaligous labyrnith which protects and supports the membranous labyrnith that contains ducts and fluid filled canals (1). The ducts in the ear lead to an area called the sacculus which contains the lagena which is responsible for receiving sound waves (1). The sacculus is lined with an epithelium of otholiths, which are made of sand grains and calcerous deposits (1). When the sharks move, the fluid follows and the otoiths send a signal to the brain to help the shark regain balance. Similarly, the human ear consists of cilia and gel that send signals to the brain to help humans maintain balance.
ReplyDeleteThe human middle ear bone and swallowing ability is an advantage over the shark retractable jaw bone. This is because in order to be able to swallow prey, sharks need to have huge powerful teeth that move in the mouth over time (Campbell). In humans, we are able to use much smaller teeth because of our advanced middle ear bone and swallowing ability. We also have the advantage of not losing teeth as a part of eating. Since sharks have retractable jaws and need to use their teeth forcefully in order to swallow food, they often lose their teeth.
Sources:
http://www.elasmodiver.com/shark_senses.htm
AP Biology- Campbell Reese
Marissa Lobl marissa.lobl@gmail.com
The sharks have ears that are somewhat different to the humans’. First of all, sharks only have an inner ear, and unlike humans, that inner ear does not consist cochlea, but consists of three chambers and an ear stone called an otolith. However, their ears still give the sharks the sense of gravity and acceleration with cilia and gel, just like the humans’ ears (http://www.seaworld.org/infobooks/Sharks&Rays/senses.html). While sharks’ inner ear only structure might have given them no problem in hearing, since they can hear better than us, as organisms moved from sea to land, they probably needed a new method of hearing, and that is where the middle ear bones come in. The middle ear boned, consisting of malleus, incus and stapes, connect the tympanic membrane and the cochlea. When the sound waves vibrate the tympanic membrane, the vibrations are passed through the middle ear bones and are sent to the cochlea, where the message is sent to the brain via auditory nerve (Campbell 1093).
ReplyDeleteSharks require tons of forces when they swallow their prey because of the shape of their face – nose sticks out and the mouth is placed in a slanted way so that lower jaw is shorter than upper jaw. This fact also gives some sharks retractable jaws, so that the lower jaw can stick out and eat their prey easily. Also, in addition to what Marissa was saying, because sharks require continuous swimming in order to insure than water flows through their gills, when they are sleeping, they use their muscles in jaws and pharynx to pump water over the gills (Campbell 707). However, the humans do not have the necessity to own a strong jaw. So, “bones that support the upper and lower jaws in sharks are used in us to swallow and hear” (Shubin 92). Because we do not need a strong jaw, some of the bones that was used to make the sharks’ jaws stronger are now being used as ear bones to help us hear better in our environment, while sharks did not need additional ear bones to help them hear better. Both of the organisms, sharks and humans, have adapted in their own way to have a selective advantage. While sharks’ strong jaws might have worked as a great advantage among their environment, its usage became useless as animals began to live above water, and for those animals, it was an advantage to have a better ear system, thus the movement of the bones from jaws to ears.
John Park (wisejsm@yahoo.com)
Shubin goes into detail about how both shark and human embryos are similar with respect to the four gill arches. Through selective advantage and evolution each gill arch becomes a different structure in humans and sharks.
ReplyDeleteThe first arch in both sharks and humans forms into the jaw, but in humans it also forms some ear bones. The second arch divides and changes into a bar of cartilage and muscle. In a shark, these two bones form a retractable jaw. In humans, it becomes three bones in our middle ear. One of the bones, hyoid, allows us to swallow and is comparable to the lower jaw bone of the shark. In sharks the bones that support the upper and lower jaw are similar to the bones that help us swallow and hear. The third and fourth arches in sharks form the tissues that support the gill and in humans these arches develop into structures that help us talk and swallow.
The second, or hyoid, arch, extended from the quadrate bone to the angle of the jaw, and acted to support the jaw. The hyoid arch later became the hyomandibular bone, which braces the quadrate bone in bony fish. As tetrapods evolved from one group of lobe-finned fish, the quadrate bone fused with the skull, giving a stronger bite. This meant that the hyomandibular bone lost its function of supporting the jaw. However, its location near the ear seems to have allowed the hyomandibular to transmit vibrations to the inner ear. The hyomandibular bone of fish is homologous to the stapes, or columella which developed in order to help humans hear. (http://sci.waikato.ac.nz/evolution/Homology.shtml)
As vertebrates came up onto land, they had no other choice but to evolve. The had to be able to hear in an atmosphere as thin as air. Again, the hyomandibular bone happens to be near the otic (ear) capsule, and bony material is excellent for transmitting sound. Because of this it was able to still act as a cranial brace but also the hyomandibular bone was initially a sound transducer in the first amphibians. “As the terrestrial vertebrates altered their locomotion, jaw structure, and posture, the cranium became firmly attached to the rest of the skull and did not need the hyomandibular brace. The hyomandibular bone then seems to have become specialized into the stapes bone of the middle ear.” The bone’s secondary function, to hear, had become its primary function in humans through selective advantage. (http://www.ncbi.nlm.nih.gov/books/NBK10049/)
I believe that the information Marissa found in Campbell was beneficial in explaining one of the major advantages of the human middle ear bone and swallowing ability over the shark retractable jaw bone. “This is because in order to be able to swallow prey, sharks need to have huge powerful teeth that move in the mouth over time (Campbell). In humans, we are able to use much smaller teeth because of our advanced middle ear bone and swallowing ability. We also have the advantage of not losing teeth as a part of eating. Since sharks have retractable jaws and need to use their teeth forcefully in order to swallow food, they often lose their teeth.”
There is also another advantage that has been proposed. Natural selection has been seen as the major cause in preservation of the structure of the middle ear in mammals. Many of the earliest mammals were quite small, and the dentition indicates that they were insectivorous. If they were "warm-blooded" (endothermic), like modern mammals, then they could have been nocturnal. This fits with the popular image of small, nocturnal insectivorous mammals surviving in niches not accessible to the large, dominant contemporary dinosaurs. The enhanced hearing, particularly in the higher frequencies, would be helpful for nocturnal animals, in particular for detecting insects. Through selective advantage these mammals would have evolved and formed the middle ear. (http://en.wikipedia.org/wiki/Evolution_of_mammalian_auditory_ossicles)
-Adnan Jahan
(adnanjahan@gmail.com)