|MadSci Network: Zoology|
Is it possible to make artifical gills for humans? As a scientist I don't like to say anything is impossible, but a human with gills would be extremely unlikely - to the point of being almost impossible. There are two reasons for this - one we can call physiological and the other we can call morphological. Since you've been studying them, you probably know more about how gills work than I do. If you think about it, an animal the size of a human would require gills with an extremely large surface area to allow for the necessary gas exchange. This would mean that a "functional" gill system for the human would have to reside outside the body and would be large (think of the hellbender salamander, that has gills almost as large as its head on either side of the neck). This would make the human gill physiologically difficult to support. And, even it they could be developed, they would be extremely vulnerable to injury - which would make the human gill very impractical. So, for a physiological basis, the human gill would probably not work. However, morphology supplies an even better reason why humans can't have gills. Vertebrates (animals with backbones like humans) are all built on the same basic body plan. Evolution is very conservative in that new structures rarely appear. Instead, evolution works to modify what is already there - and here is the morphological problem with human gills. Humans actually have gills already, or at least had them as embryos. These human gills, the branchial arches, are modified during growth into other structures, as follows: Arch 1 - The Jaws and Face Skeleton Arch 2 - The Hyoid bone and the Middle Ear Arch 3 - The Common Carotid Arteries and Some parts of the Larynx Arch 4 - The Arch of the Aorta and the Thyroid Cartilage Arch 5 - Disappears Arch 6 - Ductus Arteriosus (an important fetal blood vessel) If you were to make a human with gills, and still followed the basic vertebrate body plan, you would have to eliminate one, or more, of the above structures. Sure, arch 5 could be used, since it isn't used for anything else, but a one-arch gill system would be useless. So, morphologically, a human with gills would have to have a very different developmental pattern (no amnionic development - Arch 6) a very different circulatory system (no aorta - Arch 4) a different or smaller brain (less blood goes to the brain without carotid arteries - Arch 3), and so on. I suppose some bizarre genetic experiments could eventually cause all this to happen - but would you call the result "Human?" I wouldn't. References: Romer, AS and TS Parsons (1986) The Vertebrate Body. 6th Edition. Saunders College Publishing: Philadelphia Gilbert, SF (1997) Developmental Biology. 5th Edition Sinauer Associates: Sunderland, MA
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