Re: Is it possible to make artificial gills for humans that don't have to be...

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