Invert is absolutely correct that humans never have "gills" per se, but in the embryology stages of development of the fetus we do have "gill slits." This is an undisputable fact known to all biologists and to anyone who has studied evolutionary biology beyond a beginning level course. Almost all of the posts above refer to the fact that humans "used to have gills," and this is correct. We evolved from animals that used to have gills, and the gill slits that we still have in our embrological stage is a living testament to this fact of evolution and a fact that clearly supports the theory (fact) of evolution itself.
Actually, it is not very accurate for me to say that they are "vestigial organs" because they do not remain with us but evolve into other physical features. Pharyngeal "gill slits" are clearly visible in the human embryo. Through DNA conservation in our genome we still code for gill slits - and possibly even for gills? - but I don't know enough to state this as fact.
If this thread continues, it would not be much trouble for me to review my past course notes and post what each individual gill slit now develops into when we are born. I think there are five pharyngeal gill slits in the human embryo.
Another example is the neck region of a human embryo. These are the branchial arches containing the blood vessels - aortic arches. They develop into gill circulation and gill arch supports in fish, but eventually close in the human embryo.
http://home.earthlink.net/~dayvdanls/biology1_2/branchial_arches.htm
These traits are called "shared primitive characteristics," as opposed to "derived characteristics." Of equal interest to this subject, the following are all shared primitive characteristics that all vertebrate animals have:
1. a dorsal hollow nerve chord that develops into the brain and spinal cord
2. a notochord that exists in all chordate embryos that provides back skeletal support
3. a post-anal tail (present in the embryo, absent in the adult)
4. pharyngeal gills (gill slits) - present in human embryos
5. cephalization (brain, head...)
5. bilateral symmetry
Adult Tunicate:
Human Embryo at One Month:
Source:
http://embryology.med.unsw.edu.au/wwwhuman/Stages/stage12.htm
In this view you can see 3 pharyngeal arches, heart prominence, forebrain, notochord neural tube developing the spinal cord, and an extensive tail
See also embryo view at:
http://www.visembryo.com/baby/11.html
Also, I do not think that Walter was incorrect in any of his posts. A forum will start out with a general topic and then get more deeper and require more accuracy in terminology as it progresses. Walter was responding to Zephyr's reply that referenced a Wikipedia article called "Like-A-Fish" that describes a system to extract breathable air from water by developing an "artificial" gill. But this drifts away from the subject matter of the original post.