Why haven't we seen a single instance where a new body part has been introduced? (not duplication of body parts)
A Body Part is any part of an organism, such as liver, lungs, stomach, leg, finger, etc.
A new body part is any body part that was introduced to an organism that was not there before, either in the organism itself or the entire population of that particular species.
To get a little more on topic, completely new organs have evolved many times, but of course this takes more than one generation. This happens in at least two main ways.
One - begin with a small change, then add changes, each giving an advantage, until what you have can be called a “new organ”. Eyes are the classic example of this. First, start with regular temperature sensitive nerve cells (we can evolve their presence in steps too if you like). Now, a mutation causes them to fire when exposed to light instead of heat - not hard since light produces heat anyway. Now there are a clump of light sensitive cells, which are advantageous because sensing light is better than not sensing it - this may allow the little creature to hide under a rock, etc. Now a mutation causes that clump to be depressed in the skin - thus protecting it. Similarly, you can see that the opposite mutation - causing it to bulge up, would be selected against, so the “depressed in” mutation survives. Now the cup with light sensitive cells can sense light direction too! Mutations that cause a transparent cover of cells, and later thicken this layer to make a lens, or add muscles that allow the eye to move are all accumulations of small steps, each being advantageous, up to the eye you possess. At what point was it a “new organ”? Certainly not when it was just a few light sensitive cells! This is why your question is somewhat like “How could my family have a dozen lineages now - don’t we have to create a new lineage from scratch, and then how could it be my family?”.
The other way, as many have pointed out, is the exapatation - where something is selected for one environment, but then is later selected for another one. In addition to the classic example of the wing discussed already, the breast is a neat example. Did fish have milk- producing breasts? Of course not. Early in our mammal line, when our ancestors were much like lizards, a mutation caused excessive sweatiness, at least in one location of the body. This creature’s babies were able to lick this sweat and gain some nourishment. Obviously, mutations that made more sweat in that area in females would be selected for because their kids would have a better chance of getting fed. So small steps caused the breast to form, (note that there are still some mammal species today that don’t have nipples), finally giving us the mammal breast. Add to that that sexual selection of hominid males who looked for women that had capable breasts, and you can see where the selective pressure to add fat to “artificially” enlarge the breasts would come from. That’s why human breasts today are “fat padded” to make them much larger (in many cases) than they need to be for nursing a baby. So when did this “organ” come into being? Certainly not just when there was some sweaty skin!
There are a lot more examples too. Another good one is the poison delivery system in rattlesnakes.
Also - note that this same method works even better on a DNA level, where one can mutate a whole copy, then change the copy to make a completely new gene.
Have a fun day-
-Equinox
P. S. Is Forever around still?
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