Since I want to experiment with nonhuman shaped species, could a sentient species use its lips or tongue as primary appendages? Giraffes have prehensile tongues and many aquatic species use their lips. We can expect many creatures will have lips and tongues, but would evolution want this to happen? Aren't there many things a sentient creature would need to pick up that they wouldn't want to get in their mouths?
I don’t really see any reason why not. The elephant's trunk is proof that non standard dexterous appendages can develop. And it would make sense that they would be extensions of the face, since the front of the body is what interacts with the world, so lips and tongues developing into manipulator appendages makes complete sense.
The only problems I can see are trying to manipulate objects that are hot or sharp, such as trying to sharpen a stone tool or picking up a hot iron. Getting cut on the tongue hurts like you wouldn’t believe. So it would be infinitely more painful if they caught the hot part of metal while blacksmithing or cooking, or getting burning black powder on after firing off a shot. I can see this either limiting their development or I can see them finding ways to solve those practical problems, like some kind of glove or maybe just developing a tougher appendage through forced evolution.
Another problem I see is the limitation of a single appendage. Two hands is a really big advantage as it opens options where only a single hand limits them. Not making the situation impossible, just acknowledging how difficult it can be. One way I can see of carrying something while keeping the manipulator free would be saddle bags of some sort. Another thing that might happen could be splitting the single appendage into two.
There's a lot of things that we need to pick up that we don't want to get in our hands. Obviously this would be a larger issue for the mouth, but a mouth structure that makes it hard for things to accidentally get swallowed, and is relatively tough in itself (and can grow calloused), would help. Tools obviously would be developed for things that are dangerous even then.
If the creature normally pokes its nose into things, the tongue would be the logical part to develop. An evolutionary pressure to ensure that its feet can not develop into arms -- perhaps it needs the speed, perhaps it needs the stability because it, oh, lives in a region with many earthquakes -- would help, but fundamentally, starting to manipulate things with its mouth could evolve much further.
While I think the evolution of lips and tongues as manipulative tools is hard to imagine achieving the same level of sophistication as hands, there are certainly animals that do it, as you pointed out, and all you need to have something evolve is pressure and time.
I can think of some challenges with this evolution, but there could also be advantages. A tongue would likely be damp, but wouldn't absolutely have to be. A damp, highly flexible appendage might give very fine, gentle control of small objects, and the same could be said for lips. Certainly apes (including people) use lips as manipulative digits, with teeth as universal grippers for almost every species I can think of. The mouth naturally lends itself to manipulation, and combines the sense of taste with touch (with smell close by) to analyze materials being manipulated. The eyes are close by, but this might be a hindrance for species without eyestalks, because of the challenge of looking at something in the mouth. A dry mouth might help with not absorbing toxins and contaminants from the environment. Being able to rinse and spit out contaminants would be good.
Increasing use of the mouth as a complex manipulative organ could drive some interesting adaptations I can only guess at. Eyestalks and eyes wide and to the side may help seeing what your are eat/mouthing. I have a vision of the nuts you are carefully manipulating in your mouth and cracking, then pulling the meat out with a tongue and chewing it up, spitting it back into the shell and closing it to teach babies how to eat out of a nut. Two-headed animals exist, and are usually developmental abnormalities, but a species using the mouth as a manipulator could be advantaged by having a second head (perhaps not connected to digestion?) that manipulates things so the two "mouths" could work together like arms do. I'm guessing a flexible neck would be quite useful.
On the downside, if you want to manipulate anything, you are sticking your head up to it. How many people have gotten a hand crushed? A head crushed is lethal. A keen sense of taste to detect toxins would be important, and could be problematic for everything that was (literally) distasteful. Arms provide leverage and are in pairs, while multiple heads are rather speculative, and a spine in a long neck would, I'm guessing, have to be rather rigid to lift things in the same way as arms. Violent gyrations of the head while manipulating something vigorously just scream "brain injury" to me.
So I definitely think it could be done, especially as a fine control manipulator. There would be potential downsides, but creative design and thought to evolutionary process could likely correct for these. So why not a two-headed dragon-like creature with lips resembling a moustache, sorting coins and telling which are real and which counterfeit by taste, touch, and smell? Good luck with your idea!
Aren't there many things a sentient creature would need to pick up that they wouldn't want to get in their mouths?
Certainly, but that's hardly a barrier. After all, there are plenty of things that we need to pick up that we don't generally want to get on our hands. So, what do we do in those cases? We use gloves, or handling tools like forks and tongs.
It is obvious that animals in general can evolve to use lips and tongues as dexterous manipulators, as exemplified by giraffes, which you mentioned, as well as tapirs, elephants, various birds (if beaks can be considered roughly equivalent to lips), etc. Insects also use their mouthparts as manipulators--notably, that's how ants carry food. Even dogs carry things in their mouths, if not especially dexterously--including things like stick and tennis balls which (adult) humans would generally prefer not to put in their mouths. Evolution doesn't want anything, it just uses what is available--so if such a creature were to rise to human levels of intelligence, there is no reason to suspect that they would magically stop using their mouthparts and preferentially evolve new manipulators when they already have perfectly good ones. And while, if they use tools at all, they would certainly use tools to avoid touching things that are dangerous to touch with one's mouthparts, the example of dogs, who will put just about anything in their mouths, should be a reasonable indication that such a species may not need special manipulatory tools quite as frequently as we might initially assume based on our own preferences for not sticking gross stuff in our mouths.
There is a good example in Mulefa from the His Dark Materials series.
They develop alongside seeds they use as wheels, which then precludes the development of hands for manipulating objects.
They instead use dextrous trunks, and work together for complicated tasks like tying knots to make nests.
Given this example it's more than plausible for this structure to occur, but a) it has to be driven by some limitation to using hands, and b) comes with complications that should be accounted for.
While it's possible, there is a lot of evolutionary pressure against it. It has to be viable before they become sentient/sapient enough to invent gloves and other protective gear or they never get to the sentient/sapient stage.
- Some things we are handling can be toxic. Grabbing on a toxic bug while climbing a tree is not a big deal with human hands, you wash them off or simply not lick them and you're done with it. Having toxic things come close to your digestive system is not something you want to happen.
- Other things we are handling can lead to injury. Think of herbs with thorns, branches with splinters and such. A human hand injury can be bad enough. If that injury makes it more difficult or even impossible for you to eat, that's bad news.
- Hygiene is a big deal if you put your mouth bacteria and viruses everywhere just by touching things. Epidemics would spread very quickly, leading to increased mortality. It might lead to a strong immune system to compensate, but viruses and bacteria would evolve with it and stay dangerous. In current times, how would a face mask for mouth-manipulators even work?
So to evolve to a sentient species, they'd need to be very toxin resistant, have thick skin in their mouth that would prevent them from actually tasting things and have an impressive immune system.