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First, all animals evolve at a similar pace. If you got A->B->C->D, then D got extinct, A, B, C won't exist anymore, so there's no C to evolve back into D.

Second, mutations are random and the number of possibilities is extremely large, enough that we can consider it almost infinite. So the probability to have mutations that bring back the same species that got extinct before is almost 0.

What does happen is convergent evolution in which a species evolve to be morphologically similar to another species (extinct or not) because they occupy a similar ecological niche (what they eat and the environment they live in). Though they are morphologically similar, they'll still be as genetically different as expected for species that diverged when they did. They'll still have a lot of differences due to those genetic differences.

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Not to mention if a species became extinct due to natural forces those would preclude evolving into that form in the first place. That wouldn't necessarily apply to human-caused but they are too recent to know.

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While it's theoretically possible, it's unlikely.

First, organisms evolve based on mutations and differential reproduction, so you would need the same sorts of mutations and the same sort of selection pressures. Both of these are unlikely, the conditions just are not the same. Also, why did D go extinct? Because they died out because of over competition in their niche, some parasite, etc. ; well, it would affect a new D too. And of course mutations are random, so it's pretty much impossible to exactly replay.

That said, we do have lots of examples of convergent evolution, where different organisms have evolved to fill in niches in different areas. See: https://en.wikipedia.org/wiki/List_of_examples_of_convergent_evolution . Let's say there is a land that doesn't have a large diversity of birds (say, the Galapagos). The first birds that arrive will radiate (diversify through evolution) to fill lots of different niches, such as eating nuts, eating fruits, eating insects, even though they had the same progenitor species. Interestingly, the evolved organisms filling the niche don't do it quite the same way, because evolution adapts what is at hand.

Bic answered the first half of your question. I don’t know what lens you are coming at this from, so I’ll dump a little bit of medical and biochem in the answer.

As for the second half, if turnover rate goes down, neurotransmitter duration of action is increased. One such drug class that has that action are cholinesterase inhibitors (-stigmines). Their action increases the synaptic half life of acetylcholine by inhibiting acetylcholinesterase, and that manifests as increased cholinergic activity (SLUDGEM or DUMBBELLS are two menonics you can look up if you’d like). Stigmines are reversible and competitive (until they aren’t…they eventually can “mature” into irreversible) so they only impact Vmax, not Km.

Anti-cholinergics do the opposite action at those synapses. They bind the ACh receptor without producing an effect increasing accessibility of ACh to acetylcholinesterase. I can’t comment to what degree this effect increases the turnover of ACh. The Km of acetylcholinesterase from a quick google is somewhere in the realm of 10 mM making me think it does not operate in a saturated fashion, but it’s been a decade since I’ve learned this stuff. It seems increasing substrate supply would increase rate too.

Hope this helps.

Yes viruses have DNA, and a protein body to aid in moving around and injecting into a target cell that's it. That's why they're not really quite an alive entity Because there are no self sustaining cellular organs that bacteria and etc have

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Thank you.

Thank you.

Thank you.

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that's how I see it too. On a molecular level, there are no "alive" tags so where's the line? With things like viruses and prion proteins, it's difficult to argue there really is a reasonable line.

There are tons of cellular obligate parasites that render the discussion moot

At a certain point maybe it's not so useful to think of something as alive or not.

They do have DNA / genetic material. That means, whatever you want it to mean as far as if theyre living?

Good point.

AcCh turnover increase means more is degraded by the enzyme, decrease the opposite.

Depending on the location of the turnover (central nervous system or e.g. In skeletal muscles) this could lead to prolonged or shorter duration of AcCh.

One cool example of a drug working by decreasing AcCh effect (ultimately) is botox, leading to temperary paralysis on the injection site.

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