I get it, you're alone and bored in the universe and you have to come up with things to do to entertain yourself. I normally don't kinkshame but flying around and masterbating into every ocean you come across hoping life takes hold is kinda fudged up.
It's probably implied somewhere that the KT asteroid was part of their life seeding program that started by eliminating predominant species. They would most likely look for the right conditions on a random planet to support life and then inject the biosphere with their creation.
Not quite Star Trek, but Rick & Morty did an episode with that as a plot point recently in the season where they only travel the galaxy, not the multiverse. Every planet had some form of prehistoric life, with the interpretations of what they found having more or less the same effect on society, but with different silly names.
Was just fun to see the idea explored. Trek would have probably taken it a bit more seriously though.
Well some trek aliens are still closer to dinosaurs than mammals.
But I think that more likely no. Generally the evolution of trek species is meant to be more straightforward than that of humans. I believe Klingons had some ancient large creatures equivalent to our dinosaurs, but they were mammals too. And Andoria doesn't really change much at all.
Can't recall where I have this stuff from though. Probably non-canon sources.
I'm not sure exactly what you mean, but i would imagine any place with life will have mass extinction events and that will make it likely that there has been at least one period before the current sapient species where there was a different group of animals in prominence.
The problem is that it isn’t how evolution works at all. I think this episode and the devolution one were probably the worst, science-wise. I mean, I know they play fast and loose with everything from basic physics to computer science, but for a biologist it’s kind of the equivalent of those stories that speculate that atoms are themselves tiny solar systems with the electrons as planets that have tiny civilizations on them.
They probably came up with the idea because computers using a pseudo-random number generator can produce the same series of “random” numbers if they start from the same seed. We know that evolution is a random process. Therefore, we should imagine that we could control evolution, even over the span of billions of years, if we control the seed. 
First, we have to define our terms. When I say “evolution,” I am referring to the phenomenon that describes the change in the genetic makeup of a population over time. When we talk about evolution more colloquially, we are usually referring to (to borrow a phrase) the origin of species by means of natural selection. In other words, it is the process that explains the diversity (and homogeneity) of life from genes through ecosystems to the biosphere. It takes place simultaneously at multiple levels and the number of non-pseudo random number generators (if we want to consider them that way) is incalculable.
One of the central principles of modern theoretical biology is that you can’t rewind the evolutionary tape. The further back you rewind it, the less like our present the outcome is likely to be. Wind it back far enough - say, to the very beginning at t=0, and not only do you not get humanoids, you’re not likely to get technological intelligence. I’d be surprised to see four-limbed animals or even vertebrates. The randomness comes in not just from things like the random genetic recombination that resulted in the randomly selected gametes that resulted in the individual in a process that traces itself back through the beginning of evolutionary time. It also affects which of the resulting organisms will survive, based in part on every other living thing that’s also undergoing those same processes.
The less random part is the fitness of an organism is the contribution of its generic material to the next generation. Picture two leopards, one faster than the other due to fortunate genetics. One has a 50% chance of propagating genes to the next generation, the other 25%. If the fastest leopard is a lucky mutant, there’s still a very good chance those genes will disappear. Over time, and given some lucky rolls of the dice, we’d expect the faster gene to spread, but it can be wiped out by an unfortunate event, like breaking a leg while hunting or an unfortunate fire or flood or a mutation that gives it cancer.
Sometimes we wind up at the same place from multiple routes. Eyes evolved independently somewhere around a couple dozen times. We can use that to say that sensing light is a pretty good idea. I would not be surprised if life on other planets evolved light sensing. I would expect the underlying mathematics of evolution to be the same, modulo whatever they use for reproduction.
The field of exobiology studies how we can abstract from our single example of a biosphere and our knowledge of evolutionary dynamics and apply it to try to conceive what non-terrestrial life is like. They ask questions like “What else could take the place of DNA?” and even more importantly “What is life?”
This one I can kinda let slide, as it's really the only sensible explanation why aliens from across the galaxy all look alike, and maybe you can encode DNA with so many error correction mechanisms, that on similar M/L class planets it would give similar results. I mean... There are sillier explanations for sci-fi concepts.
Buuuuut Genesis (the de-evolution one), now that's bad, and honestly for me it's the worst ST episode in terms of concept. Threshold gets so much flak, but everything there we've seen before. And I'll much rather believe humans can evolve into salamanders eventually than devolve into spiders just due to some radiation or whatever the reason was.
Genesis was worse, but they’re both so far off the rails that I really can’t let either one go. I’m a theoretical biologist, but I can watch ST and say “Oh, a sub space spore network? Cool! I hope he can make things work out with his doctor-boyfriend.” Paul Stamets, the Disco mycologist, is actually named after the real-life Paul Stamets, mycologist.
One of the things that I found really helpful to learn was that George Romero showed that you don’t need to tell them where zombies come from. Leave it up to the hard SF people to worry about what aliens might actually be like. It’s accepted in science fiction from Star Wars to the Roswell conspiracies that aliens are hominids. They might be squid-people or lizard-people, but aside from Douglas Adams and his hyperintelligent shade of blue, they all look like some kind of terrestrial life.
I’m just saying that if you’re playing a bit fast and loose with the science part of your science fiction, you’re better off with “This is a Klingon. The Klingon says ‘Grrrr!’”
You can get all interesting about the Changlings living in the ultimate retirement home of Lake Havasu (I might be getting mixed up - I just rewatched Falling Down), but that’s more social than biological and they don’t need to get more into it than that.