Say we have an advanced alien civilization with advanced technology. They encounter a planet teeming with early life (microbes). They want to speed up evolution on the planet and have sentient and non-sentient species to develop their within the span of maybe 100 years, as opposed to 100,000 years. What chemical would a planet have to be bombarded with to achieve this desired effect? It can be a hypothetical chemical, or element
closed as primarily opinion-based by RonJohn, L.Dutch - Reinstate Monica♦, Tim B II, JBH, Palarran Apr 8 '18 at 20:51
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My answer may be a cheat if relative time does not count...
Well, unless the aliens have vats of Handwavium (RonJohn) or containers of unobtainium(D.J. Klomp) to make the problem away, I don't think they could speed up evolution in any meaningful way without a lot of direct intervention over time. Although it seems from your question that the aliens don't want to do a lot of effort, but still see how things will happen over time.
One way that your group of aliens could see a planet evolve however, is for the aliens to go really really fast. If the aliens have access to advanced enough technology (I'm assuming they are flying around in star trek like ships discovering planets and such) the aliens don't need any chemicals to see the evolution. They merely need to abuse time dilation.
So we want to turn 100 relative alien years into 100000 years or in other words, speed up time by 1000 times. This would allow your aliens to speed up the relative time of evolution and be controlled. So say for instance the aliens observe the planet, try a few different experiments, expose some bacteria to different radiation etc, then leave. Fly away for a x years, see what happens, and then play more with the evolution each and every time. There isn't really one chemical that will speed evolution up (to my feeble meatbag brain), but if your aliens played with the environment or did some artificial selection every hundreds or thousands of years it could definitely make for an interesting playground that would change how things evolved.
HOW FAST TO DILATE BY 1000 TIMES THOUGH?
Roughly 299792.30812 kilometers a second or 99.999950005413% the speed of light
There's some interest info about the topic here
I used a tool located here to assist in the calculation
For more conversions (if your aliens want to speed up time in a different manner) here is a handy chart to show you Percent Speed of Light and how many days/years that would be if you spent 1 day going at that percent speed. For example 0.99999999999999 c, for every day on board the alien ship, 19380.535 years have passed on the planet.
Given that the planet already has microbial life, they do a series of drops. First they drop seeds of small, fast-reproducing plants, plus smaller animals that can survive on eating microbes. Once those are established (which should take only a few years), they continue with a succession of larger plants & animals, until they have a complete biosphere close to that of their home planet.
What they won't have is the native microbes somehow evolving to a complete ecosystem of large multi-cellular creatures in anything less than tens of millions of years. See https://en.wikipedia.org/wiki/Cambrian_explosion
They need to drop microbes.
Here is what the aliens are going to do: They will collect a few samples of the microbes already living on the planet, digitalize them and evolve them in a computer simulation for a few simmulated millenia, have a genetic algorithm pump a few hundred variations of the microbes and then reinsert them on the biome.
Wait a few generations for them to adapt to the planet and repeat the process until you have the first sapient animals.
Now comes part 2: Civilization.
Your primitive sapients will need access to certain resources to evolve, so give them some modified crops and ease access to metals so they can start their civilization rolling.
It will depend a lot on how much interference you want, you can have a direct radio link to the ship and full access to Alien!Wikipedia or maybe a few chosen prophets each new generation bringing knowledge from the gods.