When I'm doing this, I'm naturally giving some leeway in the exact times. For most of these eras, we only have approximate estimates for when things happened anyway. So I'm basically taking it to be "Comet Hale-Bopp has shown up, something big has happened or will happen within about 50-100 years or so".
Filling in some of the blanks:
10500 BCE: This is the approximate time that the first settlements began at Jericho. Start of the Holocene Epoch.
8000 BCE: I'd say you're good here. Agriculture is the main thing.
5500 BCE: This was about the time that the Sumerian civilization began, leading to irrigation and the first known invention of writing (though writing wouldn't be for another ~2500 years). Smelting of copper, leading to the use of metal tools.
3000 BCE: Approximate invention of writing in Sumer (this is the commonly-accepted distinguishing point between history and prehistory).
500 BCE: I'd say the biggest thing here was the beginning of what we would now call knowledge/philosophy; within a hundred years, there was Buddha, Confucius, Socrates, Plato, Euclid.... This was also the time of the beginning of the Roman Republic, aka the time where Rome became the powerhouse that we remember it as.
2000 CE: Woo boy. In this general time frame, we have the first use of atomic energy, the computer age, globalization, and the beginning of the space age. Thinking to the future, it's possible that within the next 50 years or so we make contact with extraterrestrial life; learning that we aren't alone would certainly
4500 CE: Honestly, I'm not going to even try to guess what will be happening to humanity in 2500 years. I mean, 2500 years ago we had only just reached the point where we were advanced enough that people could spend time doing "pointless" things like math and philosophy (where by "pointless" I mean "things that don't directly contribute to your immediate survival"). Advancement is currently on an exponential curve - how many thousand years from the wheel to powered flight, and then 66 years from powered flight to walking on the moon. As such, there's really no way of predicting where we'll be in 4500. It'd be a pure guess, and such wild guesses that any guess would be equally valid. If you want something to fit in with your theme, maybe say that this is the founding date of the Interstellar Terran Republic, or something.
Now, what's the point? What's the trend here? The trend is slow uplifting. From nomadic hunter-gatherers to city-dwelling hunter-gatherers. From city-dwelling hunter-gatherers to farmers. From farming with stone tools to forging metal, from metalworkers to writers, from writers to mathematicians/philosophers. From city-states to large empires to a global society to an interstellar civilization.
Why would they do this? Maybe they're lonely and want someone to talk to. Maybe they're breeding us to be soldiers for their wars. Maybe they've destroyed all the other intelligent life and want someone new to fight. Maybe we're a massive supercomputer trying to figure out the Ultimate Question. Depends on what the story calls for.