You can change an event in human prehistoric times. What will you change, and why?
Here is one I would like to see played out:
Cro-Magnon man takes a detour out of Africa and arrives in Europe, say, 10,000 years later. My thought is that the delayed competition would allow Neanderthal man to better adapt and thrive in post ice-age conditions, resulting in a population strong and stable enough to resist extinction from the eventual introduction of Homo sapiens.
Having two species of humans co-exist through to modern times is, in itself, a fun prospect, but let’s extrapolate further. I’m going to add the caveat: Neanderthal man and modern humans cannot crossbreed (their peckers are too big to fit into our women. Result: Lots of very aroused ladies and vain attempts, perhaps…but no kids. Conversely, Neanderthal ladies would simply laugh at my peers shortcomings***…again, no offspring). Would two separate, yet stable populations of humans eventually merge into a harmonious civilization? I think, yes – after a period of hostile adjustment. Would this civilization be superior to the one we have now? I think yes, based on the assumption that a little competition trims the fat. In the same sense that a two-party political system is better than one, a two-sentient species civilization could be advantageous.
Would a merged civilization be better for both parties concerned? Again, I think yes. As a whole, our civilization appears to be enlightened and finally willing to take steps toward racial/ethnic/cultural equality and we have arrived at this point in a relatively short time period (in the context of evolutionary timeframes). Given tens of thousands of years (since first contact), I believe enlightenment toward bi-species equality would have already occurred, and, as a bonus, this may have lead to a much earlier sensitivity toward racial discrimination. Of course, the differences between two races is essentially, merely skin deep; differences between two species would be more basic and profound. Discrimination would be unavoidable, but perhaps it would evolve into a mutually beneficial form of discrimination. Our relationship would be symbiotic. I think a symbiotic relationship between sentient beings could be not only harmonious, but rather enjoyable:
*Yeah, Bill may be a frontal-bossed oaf, but he’s my friend, and boy-o-boy, does he dig a good ditch!
Chad? Sure, he is indeed a panty-wipe brainiac, but he’s my buddy and he finds a boatload of deductions when he does my taxes. *
In other words, there would be a more distinct division of labor than what we see today, but because the division would reflect self-evident, unquestionable innate differences it should be readily accepted. All varieties (race, ethnicity etc.) of Homo sapiens would long ago have drifted toward brain-intensive occupations, while the Neanderthal branch would have drifted toward muscle-intensive endeavors. (I’m smarter, your stronger…let’s just deal with it). The enlightenment that I spoke of would manifest itself not as absolute equality (as is possible with regard to race), but rather in a sense of fair play and equitable compensation (I teach physics, Tom picks oranges, but we both draw the same salary and belong to the same country club).
Would I like to live in this society? Yes, I think I would. What about you?
Rose colored glasses?…yeah, maybe. Perhaps delayed contact would only delay the inevitable annihilation of one or both species. Lets hear your thoughts on the matter. And, I’m also interested (yawn) to hear your original “event changes”. No, really, I am! 
Mr. Tibbs
***Not mine personally, you understand…I’m an exception to the rule. Hey!…stop laughing…