What has Christianity truly contributed to humanity

[Variachial]

"Well I think it's obvious that according to lots of different measures, certain people are better at certain things than others. 

I am inferior to many many other people in athletics, musical talent, organizational leadership ability, financial acumen, etc. etc. many etc. "

Yes but those are skills you're able to build up in life (for the most part). Everyone is born with little-to-no musical talent, that's something people achieve.

"The question of why people of different degrees of ability are assumed to have equal rights, or equal value in a society, is the question to me. How did we decide this? What makes us believe that regardless of, say, intellectual capacity, we are all equal?"

Not everyone is necessarily equal physically or mentally, however we are all people so even to those who are inferior in those ways, we should still treat them as human and not property.

"Please notice that I am NOT saying equal rights are bad. I'm FOR equal rights. But how we got from, say, the Roman Empire, which didn't believe these things, to today, in which they are widely believed, is a historical question. I don't believe it's 'just obvious.'"
 
I just think society changed for the better and started to focus more on the people.

"What science did the Ancient Greeks or Egyptians use to justify slavery?"

I was more referring to American slavery and how they used untrue racist science to justify enslaving Africans.



"If infanticide is so clearly against human nature, why was it practiced so widely and for so long? Has human nature changed?"

I do believe human nature has changed with society, natural human behavior tells us to care for children no matter what. Then people got desensitized (for lack of a better word) to death and war, so they stopped caring about babies, and stopped seeing them as helpless and innocent, but as things that should be killed for whatever reason they had. (I may be wrong on this)