What has Christianity truly contributed to humanity

[Belacqua]

When I use the term "basic human morality" I'm referring to viewing other people as equal, and seeing they should have the same rights as everyone else. 

Understood. That's certainly a widely-held view of morality today. I'm not sure how we could prove that it's "basic" in other times and places. 

Such as in most cases of slavery, the "masters" have seen their slaves as inferior (mentally and/or physically) to them, so therefore they should not have the same rights, this can also be used in some cases of infanticide. 

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. 

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? 

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I get that for years, outdated science has told people that others are inferior to them, so that slavery is justified. However, even then people were stating how they were all equal 
(ex; The Constitution stating everyone is equal, while slavery was legal.)

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

The Constitution was a revolutionary document, and, as you point out, inconsistent in its application of equality. 

And for infanticide, this is built-in human morality. Our brain tells us to care for our newborns, and to protect them.

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