I do not believe we are far from our ancestors either, neither biologically nor psychologically (Jung's archetypes...). I don't disagree on that; I disagree on using that argument as an excuse to resign to our so called "nature", which apparently is evil and selfish by definition though we can't define it by any means (do you notice the contradiction...?). And if you think science and genetics will tell you what our nature is, you should come to notice that, though they do influence, they are not the whole. Neither of us is only a genetic disposition, but a big amount of factors, among the which genetics is only one, and perhaps not even the most important. So... We can keep telling ourselves we know something about human "nature" as a defined thing. The true is that we don't.
Now, if you'd say that we are usually capable of evil and selfishness, I wouldn't deny it, and I would admit it's a huge concern regarding communism; mostly because it pretends to deposit on one individual the power of "the people". And who can say that individual is righteous? It may be Ernesto Guevara, in which case we are lucky, just as it could be Stalin... in which case we are not. So, as you can see, I agree with this argument; but to identify the possibility of greediness and selfishness with the nature of human being is wrong; it's a possibility. If that's the logic, it would be equally fair to say we are good and kind by nature, since we are capable of goodness and kindness; this is, since those are a possibility. Superficially I would say I agree with your concern; but I couldn't say the same when dealing with your argument's strength.
Short notes: Capitalism is not equality of opportunities; a labour's man's son with a bad education and forced to work since early age has not the same opportunities that the son of a wealthy man does. That's a reality nobody can deny.
Communism is not equality of income. Where did you even get that from? Haha. In the most favourable case, it's superficial propaganda against communism, but has no contact with communism as a theory. That's why people should read Marx's works and see what Marx's proposals were before attribute its theory fake aims and bases. (Actually, with Jimmy we discussed this prejudice before; you can read on those posts the thing with income in communism (basically people getting what their work is worth, and not loosing their work's value in the hand of others...)).
We developed "social contracts" as a consequence primarily of "religion", in order to create "civilizations". Well... very arguable. And you didn't give any argument, just throw the enunciation by itself. Don't even know if it's worth the debate anyway; wouldn't be sticking to the point. If your point was that communism handled the religious phenomenon very badly, I agree a hundred per cent: people should be free to practise any religion they want. But, again..., Marx didn't want to kill religious people, or repress them, but hoped humanity would overcome religiousness as a repressive form of life. I don't agree with that point of his, but come on, it's not even the important part of communism, haha. And, in theory, we were debating his ideas, not the mistakes of Castro or Stalin. So... well... that's it.