They've relied on there not being any way to know for SURE what Human Nature is for most of that millenia, but I have GREAT NEWS!
We DO know for sure now. We have done the science and been able to observe the data. I only know a little bit about the first experiment that was done (I read about it quite a number of years ago, my memory is a little spotty on how the experiments were done, all I remember is what we learned).
Altruistic! Which is a Neutral and Scientific way of describing what was observed (science is very big these days on trying to NOT put morality words on the data).
So, first of all, it's very hard to answer this question at all, because to do so you have to observe humans before any culture is given to them, and that happens VERY early indeed, almost at birth, so we have to observe babies, and that means they cannot meaningfully consent to a lot of things. So we have to be careful with them so we don't hurt them, and that means we have sample sizes that are a bit smaller than is usually wanted. That being said, what the current amount of data seems to strongly suggest (again, more Neutral Science Words) is:
How did we test this at all? Adults the babies did not know dropped something and pretended to have trouble reaching it. Adults the babies DID know did the same thing, just to make sure. Both times, the adults did not specifically look at or address the babies at all, to make sure the babies and adults were not communicating. And the babies immediately tried to help the adults, whether they knew them or not--and, iirc, whether the babies could or not.
Since then, lots of science about Altruism has continued to be done--we have better and more secretive cameras now, so we can make better experiments where more and more types of animals can be observed without them knowing they are being observed. We've observed chimpanzees and animals very like ourselves, on down to even animals as dissimilar to us as bees.
Social animals are altruistic. It seems to be part and parcel with being cooperative and able to work together. Which seems obvious, but Science likes to have Hard Evidence, you see--Data that does not rely on interpretation. And the hard evidence seems to be that humans couldn't have become something as complicated as ourselves without really strong altruism overall as a species. Now, this doesn't mean we all have the same amount, or that we're born with the same amount. But it means that across our species, it averages out to a very high amount indeed--and, human brains being plastic, it means we can learn more or less of it as we age.
Christianity and Aristocracy and other people who believe in Thin Veneer Theory really hate Science for answering this and moreover, for continuing to run experiments and pile up more and more evidence about it. But that's Science's job you see, is to Keep Asking. "What is the nature of Humanity" is no longer a question you can toss at Philosophers in order to keep them busy. We have an answer now. The answer is, put into Regular Words: