Science is observational. It doesn't give us rules we must follow, it describes the behaviour of things in the universe.
So if someone* says, "People do selfish things, that's science," true!
But if they also say, "Therefore, it's ok to kill your neighbour and take their house, see all of recorded history, colonialism, &c." I stop them. Science teaches us that people kill each other and take their land.
But science also teaches us that people coöperate and build rockets to Mars. Science is not prescriptive, and with respect to morality and ethics, science teaches us that there are many different strategies that people use to accomplish their goals, some of which may result in the replication of the information encoded in their genes.
Selfishness "Just is?" Yes.
Therefore "______ is ok and we must accept it?" No.
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* I am not putting these words in your mouth or arguing with you, just picking up where your statement left off.
"Humans are the greatest species of Life on Earth, according to all of the metrics that humans have chosen to measure greatness."
Science also tells me that insects are better than people, if I pick a different metric. I suspect we agree on this.
No but the fact that we have morality (and the possibility that other animals do not) can be explained via evolution. In reality probably many genes contribute in a complicated way to making us social animals.
What makes it true?
The more the human mind finds answers, the more questions it reveals - such is the nature of an infinite universe and our struggle to perceive it. If "God" doesn't exist we humans sure do spend a lot of time attempting to become one. "God" may not be a "he" sitting "on a cloud", but may indeed just be the New Question beyond every Old Answer. This fact seems to be rabidly overlooked by the Dawkins cult, which prefers to define God in its own, limited terms, in order to find fault in their chosen pariah cultures.
The jury is still out. On a scale of 1 to 7, 0.1 is just enough uncertainty to allow for yet more unanswered questions .. Dawkins, himself, has at least a little of the humility required to admit that.
Excuse my frankness. That's not a fact. That's unfalsifiable woo-woo.
Without the woo and whimsy and make-believe, what have we got left in our cultures? If we kill religion we may as well kill theatre and literature and all the other things which require this unmeasurable substance in order to be viable ..
Let me use a specific example. I often say that capitalism is terrible for workers. Yet I work for a capitalist company that is--in my opinion--quite good to its workers.
Being a "realist" has told me that when looking for a job, I should be careful to avoid toxic cultures, exploitative management, and being underpaid relative to whatever I consider the fair market value of my labour.
But being a realist has also told me that toxicity, exploitation, and stinginess are unevenly distributed, and therefore it is a wise thing to shop around and negotiate for what I want.
I would NEVER say, "Toxicity is just how all businesses work. Demeaning and bullying people is just how company cultures are. Exploiting workers is what businesses do. If you can manipulate someone into working such long hours their health suffers, good for you. And negotiating is impossible, all companies push their workers around."
Being a realist means acknowledging what is in the universe, but it also means acknowledging that the universe contains variety and that almost everything is unevenly distributed.