Humans are (a type) of animal. Would be the best translation in my opinion when you consider the following:
"Human" in this sentence is meant to refer to all of humanity. So, the translation just isn't right as is. You would say: "Humans". Since it is now plural, you must use "are". So, "Humans are..." But, if you were to put "Humans are animals" you would have a technically correct sentence but give the impression this is more a manifesto on the morality of humanity than a biological fact (which I presume is the intent of the sentence). So, to correctly convey that I would put: "Humans are a type of animal"
Dude ignore it, people seem to downvote a lot based just anything in this thread, the point of Duolingo is that we discuss and work out the issues we have with learning a language, don't let some troll scrolling through and hitting a down arrow derail that, your doing good by trying to keep the learning and conversation going, ignore the haters.
Z biologicznego punktu widzenia, człowiek, istota ludzka (human, human being) jest zwierzęciem, bo należy on do gatunku ludzkiego, jest w całym królestwie zwierząt gatunkiem, lub jest jednym przedstawicielem, osobnikiem gatunku homo sapiens.
Człowiek to zwierzę - A human/A human being is an animal/Humans are animals
I don't know if there is any difference at all. (I'm Polish). "To" is very slightly less formal , so in this particular sentence, which has double meaning. TO ZWIERZĘ feels more about bestiality of human race, while JEST ZWIERZĘCIEM feels more about humans belonging in animal kingdom. And jest to is sometimes very formal. It feels like a beginning of longer definition.
Well as it is interchangeable in this case, It's not always. As a rule of a thumb you can safely assume that you can use "to" as "być" in cases when you describe something as a part of a bigger whole/set/collection. Example: You can't say use "to" when you want to say: Mleko jest białe - Milk is white, there is no way to use "to" here. However when you want to say: Mleko jest napojem. - Milk is a drink. You can say: Mleko to napój - Noun+"to"+verb nominative case. Sun is a star - Słońce "to" gwiazda, same principal here. Hope it helps
It seems to me, and granted I'll probably get down voted, as all comments on this thread seem to be, that polish does not translate exactly into english and will not ever make gramatical sense if you attempt to force it. Polish does not seem to have any form of "a" as we are so used to using it, you kind of have to fill in the blanks as you translate in your head, so "human is animal" would be "a human being is a type of animal".
Perhaps this comment can shed some more light: https://forum.duolingo.com/comment/19428542
But in general yes, I agree. "Osoba to zwierzę" would mean "A person is an animal" so that's a different sentence and a strange one. But "Elrond jest osobą" means "Elrond is a person" and is definitely correct although of course Elrond is not a human being.