、。
That's because humans are mammals.
Literal
That [topic-は], humans [subject-が] mammals [copula-である] thing [to-に] derives [progressive-ています].
に由来する ('to derive from / to originate in') is a formal, written expression. The nominalized clause 人間が哺乳動物であること ('the fact that humans are mammals') serves as the object of 由来する via the particle に. The progressive ~ています suggests the derivation is an ongoing, established fact. 哺乳動物 (mammal) breaks down as 哺 (nurse) + 乳 (milk) + 動物 (animal) — literally 'milk-nursing animal.'