I've started thinking about this recently. The Cosmere has humans, animals, and plant life that are very similar to what we have on Earth. Horses. Parrots. Other types of "chicken". Dogs.
I used to think that this was sort of "handwavium". Obviously, the same species would not all evolve in another universe. But, it's easier to write and read a book if there are elements from the real world that we can relate to.
Recently I started to wonder if something else was going on. What if the Spiritual Realm has archtypes for all these different species that all the creatures running around in the Cosmere are based on?