"Personhood" seems a very arbitrary (and not at all clearly defined) qualification here. "Understanding" arises from thinking, not from "personhood". And surely, there are non-person minds that understand less complex things. That they can evolve to achieve person-caliber capabilities incrementally and over long periods of time is well-accepted.

As an aside, we can't communicate very well with the smartest animals, so we don't know how much they can understand. It could be more than most of us think.