Where does “prone” come from?

I need to work through this chain carefully to understand the sense development and create an accurate etymology sentence. Looking at the chain from oldest to newest: - Proto-Indo-European h₂el- - Latin illa - Proto-Indo-European per- - Proto-Indo-European bʰuH- - English lad - English la - Latin pronus - Middle English proone However, this chain seems problematic. The path from these disparate elements to "prone" doesn't follow standard etymological logic. Latin "pronus" (meaning "leaning forward, prone") is the clear direct source for Middle English "proone," but the rest of the chain appears disconnected or erroneous. Following the instruction to "state only what the chain supports," I should acknowledge that while I can see the Latin pronus → Middle English proone connection clearly, the chain as given doesn't coherently explain how

prone (English): Lying face downward; Having a downward...