Wikipedia Translation Question I found the following sentence on a Wikipedia page in the paleontology subsection: "Los fósiles que podemos encontrar en un estrato no tienen por qué provenir de organismos que vivieron juntos, que compartieron el mismo ambiente e interactuaron, ni siquiera ser de la misma edad geológica." (http://es.wikipedia.org/wiki/Tafonom%C3%ADa). I'm currently reading that as "The fossils found in a stratum do not need to originate(?) from organisms that lived at the same time, that shared the same environment and interacted, nor do they need to be from the same geologic age." Specifically, I don't understand the role of por qué in this sentence (as in "tienen por qué provenir"). My understanding of the phrase is that it means "why" or "for what," but I'm having trouble understanding it in context.