State one reason why some species might have similar body structures even if they are not closely related.

A reason would be the function: some body structures are good for some functions.

For example, a Giraffe is tall to be able to eat from the tree tops. Another animal might get a selective advantage of being tall and might develop a long neck, because the leaves from tree tops might support the taller individuals in survival.

So some structures are adaptive in some environments and might arise independently.