Ans.
According to theory of evolution, all living organisms are evolved from a common ancestor and simple life forms progressively evolved into more complex life forms. It means, evolution represents a gradual process, in which diversification of simpler organisms into complex life forms takes place.
As fossil of organism X shows a more complex bone structure than fossil of organism Y, fossil X must have evolved after fossil Y.
Thus, the correct answer is option). 'fossil X evolved later in Earth's history than fossil Y.'
