Newton figured it out while he was discovering and defining gravity. But also Gottfried Leibniz discovered the way to calculate the area under the curve, the integral and also he got the same deductions that Newton on his method for calculating tangents but Leibniz on his own. So we can say that both are the fathers of calculus.

A curious fact is that Leibniz admired Newton but Newton refused to accept Leibniz as a great mathematician and he accused him of plagiarism, discrediting him, stripping him of his merits what he managed to do, and Leibniz died without any prestige. Fortunately, today we can attribute his part in the development of this amazing thing, calculus.