Quite complicated, as we only have skeletons, tools and some wall paintings. The modern human is about 200,000 years old, but the first steps in evolution that made humans possible started, depending on what you see as "distinctly human", between 7 and 2 million years ago. What we're talking about in this case is mate choice, which has probably been around for 300-400 million years. Obesity is simply disease and degeneracy, any argument to the contrary is unspeakably ridiculous in evolutionary terms.