Really far back in time, there wouldn't be any legs at all, just fins for swimming.
Lead by scientists like Buffon, George Cuvier and James Hutton, the men (few women could participate then) who studied rocks and fossils accepted that the Earth had been around for thousands, if not millions, of years before people came on the scene.Biological evolution is change in the characteristics of living organisms over generations.Despite your vastly superior tastes in music and fashion, you probably look vaguely like your parents, just as they look vaguely like their parents.Only later came Noah's Flood, which was held responsible for depositing all those weird remains, like shells, in rocks on top of mountains.
If that actually happened, human remains should have appeared in the oldest rocks at the bottom of the heap, but they didn't. The oldest layers of rocks held different creatures, and the further down in the heap one looked, the weirder the creatures got.
A simple way to think of evolution is "descent with modification" — over many generations, organisms change into something different.