I do think ancestors did stretch: nobody ever learnt me to stretch yet I've been doing it when waking up before getting out of bed since I can remember and I'm definitely not the only one, on the contrary. Some other animals do so as well. I know these are mere observations and no hard evidence, but I would be surprised to learn I'm wrong on this point. Whether they did actual stretching exercises is another thing though. I would guess they did if they were able to figure out by experience stretching had positive effects. Also sort of depends what you mean with ancestors, i.e. how many centuries ago.