This is a question inspired by Darwin and the "Glutton Dining Club" where he enjoyed eating all sorts of different animals.

I'm intrigued to know why do more recently evolved animals generally have tougher meat.

Mammals are generally tougher than birds which are generally tougher than fish which are generally tougher than invertebrates. (Ive not eaten reptiles so don't know how tough their meat is)

If it was a simple matter of protection or strength then I'd have assumed that tough meat would have evolved very early. If soft meat was advantageous to speed or maneuverability then jellyfish, crustaceans, and mollusc's wouldn't need such soft bodies.

What would you think?

I am not sure your hypothesis is entirely correct and am unaware of specific studies that have accurately measured and then compared "toughness".

If we assume however your hypothesis is correct then one answer could be that larger stronger animals have more densely packed muscle fibrils with more fibrous sheaths for added tensile strength which in turn make the meat tougher and thus has to be cooked for longer. see
https://www.exploratorium.edu/cooking/m … juicy.html