Cardiac muscle can expand and contract almost indefinitely without getting tired whereas skeletal muscle tires rapidly (more rapidly in some of us than others). So my question is: Why didn’t humans or any other mammals evolve to have cardiac muscle in place of skeletal muscle, allowing them to run and fight indefinitely making them better hunters?
Because maybe cardiac muscle requires much more food and oxygen than regular muscle, and if the entire muscular system was cardiac muscle the animal wouldn’t be able to take in enough food and oxygen to run the body? Just a WAG, but biology isn’t infinitely efficient and there always seems to be trade-offs.
Here is a comparison between skeletal and cardiac muscle which makes it “clear” that as Broomstick says, ther are tradeoffs.
The biggest problem is that cardiac works as a single unit. The heart acts as though it were one individual muscle fibre despite being numerous cells. That’s good if you want something that produces a steady relatively slow beat. It’s useless if you want to have any sort of control over the muscle.
Imagine if you could only contract your muscles completely and then relax them, and always at top speed. You could never for example lift a glass of water gently to your mouth, you could only ever throw it at full power over your shoulder. Every time you want to shake hands you would have no choice but to poke the other person as hard and fast as you can in the chest and so on. Obviously it’s never going to be practical for an organism to have cardiac muscle as the motive system.