I'm generally quite knowledgeable about how certain complex traits evolve, such as the eye etc, but one thing has always confused the heck out of me, and thats the evolution of the heart and veins. Surely the purpose of veins will always have been to carry fluids through the body to muscles etc, but veins can't have just suddenly appeared as hollow in a creatures evolution carrying blood? Also the same with the heart, surely it has no advantage before its beating? I guess what I'm bringing up is the argument of irreducible complexity, but I would love to thwart this argument yet again and have some answers to this. Possibly its an unanswerable question but if anyone could give me their opinion I would be really happy

The most primitive form of these organs would have been tubes allowing bodily fluids to disperse through the body; then a form that use peristalsis (local contractions) to move the fluids more efficiently; then a more complex network of tubes; then an increasingly specialised a powerful perstaltic region that eventually evolved into a heart. Each step has survival/reproductive value on its own.