Eli5: How does the heart keep beating, and how did it start beating in the first place?
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Two step answer here.
1. Heart beat is AUTONOMOUS ELECTRICAL MUSCULAR activity, pretty much the building block of all nervous systems. Where did that start? Most likely with something super simple like the [Bacterial Flagellar Motor](https://www.youtube.com/watch?v=cwDRZGj2nnY)
2. But what about hearts themselves i.e something that pumps liquid across the body in steady intervals like a clock? We think Jellyfish are the earliest organisms to have evolved something like a heart about 500 million years ago. Jellyfish have a simple cardiovascular system consisting of a muscular sac that pumps fluid, which is believed to function similarly to a heart. The sac, called a gastrovascular cavity, circulates nutrients and oxygen throughout the jellyfish’s body and also helps to remove waste products. Perhaps not quite as steady as human hearts but the starting point nonetheless.
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