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According to this

New understanding of heart evolution

Humans, like other warm-blooded animals, expend a lot of energy and need a lot of oxygen. Our four-chambered hearts make this possible. It gives us an evolutionary advantage: We're able to roam, hunt and hide even in the cold of night, or the chill of winter.

But in the other hand, there are heartless JellyFish of 2.3 meters with up to 70-150 tentacles per one of each 8 lobes of up to 37 meters in lenght. Lion's mane JellyFish

Having humans spending a lot of their energies in moving limbs, even though I understand JellyFish move in another environment, I'm having a hard time believing moving our limbs spend more energy than moving 70-150 tentacles per one of each 8 lobes of up to 37 meters in lenght.

What's the evolutionary purpose of having hearts? Is it for animals with it to be able to expend more energy or is there any other reason for it?

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  • $\begingroup$ Perhaps the currents help move those tentacles... water is approx 1000 times as dense... $\endgroup$ – Solar Mike Aug 9 at 16:13
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    $\begingroup$ Evolution has no purpose. BTW, bacteria also move and they don't even need nervous system for doing so. $\endgroup$ – WYSIWYG Aug 9 at 17:18
  • $\begingroup$ Have you tried looking up the energetics of movement for vertebrates? ——— Note also that a jelly doesn't need a heart to transport oxygen to its tissues since it is very thin and absorbs oxygen from the water. This makes the comparison to the energetics of a jelly completely irrelevant! $\endgroup$ – tyersome Aug 9 at 18:28
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There are very important advantages (not purposes) of having heart.

Animals that have heart are more agile and can be much larger than others. That is because they can move nutrients, gases and waste products faster and efficiently between parts of the body. Without heart and circulatory system, all the necessary material must be moved inside the body through diffusion which isn't efficient especially at longer distances and fast.

All of the largest animals that live and had lived on earth have heart. An animal which doesn't have heart can be large and can be very long, but will be thin to get a very large surface area which will increase the efficiency of diffusion between the body and the environment.

On terms of advantages that they provide, having a heart (and circulatory system) is better than not having one, having closed circulatory system is better than having open circulatory system.

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