No, it would not be a problem at all. The steps are even smaller than you think, eventhough the human heart as we now it now is an evolution marble off course, it has developed over million of years.

The most simplest kinds of hearts present now are those of some invertebrates such as certain kinds of worms. It's just a muscular tube which sqeezes rythmically and moves blood-like liquid by peristaltic contraction.

The role of invertebrate circulatory system is not necessarily respiratory exchange, but rather nutrient transport (which does not require a rigid and systematic circulation contrary to respiratory exchange).

With this in mind, it would be logical to assume that infact quite a lot steps in between happened, before going from one tube like muscle to a two chambered pump.

Evidence for this can be derived from what we see around us now too, just look at the anatomy of other animals.

Anatomic studies have made it more than plausible to conclude that the most likely way the evolution of the heart for vertebrates went is like this. Fish to amphibians to reptiles and to mammals. Any systematic drawing of all those hearts next to eachother should be enough to draw the exact same conclusion.

Fish:

Amphibians:

Reptiles:

Mammals:


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