The heart that now pumps blood through your body is a marvel of evolution which has taken millions of years to perfect. Below is a brief account of how the organ changed from the exceedingly simple heart of a worm to the amazingly efficient, four chambered heart of a human.
The simplest kinds of hearts, present in some invertebrates such as some types of worms, consist of a muscular tube which squeezes rhythmically and moves blood-like liquid by peristaltic contraction (in sea squirts such heart can actually pump blood several minutes in one direction and then reverse the flow into the opposite direction). On the other hand, some mollusk hearts have quite a complex structure, which may include four atria and one ventricle(Nautilus), or be composed from multiple (seven and more) individual hearts as in annelid worms. Such diversity in the structure and number of invertebrate hearts can be explained by the relatively small size and low metabolic activity of these animals as well by the fact that the role of invertebrate circulatory system is not necessarily respiratory exchange, but rather nutrient transport (which does not require as rigid and systematic circulation as does respiratory exchange).
In
vertebrates, however, one of the primary roles of the circulatory system is
transporting oxygen to the cells and a certain generalization about circulation
in the different phyla and stages of evolution of the heart are possible. As
a result, we can follow the heart as it develops from fish
to amphibians
to reptiles
to mammals.
