Re: At what point in evolution did the circulatory system and heart develop?

At what point in evolution was a circulatory system and heart developed?

First, I have to address some very important misconceptions you seem to have about evolution and biology.

You state: If you view evolution as progressive from the simplest organisms to more complex. . . This statement reflects one of the most fundamental errors in understanding life. The study of evolution is not the study of progress. Evolution is the study of change – that’s it. Not better, not worse, just different. There are plenty of evolutionary examples where lineages evolved from “complex” to “simpler” forms of life.

There is no progression toward complexity here. People like to think of humanity as the pinnacle of evolution – we’re not. Humans are merely one more twig on a very bushy tree of life. The only trend that can meaningfully be ascribed to the evolution of life on Earth is that of increasing diversity. There are certainly more “types” of creatures alive today then there were to begin with. And, having just written that, I can think of several examples were an evolutionary process resulted in the loss of diversity.

You say you don’t know whether to be looking in biology or evolution.” Well, here’s your problem. They are the same thing! As Dobzhansky once said: “Nothing in biology makes sense except in light of evolution.” Biology without evolution is merely a descriptive exercise and would barely qualify as a science.

Now, to your main question. Circulatory systems are a requirement of body size. The smallest creatures can obtain nourishment and eliminate waste through simple osmosis. As body size increases, some type of circulation is needed to aid in osmosis. Smaller creatures can achieve this result by living in a current (a stream of water) or by creating a current through fanning, or some other process. As the body gets even large, water becomes a less effective means of transport, and so a substance like blood and a means to pump it (a heart) becomes necessary. Since we are talking about very small creatures here, and the soft parts of those creatures, it is very unlikely that direct evidence of a heart and circulatory system would ever be found in the fossil record. Instead, you need to look to comparative physiology to determine the conditions that make a heart and circulatory system necessary.

Moderator's "Note added in Proof"
The circulatory systems of most arthropods and many worms are open: they consist of a single, open-ended, muscular tube that slowly pumps the interstitial (between cells) fluid, called hemolymph in insects, around the body. Genetic studies have demonstrated that the genes responsible for the initial formation of the human heart tube are also responsible for forming this simple arthropod "heart", suggesting that the common ancestor of arthropods and vertebrates also had this linear heart and open circulation.