Posted: Sep 09, 2010 3:47 am
by hotshoe
It's good to note that tube-shaped digestive system did evolve before lungs, complicated nervous system, backbone, etc. Tube-shaped digestive system organism is the common ancestor of most animals -- that is, not just the large furry things we think of as "animals", not just all the vertebrates, but the simpler creepy crawlies, too. As it was more successful than simpler animals which didn't have a tube, it gave rise to many descendants which spread, were in turn successful in various places, and started their own families, who diversified into many of the different phyla - still existing or extinct - which non-scientists never learn of (because we generally pay attention to the ones that are at least a little like us, the ones who at least have backbones and central nervous systems ...). Tube-shaped digestion predates the beginning of our phylum, the Chordata, by at least 500 million years.
So that was eons in which to add the next systems of interest to us, like nerve cord and closed blood vessels. Whichever tube-digestion ancestor managed to evolve the first primitive nerve-cord is the one who is the ancestor of all the chordates; every other tube-digestion animal is either the ancestor of a different phylum than Chordates, or else died out completely.