630-600 million years ago


The rhythmically contracting pharynx of ancestral higher worms utilized genes which their descendants would use in the development of the heart.

In primitive invertebrates that possess a primitive circulatory system (such as nemertine worms), the blood is pumped by contractile blood vessels whose muscle contracts in peristaltic waves, similar to those waves of muscle contractions observed in digestive, urinary, and reproductive tracts (Hoar, 1983).
While nematodes lack a heart and circulatory system, there is evidence that the rhythmically contracting pharynx is relevant to the evolution of cardiac muscle. Like cardiac muscle, the muscle in this area does not involve MyoD genes (unlike skeletal muscle) and can contract without nervous input. Nematodes with mutations in the homeobox ceh-22 display defects in pharyngeal muscle but these mutants can be rescued with similar vertebrate homeobox gene nkx2.5, which is specific to the vertebrate heart (Haun, 1998).