An evolutionary perspective on the origin of the cardiovascular system of vertebrates

The origin of the cardiovascular system of vertebrates is inferred from comparisons of basal chordates but must also encompass bewildering discrepancies. Basal chordates like lancelets (cephalochordates) have a vascular pattern similar to that of a vertebrate embryo, but without a recognizable heart or myocardium. Instead, the ‘venous’ part of their circulation contains contractile vessels, located upstream and downstream of the liver. Tunicates (urochordates) have a tubular heart containing cardiomyocytes and enclosed by a pericardium. Their circulation is open and the dominant pacemaker activity can be at either end of the heart tube, causing blood flow to reverse periodically. Recent molecular investigations have proved that urochordates rather than cephalochordates are the closest living relatives of vertebrates. This implies that the cardiovascular peculiarities of lancelets may be primitive ancestral qualities and that the original building plan of the vertebrate circulation featured a post-hepatic as well as a pre-hepatic cardiac pump.