The human heart is unlike those of primitive vertebrates.  Primitive vertebrates possess a tubular heart with 2 regions which are not found in adult human hearts (the sinus venosus and truncus arteriosus), a single ventricle, a single atrium, and sinoatrial valves.  These chambers are evident in the following photos of the hearts of a hagfish,


shark HEART

and frog.

frog HEART

frog heart

These primitive features are present, however, in the hearts of human embryos.  Human embryos possess a tubular heart which possesses a sinus venosus, truncus arteriousus, a single ventricle, a single atrium, and sinoatrial valves.  This primitive condition is subsequently modified to take on the characteristics of the adult human heart.

        Hearts are not only known in vertebrates.  Genetic evidence supports that protostomes and deuterostomes utilize modified versions of the same ancestral body plan given the considerable number of developmental genes are shared.  The last common ancestor of protostomes and deuterostomes probably possessed some type of primitive heart since homologous homeodomain genes are involved in the earliest formation of the heart (Msh-2 in Drosophila and csx in mammals) (Komuro, 1993).  tinman/csk is expressed in heart progenitor cells in protostomes and deuterostomes but on opposite sides of the body (Gerhart, 2000).   Several vertebrate proteins related to tinman are also involved in the development of the embryonic vertebrate heart (Park, 1998).   In flies, Dpp functions with wingless in the formation of the heart tube.  Their vertebrate counterparts (bone morphogenetic proteins and Wnt/Wg respectively),   function in the formation of vertebrate embryonic hearts (Nakamura, 2003).   

     In the study of primitive chordates, some classify the rhythmically contracting ventral blood vessel in lancelets as the branchial artery, others consider it as a one chambered heart which may be homologous regions of vertebrate hearts (more specifically to the truncus arteriosus or to the sinus venosus with a conus arteriosus). (Prosser, 1973; Weichert, 1970; Willey, p. 47).   The growing body of genetic evidence suggests that this structure in lancelets is homologous to the vertebrate heart.  Although the Amphioxus heart possesses no separate chambers, valves, endocardium, or epicardium, it does express amphiNk2-tin, a homolog of vertebrate NK2  and Drosophila tinman genes which are expressed in the developing heart.  Vertebrates and amphioxus also express members of the BMP family, TGFb, GATA, MEF, and FGF in the developing heart (Holland, 2003). 

     Comparative anatomy suggests that the primitive hearts were contractile blood vessels.  The lamprey embryonic heart forms from the anterior part of the subintestinal vein during embryonic development, and the subintestinal vein forms originally from a fused pair of vitelline veins.  (Hardisty 242-5).  The human embryonic heart forms from a pair of endocardial vessels which fuse.

fusing tubes
heart formation
     The straight tube of the early embryonic heart divides into four regions: a sinus venosus, a single atrium, a single ventricle, and a truncus arteriosus.  This tube then folds on itself and some portions become more muscular.

  When the pericardium forms, the atrium and sinus venosus are located outside the pericardial cavity (Sadler, p.182).  The sinus venosus remains paired longer than the other regions of the heart (Sadler, p. 185).  The sinus venosus originally opens into both atria.  It later opens into the right atrium only and finally becomes part of the right atrial wall (shell 367). Initially there are valves around the sinoatrial junction (Sadler, p. 186).  The first contractions of the embryonic heart begin in the ventricles (in rodents, these contractions may actually begin before the heart tubes even fuse).  The atria then begin to contract and with a faster rhythm than the ventricles so that they become the pacemaker.  Finally, the sinus venosus begins to beat with even a faster rhythm so that it becomes the primary pacemaker. (Torrey, 1979, p. 415).  Birds still retain a portion of the sinus venosus separated from the right atrium by a sulcus in addition to the internal valves of the sinus venosus (Torrey, 1979).


sinus venosus
During the development of the mammalian heart, the single atrium divides into a left and right atrium; the single ventricle divides into a left and right ventricle.
6 mm heart
division of heart
The truncus arteriosus divides to form the pulmonary artery and aorta. 
division of truncus

     Congenital cardiac defects occur in 1% of live births and an estimated 10% of stillbirths (Schneider, 1999).  There are a number of problems which can result from the unequal division of the ventricles (interventricular septum defects) or of the truncus arteriosus.  The division of the truncus can be unequal in the tetralogy of Fallot.  Eight in 100,000 births possess a persistent truncus arteriosus (Sadler, p. 200-4).

normal heart heart abnormalities