The human heart is composed primarily of cardiac muscles whose contractions pump blood throughout the body.  Although the heart muscle cells are autorhythmic and can generate heartbeats on their own, the heartrate can be regulated by nervous input and hormones.  The human heart is divided into four chambers (left and right atria and left and right ventricles).  The right atria and ventricle are part of the pulmonary circuit which delivers deoxygenated blood to the lungs while the left atrium and ventricle are part of the systemic circuit which delivers oxygenated blood to the tissues of the body.  These characteristics developed over a series of transitions from more primitive ancestral conditions.  The heart has a long evolutionary history, beginning as a contractile blood vessel, developing into a series of chambers which delivered blood to the gills and then onto the bodies tissues and finally a pair of circuits which separated oxygenated and deoxygenated blood in land vertebrates which lacked lungs.

     Hearts were not required organs for the first animals.  The most primitive animals lack circulatory systems and rely on diffusion alone to disperse materials through their bodies.  Placozoans nave no nervous or muscle tissue.  They are the simplest animals with only 4 types of cells. In placozoans, epithelial cells are joined by desmosomes, a type of junction which is also found in vertebrate cardiac muscle (Grell, from Harrison, Vol.2).  Porifierans lack classical desomosomes and gap junctions, although they do possess desomosome-like deposits (Harrison, Vol. 2). 

    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).  A number of invertebrates possess contractile vessels throughout the body including hemichordates, Amphioxus, and annelids in which blood is pumped through peristalsis.  Such vessels are even known in the wings of bats.  (Burighel, from Harrison, 1997, p. 232; Ruppert, from Harrison, 1997, p. 445-52; Prosser, 1973).  Nemertine blood vessels are line by epithelial cells and are joined by desmosomes (Turbeville, 1986). 

     Pognophoran worms possess a ventral blood vessel which is more muscular than other vessels and forms a tubular heart with pacemakers at both ends (Hickman 713).  Many mollusks possess large hearts with separate chambers and cephalopods also possess accessory hearts as well (Prosser, 1973).  Annelid hearts can beat rhythmically and can continue to beat even after removed from the body, as observed in the hearts of vertebrates. (Webster, 1974, p. 9).  Most arthropods rely on rhythmic contractions of a dorsal blood vessel or a heart which is derived from this blood vessel.  Arthropods may possess both auxillary hearts and heart valves (Hoar, 1983).  

     How many times did animals evolve hearts?  Are the hearts of invertebrate protostomes and deuterostomes homolous?  At first, it might appear that they are not given the difference in their location in the body (a dorsal position in protostomes as opposed to a ventral position in higher deuterostomes) and the difference in the direction of blood flow through the dorsal and ventral vessels.  These differences in the cardiovascular system, and a number of other differences in the body organization between protostomes and deuterostomes could be reconciled if a rotation of the body axis occurred in ancestral deuterostomes.  As a result of this rotation from the ancestral condition, deuterostomes would possess a ventral heart and the anterior-flowing blood travels through ventral blood vessels while protostomes possess a dorsal heart and the anterior-flowing blood travels through dorsal vessels (Gerhart, 2000). 

   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.  These vertebrate proteins can actually be substituted for tinman in Drosophila to promote the differentiation of the visceral mesoderm but they do not replace its function in promoting the development of the 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).    

     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).



     Hemichordates possess a heart which includes a venous sinus which receives venous blood  (Hickamn; Benito, from Harrison, 1997).  Enteropneust worms are classified as hemichordates but are similar to protostomes in possessing a dorsal heart and protostome-like blood flow (Gerhart, 2000; Benito, form Harrison 1997, p. 20).  This and other evidence suggests that the hemichordates separated from the chordate lineage at about the time the body axis rotated in ancestral chordates.  Most of the muscle in the hemichordate heart is smooth but some striated muscle also exists. (Benito, form Harrison 1997, p. 44)



     Urochordates possess a primitive heart in which the blood flow periodically reverses due to peristaltic waves which can travel in either direction over the heart muscle, even when the heart is removed from the body (Harris; Hoar, 1983).  There is no endothelial layer lining the interior of the tunicate heart and striated muscle contacts the blood directly. The muscle cells in tunicates require calcium for their contraction (as in vertebrates) and are joined by tight junctions and gap junctions. (Webster, 1974, p. 48-9; Burighel, from Harrison, 1997).  There are pacemaker cells at either end of the heart and peristaltic waves move blood in the hearts of tunicates (Webster, 1974, p. 46; Prosser, 1973).  There seems to be no nervous control of heart rate (Prosser, 1973).



     The heart is a modified blood vessel.  This raises the obvious question: at what point should a contractile ventral blood vessel be called a heart?  While hemichordates, urochordates, and pognophorans possess definitive, albeit primitive hearts, the existence of a true heart in lancelets is less clear.  While some classify the rhythmically contracting ventral blood vessel in lancets as the branchial artery, others consider it as  a 1 chambered heart which may be homologous to the truncus arteriosus or to the sinus venosus (with a conus arteriosus) of vertebrate hearts (Prosser, 1973; Weichert, 1970; Willey, p. 47).   The growing body of genetic evidence suggests that this structure is homologous to the vertebrate heart.  Although the Amphioxus heart has 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).  The heart muscle of the myocardium is not striated. (Holland, 2003).  The direction of blood flow can reverse in some species but not in others (Ruppert, from Harrison, 1997, p. 445-52)



      The earliest fish known is Myllokunmingia from the Early Cambrian.  The fossils suggest that it was a craniate, like modern hagfish, and that it may have possessed a pericardial cavity.   Modern hagfish possess a heart with 3 chambers: the  sinus venosus, a single atrium, and a single ventricle.  The sinus venosus is the site where the contraction of the heart in initiated, which is also true of the embryonic hearts of all vertebrates (Torrey). The hagfish heart is made of cardiac muscle and valves permit only one direction of flow between heart chambers (Kardong, p. 462).  The cardiac muscle fibers of hagfish and lampreys are smaller than those in mammals; those of hagfish are about half the size of those of mammals (Hoar, 1970).  Hagfish, lampreys, and all jawed vertebrates possess both an epicardium and an endocardium (Webster, 1974, p. 129). Hagfish may possess a rudimentary chamber corresponding to the conus arteriosus (Simoes-Costa, 2005).

      Hagfish also possess accessory “hearts”.  These “hearts” are contractile blood vessels (which usually possess smooth rather than cardiac muscle) and include a portal heart which pumps blood to the liver, cardinal hearts (in the cardinal veins), and caudal hearts (in the caudal veins). (Linzey, Kardong)  Hagfish also possess cardiac muscle in the hepatic portal vein heart (Webster, 1974, p. 130).

     In hagfish, unlike all other fish, there are no coronary blood vessels to the heart and the blood perfusing the heart is almost completely deoxygenated  (Weichert, 1970).  Thus the heart depends on a significant amount of anaerobic metabolism and can power 70% of its maximum power on anaerobic pathways alone (Forster, 1997).     Although there is no coronary circulation in hagfish, some capillaries do service the outer layers of the atrium and ventricles and blood from the interior of the heart does service some of the myocardium through the channels between trabeculae.  (Webster, 1974, p. 124) 

     No major nerves innervate the hagfish heart and the ANS can not directly affect the heart rate (Romer, Kardong).   Hagfish are capable of some variation of cardiac output.  Upon exertion, the blood pressure in the ventral aorta can rise 7.5% in hagfish compared to 21-35% increases in teleosts (Forster, 1997).  Hagfish use catecholamines, ANP, and neuropeptides to regulate circulatory physiology.  Catecholamines stimulate moderate increases in the contraction rate of both the primary and portal hearts (Forster, 1997).  Hagfish hearts have no vagal input and are insensitve to ACh (Prosser).  In hagfish and lampreys, the transmission of impulses from the atrium to the ventricle occurs more slowly.

(Webster, 1974, p. 54)

      The EKGs of hagfish are unlike those of other fish in displaying both a V wave (the depolarization of the sinus venosus) and a PT deflection (the repolarization of the atrium).  The EKG tracings of hagfish hearts do include P, QRS, and T waves (Davie, 1987).


A hagfish heart is pictured below.HAGFISH

     The Early Cambrian fossil fish Haikouicthyes did possess a pericardial cavity and is classified with the modern lamprey with regard to its level of complexity.

    In modern lampreys, the heart is composed of four chambers: a sinus venosus, atrium, ventricle, and truncus arteriosus (the truncus arteriosus is also referred to as the conus arteriosus or the bulbis cordis).  In lampreys, the sinus venosus collects blood from the cardinal veins, jugular vein, and the hepatic vein.  The striated cardiac muscle fibers are similar to those found in higher vertebrates, complete with intercalated disks and desmosomes (although they lack T-tubules) (Hardisty; Webster, 1974, p. 121-2).  The heart is large and may beat 25-40 times per minute.  Adult lamprey hearts possess 2 ridges in the conus which are only embryonic in other fishes (Hoar, 1970).  Lampreys possess a number of accessory hearts, including a cardinal, caudal, and hepatic portal heart (Webster, 1974, p. 55). 

     Unlike hagfish, the lamprey heart does receive nervous input from the ANS (Kardong).  In fish, amphibians, and reptiles, the sinus venosus receives this nervous input (Torrey).  In mammals, the remnant of the embryonic sinus venosus (the SA node) also is innervated although it has fused with the right atrial wall.  Although lamprey hearts, unlike those of hagfish, are sensitive to ACh, the effect of ACh is different than that observed in jawed vertebrates.  Lamprey hearts possess nicotinic cholinergic receptors which cause the heartrate to increase with exposure to ACh while all other vertebrates possess muscarinic cholinergic receptors and the heartrate decreases with exposure to ACh. (Prosser, 1973).  Chromaffin-like cells are present in the hearts of lampreys and higher vertebrates including mammals. (Webster, 1974, p. 127).

 The lamprey heart and pericardial cavity are pictured below.



          Like lampreys, jawed fish have a four chambered heart: a sinus venosus which collects the venous blood of the body, a thin-walled atrium, the muscular ventricle which pumps the blood, and the conus arteriosus which connects the heart to the ventral aorta.  In cartilaginous fish and bony fish other than the teleosts, the conus arteriosus is contractile and may possess a number of  semilunar valves (Guenter, 151).  In fish, the sinus venosus is also contractile (Hoar, 1970)

  The hearts of sharks have a greater circulatory supply than observed in lampreys, complete with anastomoses between coronary vessels (as in higher vertebrates).  Although jawed fish possess coronary circulation, the atria and the spongy trabeculae of the ventricles receive  little blood, unlike the outer layer of the ventricle. (Webster, 1974, p. 68).  Cardiac muscle cells are present in the conus arteriousus of shark hearts, unlike those of jawless fish (Webster, 1974, p. 136).  Fish not only have P, QRS, and T waves during a cardiac cycle, they can possess a V wave and a Bd complex corresponding to the electrical activity of the sinus venosus and the conus arteriosus. (Hoar, 1970).   All jawed fish have a vagal innervation of the heart and ACh reduces heart rate.  Catecholamines increase heart rate and contractibility (Hoar, 1970)

Below are images of the shark heart.

shark shark heart
shark heart
shark heart
shark heart shark heart
shark heart

    In actinoptergian fish, the conus arteriosus and ventral aorta are bulb shaped and are referred to as the bulbus arteriosus (Webster, 1974)






A perch heart is pictured below.



     In lungfish, the single atrium, the single ventricle, and the truncus arteriosus are partially divided into two chambers each (i.e. there are partial interatrial and interventricular septa).  In fact, it is not clear whether the sarcopterygian ancestors of the amphibians were more advanced in the separation of oxygenated and deoxygenated blood in their hearts than modern amphibians.  No modern amphibian group possesses the partial ventricular septum found in lungfish, although a partial septum does exist in the amphibian genus Siren.  Although frogs have two separate atria, some salamanders have an incomplete interatrial septum and the lungless salamanders possess a single atrium without a septum.  In frogs, the truncus arteriosus is subdivided into channels while in salamanders this division is reduced and even absent in caecilians and aquatic salamanders (Torrey).

     In lungfish and amphibians, the sinus venosus empties into right atrium and the pulmonary vein empties into the left atrium ( Kardong p. 465). Thus in these groups oxygenated and deoxygenated blood is largely separated in the heart despite the absence of septa which completely divide the ventricles (Webster, 1974).  In lungfish and amphibians, the bulbis cordis bends sharply.  (Webster, 1974, p. 76)  In lungfish and amphibians, the wall of the ventricle (and to a lesser extent, that of the atria) has a greater amount of cardiac muscle and contains folds called trabeculae (Webster, 1974)


In amphibians, the two atria are usually completely separate. 






    Reptilian hearts create higher pressure and greater cardiac output than the hearts of amphibians. Reptilian hearts also separate oxygenated from deoxygenated blood more completely (Kardong 467).  In turtles, the sinus venosus is smaller than that of amphibians but it is still a separate chamber and contains the SA node.  Birds retain a small sinus venosus.  In snakes, lizards, and turtles, the ventricle is partially divided while in crocodilians this separation is virtually complete (Webster, 1974; Kardong 467).  Reptiles retain a small amount of cardiac muscle in the conus arteriosus which contracts after the contraction of the ventricles. (Webster, 1974, p. 98).  A conduction system exists in the heart of amniotes which allows the heart to contract as separate units.  In more primitive vertebrates, the cardiac muscle contracts in a wave (Romer, 482).  Cardiac muscle in reptiles and amphibians is smaller in diameter and has less sarcoplasmic reticulum than that in mammals (Webster, 1974, p. 179). 

     In reptiles, not only is the truncus arteriosus divided into pulmonary and systemic vessels, the systemic vessel is separated into right and left vessels (the right systemic vessel being equivalent to the mammalian aorta).  The two systemic vessels may leave different ventricles in turtles and crocodiles, or the left ventricle only in lizards and snakes.  In crocodiles, the left and right atria empty separately into the left and right ventricles.  In all other reptiles, the left and right atria empty into the left ventricle, although two flaps of the interatrial valve helps to separate the two channels of blood.  The interventricular septum in crocodiles is virtually complete and is significantly different from that of other reptiles in some of its characteristics (such as its vertical position) (Torrey, 1979).