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The Vertebrate Animal Heart: Unevolvable, whether Primitive or Complex “We conclude that there is a design in the evolution of the venous connections of the heart, pectinate muscles, atrioventricular valves,’ left ventricular tendons, outflow tracts, and great arteries. One neglected aspect in the study of evolution is that of anticipation. Nayak, Raveen Rajasingh, “Evolution of the Ventricles,” Texas Heart Institute Journal, Vol. Before we get going, here are some exciting heart facts to start your day with: In case your momma never told you, hearts have 2 types of chambers: atria and ventricles. Atria are where blood enters the heart and ventricles pump the blood out of the heart. Given all this, there are 3 basic ways to make a heart found in animals: a 2 chambered heart, a 3 chambered heart, and a 4 chambered heart. Fish have 2 chambers, one atrium and one ventricle.
Amphibians and reptiles have 3 chambers: 2 atria and a ventricle. Hearts are very complex–they’re not just a bunch of random arteries and veins connecting tissue. Fish hearts simply draw in deoxygenated blood in a single atrium, and pump it out through a ventricle. This system is termed “single circulation”, as blood enters the heart, gets pumped through the gills and out to the body, Blood pressure is low for oxygenated blood leaving the gills. Double circulation” has an interior circuit within the heart–blood enters the heart, leaves the heart and gets oxygenated, enters the heart again, and then gets pumped out to the body.
Because “Double circulation” allows oxygenated blood to be pumped back into the heart before going out to the body, it pumps blood with much more pressure and much more vigorously than “single circulation”. Though the 4 chambered heart has 2 atrium-ventricle pairs, both pairs do not do the same thing. Oxygen rich blood just leaving the lungs is pumped back into the second atria. The 4 chambered heart differs from the 3 chambered heart in that it keeps oxygenated blood completely separate from de-oxygnated blood, because there is one ventricle for deoxgynated blood and one for oxygenated blood. In the 3 chambered heart, a single ventricle pumps both out of the heart, and there is some mixing between fresh and old blood. The 2 ventricle-4 chamber heart prevents mixing allows the blood leaving the heart to have far more oxygen than it would otherwise.
Getting a heart, period: Evolving a 2 chambered heart in the first place is very difficult because the circulatory system is irreducibly complex. Even if the subsystems of the circulatory system could be evolved on their own, evolving a freestanding 2 chambered heart on its own would be very difficult, even if it didn’t need other components of the circulatory system to be useful. A human heart is an 11 ounce pumping machine the size of a human fist which beats over 2 billion times and pumps over 100 million gallons of blood over the course of the average human lifetime. Getting a 3 chambered heart from a 2 chambered heart: Given a 2 chambered heart, experts do not know when, how, or in what lineage the alleged transition from the 2 chamber fish heart to the 3 chambered amphibian heart took place, mainly because this is a very difficult transition to even imagine. A 3 chambered heart has “double circulation” and is irreducibly complex with respect to “double circulation”. A vastly oversimplified explanation of the evolution of the heart might be to say that by simply duplicating the some or all of the chambers of a 2 chambered heart, one could easily evolve a functional 3 or 4 chambered heart.
3 chamber heart transition requires much more than the duplication of an atrium, because the interior circuit causing the “double circulation” of the 3 chambered heart must also be created. Duplicating the atrium without a closed circulatory network for “double circulation” would cause the heart to suck nothing but interstitial fluid out of the body. Double circulation” only works when there is a loop feeding from a ventricle back to the heart, and back to a ventricle. And if this new loop doesn’t connect with the gas exchange organ, then the new loop is functionless and useless. Finally, the heart muscle has to adapt to all of these changes, especially such that beating can occur to pump through the new atrium and associated fluid pressure changes. The duplication of the atrium such that fluid transport through new atrium is functional A conversion of the vein leaving the heart into an artery at the other end such that it is fed back into the heart. If any of these steps are missing, double circulation won’t work.
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And this says nothing about the many valves and other smaller veins and arteries associated with double circulation which characterize true hearts as well changes needed in the pumping mechanism of the heart muscle to accommodate a completely new atrium and fluid-pressure balance. The transition from 2 to 3 chambers requires a change from single to double circulation which involves at least 4 major simultaneous changes including the complete rewiring of how blood leaves the heart to the rest of the body. Many more minor simultaneous changes associated with mechanics of proper fluid transport would also be necessary. Getting a 4 chambered heart: With respect to hearts with “double circulation”, the 4 chamber mammalian heart probably isn’t irreducibly complex.
Basically, the single ventricle in the 3-chambered heart is split into 2 chambers in the 4 chambered heart, making 2 ventricles instead of one. The human heart has 2 atrium-ventricle pairs, which beat in succession something like pistons in a car. Only one ventricle is really needed to pump the blood. There is a huge advantage to having a 4 chamber heart. Irreducible complexity is a real phenomena, and it can be analyzed, and so in some cases it might not exist, and in the case of the 4-chambered heart case it probably doesn’t. Though the 4 chambered heart may not be irreducibly complex with respect to “double circulation”, it might still be the result of intelligent design and not evolution, and irreducible complexity doesn’t have to exist in all instances for it to exist in some.