The left-sided aortic arch in humans, viewed as the end-result of natural selection during vertebrate evolution

Rt some point during vertebrate evolution from species dwelling in water to living on land, the ancestral double or right aortic arches became single and left-sided in mammals, including humans, as the result of synchronous d evelopments in cardiovascular and respiratory embryogenesis. Since left-sid ed aortic arches are unique to mammals, hemodynamics related to the placent a, specifically the requirement for a large arterial duct connecting to the descending aorta, may have led to switching from the right-sided to the le ft-sided arch. Additionally, development of a trilobar right lung and its b ronchial tree, also unique to mammalian evolution, restricted the space abo ve the high eparterial bronchus to a single large vessel. Consequently, mam mals that mutated to the left-sided aortic arch avoided respiratory, digest ive or circulatory problems that are often associated with an isolated righ t-sided aortic arch - something which could be considered a successful mist ake. Due to natural selection, and survival of the fittest, the left-sided arch became the norm ill mammals. In congenital cardiac malformations where a large arterial duct is not mand atory in fetal life, as in Fallot's tetralogy or common arterial trunk, a r ight-sided aortic arch continues to occur, perhaps as an atavistic reversio n to the anatomy seen in ancestral vertebrates.