Human Evolution
The narrative of human evolution has moved on from a gene-centric view as more is understood of the role of external forces on organismal morphology, however an underlying mechanism to describe the transition from fish to man remains wanting. A blood-based model has been proposed and describes how a double looped human circulation can evolve from an oxygenated shunt that sets up a spiralled loop-in-loop in a single-looped fish circuit.
The adaptive circuitry transition is related to extant vertebrate anatomy and to taxonomic evolution over the past 500 million years. The blood-based model accounts for the polarity-switching behaviour of a redoxing blood circuit streaming in the presence of an external magnetic field.
Circulation of oxygenated (red) and deoxygenated (blue) blood in vertebrates represented as a one-way, closed tubular passage positioned relative to gravity (g): a) fish circuit (inert), b) fish circuit with repellent slip stream arising from crossing-point, c) mixed reptilian circuit, d) human circuit (ligamentum arteriosum shown as a relict of the primordial gill trajectory – purple). From W. Peters. Basic Science Model of Blood as an Electron-Delivery Circuit Describes the Evolution of the Human Circulation. Human Genetics & Embryology (2017)
In the single-looped circuit, blue blood flows in a laminar manner to paired gills for oxygenation, then as red blood to paired dorsal aortae for bisymmetric distribution in each of the body segments. This is the basic fish circulation.
The eleven coloured body segments are representative of the forty somitomeres that make up the body. The orange segment has bilateral slits representing the pharyngeal-based gill arches in the cervical region.
A pharyngeal-systemic capilleric shunt allows a fraction of the oxygenated gill blood to re-circulate to the heart with the deoxygenated stream from the body.
The polar-opposite red thread spirals axially on the laminar blue stream as they run through the heart, setting up a loop-in-loop that wants to scissor apart. The red thread levers into upper gill arches and the blue into the lower arches.
Loop-in-loop development causes the entire blood circuit to process to accommodate the change in angular moment.
Proportional to the fraction of spirality replacing the laminar dorsal aortic flow, blood distribution to the somitomeres adjusts toward a pentameric disposition to retain bisymmetry in the sagittal plane. The head comes up, the neck taughtens, limbs bud and tail segments foreshorten.
This is the situation of the salamander (or axolotl etc). With scissoring, the (orange) gill arches begin to involute and insufflate as alveolae (a single slit is now present in the model). A proportion of the blue blood from the 6th arches has diverted into the newly involuted pharyngeal-alveolar bed and drains centrally to the heart (pulmonary venous drainage) and enters the common atrium, a nascent interposed pulmonary pathway established. The remainder of the blue blood continues through (6th) gill arches and to paired dorsal aortae distally.
With greater loop-in-loop development and scissoring, the strengthening, spiralled red outflow selects one fourth arch into the dorsal aorta. The other arch regresses. The interposed pulmonary circuit is fully developed and in mammals and birds the sixth arch ductus to the dorsal aorta has shut (black). In mammals the persisting aortic arch is to the left of the trachea and in birds it is to the right.
In humans, the body is fully vertically disposed (precession to 90 degrees from horizontal – aka bipedalism), the chest broad and the aortic and pulmonary valves are offset ninety degrees. Perpendicular parity and a pentameric somitomeric disposition have been achieved. In quadruped mammals the chest is deeper and narrower, and the pulmonary artery is more over the top of the aorta i.e. not so unpacked, with each species finding its own adaptive equilibrium point, or centre or redox, in circuitry distribution that fits with its habitat.
Interestingly, birds are also bipedal, but not so upright. It is hypothesised that right aortic arch selection determines a pentameric somitomeric re-distribution that biases the pectoral and cloacal regions, with a long narrow neck and less blood to the head (bird-brained!) to maintain sagittal bisymmetry, predisposing the possibility of flight.
In summary, the vertebrate blood circuit is presented as a biological electron carrier archetype that can exist along a continuum from single to double-looped disposition, an accordian of possible topologies bounded by basic forces of nature.
It is an adaptive circuit, sensitive to oxygen and maintains a center of redox and thus stability of form at any point between single and double loop states, the degree of precession and bilateral somitomeric redistribution driven by fitness with the environment.
As a corollary, the notion of the blood as a one-way electron delivery circuit posits the mitochondria as the motor and the genome as some kind of electron sink and/or regulator. Interestingly, both the rete mirabilis and the genome are vastly more prominent in lungfish/salamanders/axolotls compared to other, less transitional vertebrates. Fish and bipeds represent more efficient, less flexible single and double-looped circuits, respectively, and have much smaller genomes.