Evolution: Path to the Human Heart Revealed

The heart has always been the most popular organ, be it big, warm, defining the current location of your home or simply pumping blood around your body.  Its lopsided lumpy shape might not live up to the pretty valentine's card image, but without it you wouldn't be living up to anything so it's an important subject of study.  Recent research has revealed another key step in the "build something that never stops for eighty years" program.

If you think the heart is just a big pump pushing blood around your body you could be a frog - a floppy amphibian whose simple three-chambered cardiac core is just one great big ventricle, flexing to flush out the blood arriving via two atria, mixing the literally vital fluid from the body and lungs and shoving it back out to both.  This means the blood is never full or empty, but instead always sorta-kinda half-full of oxygen as it courses around and remixes.

Mammals like us, on the other heart, have the optimized V10 of vital-juice-shovers.  A septum divides the ventricle into two chambers (for a total of four) meaning blood returning the body is all sent to the lungs, and all the oxygenated juice is sent back out again.  This gives us the energy (released in metabolic activities requiring oxygen) to do all sorts of things like run around, not live in ponds, and maintain constant body temperature.  (In emergencies you might metabolize anaerobically for a bit, but you'll pay for it in cramps and massively inefficient use of fuel.)

It turns out turtles (and other reptiles) have partly divided hearts - three-and-a-half chamber deals.  The septum which splits our ventricles in twain is only partly developed, steering a little more blood into the optimum paths (empty to lungs, fresh to body) but still allowing a lot of mixing.  Professor Bruneau of the Gladstone Institute of Cardiovascular Disease and colleagues have shown that transcription factor Tbx5 is the master regulator of the heart, a sort of switch which decides between the three-chambered basic or the four-chambered luxury model.

In mammals the factor is restricted to the left side of the heart, causing the divide, while cold-blooded animals have it constant across the organ.  Most interestingly, in turtles it fades across the heart, halfway between the two extremes.

Identifying this transcriptional switch is a boost in work to prevent congenital heart defects. At this point it's fun (or frustrating) to remember that even with this level of detail some insist that evolution simply doesn't happen, to which we can only reply: if an intelligent designer really built this intricate a simulacrum then he wants us to believe in evolution.  It's certainly more convincing than a book of mistranslated parables.

Luke McKinney

Turtles reveal secret of 4-chambered heart