Evolution of heart-diaphragm relationships in primates

In mammalian quadrupeds the heart is separated from the diaphragm by a cardiac lobe of the right lung, allowing the diaphragm and heart to move independently; in the presence of this lobe the diaphragm on a PA chest x-ray has a single dome. In humans there is no cardiac lobe and the diaphragm shows a double dome corresponding to the two hemidiaphragms; the pericardium is firmly attached to the central tendon and as the diaphragm moves the heart must follow. When and why did the cardiac lobe disappear in the evolution of primates? We compared chest x-rays in dogs, pigs, monkeys, great apes and humans. In quadrupeds there is little interaction between heart and diaphragm. In monkeys the cardiac lobe is smaller and a phrenico-pericardial ligament tethers the diaphragm ventrally in the upright position. There is no double hump in monkeys. In great apes the cardiac lobe is absent; the pericardium broadly attaches to the central tendon; a diaphragmatic double hump appears as in humans and tethering of the diaphragm appears more substantial in both ventral and mid-lateral regions. Based on the time that great apes and old world monkeys branched from a common ancestor, the cardiac lobe was lost in higher primates between 15 and 30 million years ago. The two humps of the diaphragm seen on chest xray in great apes and humans probably results from weight of the heart applied directly to the diaphragm. A selective benefit of the progressive loss of the cardiac lobe from quadrupedal to bipedal mammals may arise from (1) elimination of a cardiac lobe which in the upright position would tend to be compressed by the heart and (2) pericardial tethering of the diaphragm which in the upright position would maintain a higher station and enhanced pump function of the diaphragm.