 The second thing researchers found in trying to understand what makes some people's faces more attractive than others is that in addition to finding men and women prefer a tint of yellow in Caucasian faces, to enhance healthy appearance, people also increased skin redness, the proverbial rosy glow in both men and women. This makes sense evolutionarily. Increased skin vascularization increases skin pinkness and is associated with increased physical fitness, but is impaired in patients with type 2 diabetes, hypertension. Increased blood oxygenation, associated with increased aerobic fitness, increases blood redness. Skin blood deoxygenation is associated with cardiac and respiratory illness, and causes a dull blue tint to the skin. So it makes sense that we should prefer mates with a rosy glow. This may explain why we lost the bone in our penis. There are certain genetic diseases that affect 100% of the human population, and one extending to 100% of human males is the congenital lack of a baculum, or a penile bone, whereas most mammals and most other primates have a bone in their penis, including all the old world primates, all the great apes, including our ancestors. But over the last six or seven million years, we lost it. Maybe, thinks evolutionary biologist Richard Dawkins, due to sexual selection by females looking for an honest advertisement of good health in prospective mates. Since human males lack this bone and must rely solely on fluid hydraulics to maintain erections, erection failure is a sensitive early warning of cardiovascular disease. It is not implausible that, with natural selection refining their diagnostic skills, females could glean all sorts of clues about a male's health and the robustness of his ability to cope with stress from the tone and bearing of his penis. Without a bone, then only genuinely healthier strong males could present a really stiff erection, and the females could make an unobstructed diagnosis. Yet another reason to eat a heart-healthy diet.