What is luca cavalli-sforza's hypothesis for why skin color change came late?

Luca Cavalli-Sforza is a geneticist who hypothesized that the skin-color change came late because previous people had complete vitamin D access to the foods that they are eating. But the moment they started farming, the main vitamin sources are reduced, making a larger gain of having light skin.

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.

whats the drop down answers ?

explanation: