How could the adaption of valves in leg vains allow a human to better survive in its environment.

The answer is that healthy legs have veins with smooth, elastic walls that are perfectly designed to adapt to the changes in pressure within a vein. Veins have valves that keep blood moving in one direction: back toward the heart. As the leg muscles are activated, the venous valves open to allow one-way flow in the direction of the heart.

It should be at the center of any map going vertically: the prime meridian, as it passes through greenwich,  england, is considered 0 degrees longitude