How ?It’s because whether you evolved in the savannas of Africa or the woods of North America, the sun was always in the same place: overhead. That means more light comes from above than below. True, light gets reflected back up from the ground, but not as much as comes down from the sky. More light lands on a four-footed animal’s back than on its belly, making the back brighter and the belly darker. That’s no good for hiding. Think of it this way. If you are in a green forest, you might hide yourself by wearing green clothes. Then you’d blend in. But if your pants are dark green and your shirt bright green, you will once again stand out. By having a pale belly and a dark back, animals balance out the amount of light that reflects off their bodies. The dark back absorbs more, the pale belly reflects more. The contrast between top and bottom is diminished, and it is easier to blend in.