Hold a leaf up to the light. Some green light bounces toward your eye, while chlorophyll, the leaf's main light-catching pigment, takes in more blue and red. A pigment is a substance that absorbs some colors more than others.
Now meet a microbe with another way to catch light. It uses retinal, a small molecule tucked into a protein in its outer skin. Light makes this tiny pump push charged particles across the skin, helping the cell gather energy.
You have retinal too. In your eyes, it helps you sense light. Some microbes use it to catch green light and look purple instead. The same molecule, put to different work.
That has led to an idea called purple Earth: perhaps retinal-using microbes spread widely before green life did. Their light-catching kit is simpler, so it could have come early.
Here is the honest answer: nobody knows if Earth ever looked purple. There is no purple fossil layer or firm date, and traded genes tangle the trail. Even the green gap needs care: leaves use some green light deeper inside. Purple microbes are here today, including in salty ponds. A purple planet is still a guess.
