What it was
Pink and violet patches spread through a salt pond. Tiny cells help color the water. Some catch light with retinal, the same small molecule that helps you see with your eyes, put to a different job.
Inside these microbes, retinal sits in a protein in the membrane, the thin skin around a cell. Light changes its shape. The whole system can pump charged particles across that skin and help the cell gather energy. It takes less kit than a leaf's full food-making machinery.
When and where
These cells live now, in salty ponds and the sunlit sea. They are no vanished cast from Earth's past. Other pigments, substances that absorb some colors of light, can help tint the ponds too. A pink pool does not tell you every molecule in it.
The purple-Earth idea asks whether retinal-based life spread widely before the green kind did. Leaves catch light mainly with chlorophyll, a green pigment. A simpler light-catching system could have appeared first. That gives scientists an idea to test, rather than a dated purple layer in the rocks.
How we know
Scientists shine light on pigments from living microbes and measure what happens. They also study cells in the sea. Retinal changes shape in animal eyes too, though it starts in a different form there. One molecule can help a cell gather energy or an eye detect light.
Still a mystery
Nobody knows if Earth ever looked purple all over. No fossil layer proves it, and genes traded between groups tangle the trail back through time. The living cells give us something solid to test today. In some salt ponds, their colors spread right across the water.