An interesting theory on early photosynthetic organisms on Earth. It seems that early microbes may have used retinal to absorb solar energy, giving them a purple hue rather than a green one. Ultimately, chlorophyll was more efficient though more difficult to produce, leading to it becoming the energy source of all plants today.
It's a good thing to keep things like this in mind when searching for extra solar planets with life on them. Our sun made chlorophyll more efficient, but a star with different properties may make retinal dominant.LiveScience.com - Early Earth Was Purple, Study Suggests