Jennifer Glass

The big question, and a topic of active scientific debate, is whether the purple bacteria living on the ancient Earth had the ability to protect themselves from UV radiation well enough to inhabit vast expanses of the land surface, like plants do on the modern Earth, the spectral signature of the pigment would be much stronger if the pigmented bacteria covered the land surface.

However, in the deep past, before oxygen accumulated in the atmosphere, these purple bacteria might have proliferated to such an extent that they could have been detectable on the Earth's surface.