The most profound event during the 4,567 million years of Earth’s history happened when microbes learned to use light as a source of energy.
Long before us, and billions of years prior to the evolution of complex organisms altogether, microbes ruled the Earth. Even bacteria need to know where they are and where they should be headed. To get their bearings, they needed to know a direction, like towards the Sun or perhaps away from glowing magma that would fry them if they got too close. The first pigments probably evolved to allow organisms to sense the Sun, find direction and organize themselves according to this information.
When we admire the color of a frail green leaf, we see the discarded part of the spectrum of the Sun that proved useless to the plant. The blue and red light is absorbed and made useful by the pigment chlorophyll, while the junk light is reflected, analysed by our eye and brain, and perceived as the pleasant color of spring. Early life forms fueled their activities by the extremely scarce chemical energy that could be harvested from the geologic environments. This was a meager living, but with the invention of pigments, a brighter future under the sun was in store for the impoverished biosphere.
It turned out, that the energy from light absorbed by pigments could be put to work. Light energy could be used directly in the photosynthesis of sugar, and sugar could be used as a source of energy to perform work. This new source of energy from sunlight suddenly supplied the biosphere with an abundance of free energy to spend on proliferation and increased biologic activity. Microbes started to work, and their work changed Earth’s environments. The present atmosphere is a completely synthetic product manufactured and maintained by an endless number of minute living cells, and there are good reasons to believe that the continents, the land on which we live, also formed as a consequence of the work performed by microbes.
Since life emerged on Earth more than four thousand million years ago, microbes with the ability to exploit light energy to fuel their work, have taken on the job of constructing a world, which we now share. Our present world is built and maintained by the energy supplied by light from the Sun. Everything we do is supported by solar energy.
Professor of Geology, Natural History Museum of Denmark, Nordic Center for Earth Evolution, Copenhagen, Denmark