Beautiful planet in space. Earth-like planet and a star. Realistic exoplanet illuminated by the sun.gettyCarl Sagan described the Earth, as imaged by Voyager1 at a distance of 6billionkilometers, as a pale blue dot. This description is a consequence of the Rayleigh scattering of sunlight in the atmosphere, in conjunction with the reflection and scattering across the expanse of the ocean. The pale blue color symbolizes the cradle of life, yet it wasnt always so.While chlorophylls are universal pigments used by plants, cyanobacteria use additional pigments called phycobilins to absorb light energy. But this pigment is not very efficient under todays environmental conditions, so why cyanobacteria evolved this trait has remained a bit of a biological mystery. A new study published by an international team of researchers led by Taro Matsuo, an astrophysicist at Nagoya University in Japan, argues that this evolutionary trend made sense 3 billion years when Earths oceans had a different color.Microscopy view of filamentous cyanobacteria. Often also labelled (incorrectly as they are not ... [+] closely related) as blue-green algae.gettyThe researchers simulated early Earth and how its primordial environment affected the color of the planets oceans.Earths early atmosphere was dominated by water vapor and carbon dioxide, forming an acid environment and accelerating weathering and erosion on the still barren land. Large amounts of iron were washed out, flooding the oceans. Depending on its ionic charge, dissolved iron causes color variations ranging from brown to grey to green. Based on the simulation, triple-charged iron was likely the most common element in solution, resulting in predominantly green oceans.In this environment the evolution of the phycobilins pigments makes sense, as the cyanobacteria were able to harvest light energy not absorbed by the green water.Earths green oceans likely lasted from around 3 billion years to 600 million years ago. Over this vast time span, cyanobacteria used the energy harvested from the sun to split water into its component hydrogen and oxygen. Oxygen finally started accumulating in the atmosphere between 2.4 billion years to 400 million years ago. The free oxygen reacted with the dissolved iron, forming ironstone formations. As iron was constantly removed from the oceans, they started to change color, finally becoming blue. The blue oceans favored algae using chlorophyll, eventually leading to the evolution of green plants.The open access study, "Archaean green-light environments drove the evolution of cyanobacterias light-harvesting system," was published in the journal nature ecology & evolution.