Why is the sky blue? Blame rocks

Early Earth’s changing landscape…and skyscape

If you traveled back in time to about 2.5 billion years ago, you wouldn’t recognize much of what you saw. The dawning, living planet back in the day sported skies of orange, shaded by an unbreathable atmosphere awash in methane gas. But through the longĀ Proterozoic Era, those skies changed from orange to blue. Usually, we give our thanks for this change to oxygen, but a recent review traces the original actor in the color change drama to rocks. Specifically, to the phosphorus in the rocks.

Phosphorus? Really?

Phosphorus is one of the critical of elements of life. It’s a primary component of a DNA or RNA building block (nucleotide = sugar, phosphate, base). It also happens to be the primary component of the “energy currency” of cells, ATP, or adenosine triphosphate, which is really a nucleotide with three phosphates on it. Many organisms use ATP, and all organisms use phosphorus in their genetic code and their RNA.

And, a major source of it is rocks. So, yes. Phosphorus, really. Read on.

Geochemical cycles and a whole lot of gas

Several independent lines of evidence have shown that oxygen levels rose in two lengthy bursts coincident with bursts of life on Earth. The first gassy increase happened between 2.5 to 2 billion years ago. Fittingly, scientists refer to this rise in atomspheric oxygen as the Great Oxidation Event. One of the effects of the increased oxygen is that rust started showing up in the geologic record. During this Great Event, the single-celled organisms that had thrived under a presumably orange sky grew larger, and mitochondria may have arisen as a result of endosymbiosis. These cellular powerhouses are, in fact, responsible for completing energy extraction from organic molecules and using the energy to build…ATP.

The second big burst of oxygen happened about 1 billion to 540 million years ago, this time coincident with the rise of multicellular organisms and culminating in the blast of diversificiation known as the Cambrian Explosion.

What does oxygen have to do with phosphorus?

The two often hang out together as phosphate, but the real connection here is about phosphate’s contribution to life’s explosions. It may be that geologic processes, such as erosion, caused a gradual but abundant release of phosphorus from rock into the Earth’s seas. With this influx of a key component for building life, the phosphorus facilitated the early Earth equivalent of enormous algal blooms.

And guess what those algae did…and still do? Photosynthesisis. And one of the main byproducts they release from all that busy light capturing and sugar building is…oxygen. That’s the phosphorus-oxygen connection.

So, if a child ever asks you why the sky is blue and you just can’t think of the answer, you can distract them by asking them, “Did you know that the sky used to be orange?”

For your consideration

Double-membrane organelles like mitochondria are thought to have arisen through a process called endosymbiosis. What is endosymbiosis, and how could it have led to the presence of organelles like mitochondria in cells?

The algal blooms described in this paper were enormous and their influence may literally have changed the color of the sky back in the day, but the oxygen buildup and phosphorus release happened over long stretches of geological time. Today, we experience algal blooms, too. Can you identify the causes of these blooms? How do these blooms affect today’s life on Earth? Do the effects seem to be beneficial or adverse?

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