LIFE × EVOLUTION

The First Photosynthesis

~3.4 billion years ago

Archean · Paleoarchean

Photosynthesis builds living matter with the light of the Sun, but the chemistry has a cost: it must strip electrons from something. Today's plants strip them from water, the hardest source of all, and oxygen escapes in the process. The first photosynthesisers turned to easier sources, hydrogen gas or the iron dissolved in the sea, and so gave off no oxygen at all. They fed the biosphere for hundreds of millions of years without touching its methane; and those that drew on iron returned it to the sea as rust, so some of the banded iron formations may be their work.

Why it matters

It shows that harvesting sunlight is the easy part; splitting water, the step that would eventually fill the air with oxygen, is the hard one.

Dating & uncertainty

Anoxygenic photosynthesis leaves no unambiguous fossil, and its age is inferred from the branching order of the photosystems and from what the early ocean was doing chemically. That it preceded the oxygen-splitting form is agreed; by how long is not.

Sources

1. Ozaki et al. (2019), Nature Communications · Anoxygenic photosynthesis and the delayed oxygenation of Earth's atmosphere
2. Thompson et al. (2019), Science Advances · Photoferrotrophy, deposition of banded iron formations, and methane production in Archean oceans
3. Catling & Zahnle (2020), Science Advances · The Archean atmosphere

Earth history as one day

06:01:3517 h 58 min before midnight

if Earth’s 4.54 billion years were compressed into a single day