Waves wash up a beach on early Earth. It looks like a place you could stand. The chapter gives you about a minute before the lack of oxygen brings you down.
The air's exact mix is still argued about, but the lack of breath is more than a guess. Sunlight changes sulfur in an oxygen-poor sky and leaves a telltale pattern in rock. Oxygen would wipe that pattern out.
The water is harder to judge. Oxygen in chert, a rock rich in silica, once pointed to a very hot sea. Tests that added hydrogen to the picture backed cooler water. Scientists still have to make the clues agree.
Look down at the dark film on the shore. It is a mat of microbes, like a tiny lawn you could peel with a fingernail. Cells grip sand grains. Folds and wrinkles in ancient rock can preserve traces of that grip.
And the sky's color? Nobody knows for sure. Methane haze could change the view, but the rocks do not hand us a paint chart. The shore holds signs of life in the shallows. The waves can look familiar while the air above them holds almost nothing you can breathe.
