Earth & Deep Time

Earth is 4.54 billion years old, and for most of that time it was uninhabitable to anything that breathes oxygen. The atmosphere we depend on did not exist for the first two billion years of the planet's history. It then took another billion and a half years to stabilize at the levels modern human metabolism requires. And a 2021 paper in Nature Geoscience calculated, with uncomfortable precision, that the oxygen window is closing — in roughly 1.08 billion years, photosynthesis will collapse and the atmosphere will return to the methane-dominated chemistry of the ancient Archaean. We are living near the end of the breathable era, not the beginning.

The intervening four and a half billion years contain at least five mass extinction events. The worst is not the one that killed the dinosaurs. The Permian-Triassic extinction of 252 million years ago — the Great Dying — erased 96 percent of marine species and 70 percent of terrestrial vertebrates. The cause was not an asteroid. It was the Siberian Traps, a slow-motion volcanic eruption that lasted two million years, released enough carbon dioxide to acidify the oceans, and crashed the global oxygen supply. The closest geological analog to what is happening now is the worst day in the planet's history, and it lasted longer than our entire species has been around.

The deep-time picture frames a sharper question: how far back could a hundred modern humans, armed only with stone-age tools, actually survive if dropped into Earth's past? The honest answer, based on atmospheric composition and oxygen partial pressures reconstructed from the GEOCARB and COPSE proxy models, is roughly 298 million years ago — the late Carboniferous. Anything earlier, and the air itself becomes lethal. Anything earlier still, and there is no breathable atmosphere at all.

And then there is the question of what could end us next. The list is well-catalogued. Supervolcanoes. Asteroid impacts. Gamma-ray bursts from nearby supernovae. Close stellar passes — the star Gliese 710 will pass within 0.06 light-years of the Sun in roughly 1.3 million years, almost certainly perturbing the Oort Cloud and sending a wave of long-period comets sunward. The articles below cover each entry on the inventory, with the timescales attached.