Earth has been habitable for humans for about 1.5% of its existence, and comfortable for far less. Below: every interval, what the air and the food and the weather were actually doing, and how long that gives you.
The survival times on this site are reasoned scenarios, not measurements. They come from published reconstructions of the air, the climate and the food, and from what is known about how a human body fails. Nobody has run the experiment, the reconstructions themselves are disputed, and the answer would change with the date, the latitude and the weather. Every page ends with a section on which of its numbers are soft.
Pick a moment at random from the whole 4.54 billion years, with no weighting of any kind. Because the four lethal intervals are also the four longest, the odds are not close. Roll it ten times and watch the tally.
Every row below is sized by its true share of the 4.54 billion years. This is the part that never survives a textbook, where each period gets one tidy chapter regardless of length. The Quaternary (ice ages, mammoths, all of human existence) is the sliver at the bottom.
The same sixteen, reordered. Chronology and survivability track each other loosely at best: the Carboniferous beats the Jurassic by a wide margin, and the Triassic is worse than the Silurian, 200 million years older.
| # | Interval | You last | The limiting factor |
|---|
Two thresholds, crossed 130 million years apart. Consciousness fails somewhere below about 10% oxygen, and Earth stayed under that line until roughly 550 Ma. Fire needs about 16%, which does not arrive until roughly 420 Ma. Before the first line everything is a suffocation problem. Between the two the air is breathable and nothing will burn.
Breathable air arrives long before edible land. No grass until roughly 55 Ma, no fruit until the mid-Cretaceous. A diet of lean meat alone starts to fail somewhere past roughly a third of calories from protein, though that threshold is soft and depends on total energy, available fat and how long you keep it up.
Pathogens co-evolve with their hosts, so most of what is out there would probably fail on you. That is not immunity: host jumps happen, and free-living organisms in soil and warm water infect people with no co-evolution at all. What reliably kills you is an ordinary infected wound, in every interval, with no antibiotics anywhere.