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Let's imagine that a human (male or female) could be sent to the past, anytime after the birth of our planet. What would be their survival probability over time?

If this question is too hard to answer, I would also be interested in the first time (most distant) that a human could have survived their lifetime on Earth.

Assume that this human is someone like Bear Grylls, who is well trained to survive in nature.

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    $\begingroup$ Bear Grylls would have lasted 23 seconds in the Jurassic: about the time it would have taken an Allosaur to assess, chase, kill, and swallow him. $\endgroup$
    – Knob Scratcher
    Feb 16, 2017 at 19:56

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One needs breathable air, drinkable water, and food to survive. Food as we know it was nonexistent prior to the Ediacaran, which began about 635 million years ago. Life was exceedingly primitive (single celled) prior to that. Breathable air is a more significant challenge. A breathable atmosphere means sufficient oxygen and not too much carbon dioxide. Oxygen levels rose to above half it's current amount in the atmosphere about 850 million years ago, but carbon dioxide didn't fall below 5000 ppm until about 450 million years ago, when it dropped precipitously and caused an ice age and an extinction event. Extinction events would be something to avoid, so about 440 million years ago.

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  • $\begingroup$ Would 5000 ppm (0.5%) be hard to breathe? $\endgroup$
    – gerrit ♦
    Feb 16, 2017 at 10:06
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    $\begingroup$ @gerrit According to wikipedia, with that concentration of CO2 you might suffer of headaches, lethargy, mental slowness, emotional irritation, and sleep disruption. en.wikipedia.org/wiki/Carbon_dioxide#Toxicity $\endgroup$
    – Santiago
    Feb 16, 2017 at 12:38
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    $\begingroup$ @Santiago Sounds survivable, at least temporarily. $\endgroup$
    – gerrit ♦
    Feb 16, 2017 at 12:41
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    $\begingroup$ "Food as we know it was nonexistent prior to the Ediacaran, which began about 635 million years ago". Food, as we know it, didn't arrive until well after the appearance of land plants in the Ordivician. Until perhaps the Permian, you'd have been limited to foraging the seashore for food, and shelter would have been caves, or digging holes underground. $\endgroup$
    – Knob Scratcher
    Feb 16, 2017 at 19:50
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    $\begingroup$ @KnobScratcher -- This is about what a Bear Grylls would think is edible. $\endgroup$
    – David Hammen
    Feb 16, 2017 at 20:34

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