<p>850 million years ago and earlier oxygen levels were below 6%, which is the  minimum for human survival. But at oxygen levels below 14% we start to be significantly impaired, and we have evolved with current levels of 20%. 14% was attained about 500 million years ago, but there have been significant low oxygen events since, notably at the start of the Jurassic 200 million years ago when levels were 60% of current concentrations.</p>

CC BY-SA 4.0
2021-10-03T13:46:37.347Z
2021-10-04T14:24:02.550Z
2021-10-04T14:24:02.550Z
Andy M
<p>Roughly 3,000,000,000 years ago Earth was a water world. This means terrestrial flora and fauna was 0%. Not much fun breathing in the atmosphere with limited photosynthesis from plants. And a nitrogen cycle that’s unrecognizable.</p>
<p>To be fair anything before the Cambrian explosion would have been a harsh, most likely inhospitable environment.</p>
<p><a href="https://www.science.org/content/article/earth-s-ancient-atmosphere-was-half-thick-it-today-rev2" rel="nofollow noreferrer">https://www.science.org/content/article/earth-s-ancient-atmosphere-was-half-thick-it-today-rev2</a></p>
<p><a href="https://www.nhm.ac.uk/discover/news/2018/february/plant-life-on-earth-is-much-older-than-we-thought.html" rel="nofollow noreferrer">https://www.nhm.ac.uk/discover/news/2018/february/plant-life-on-earth-is-much-older-than-we-thought.html</a></p>
<p><a href="https://www.nationalgeographic.org/media/age-earth/" rel="nofollow noreferrer">https://www.nationalgeographic.org/media/age-earth/</a></p>

CC BY-SA 4.0
2021-10-03T12:12:17.523Z
Thanks. I appreciate the answer.
Lord Nobody
CC BY-SA 4.0
2021-10-03T05:10:42.600Z
2021-10-03T22:17:09.513Z
2021-10-03T22:17:09.513Z
Science nerd
<p>Not counting temporary events, like mass extinctions.</p>

CC BY-SA 4.0
2021-10-04T02:03:29.457Z
The Ozone Layer didn't develop until around 600 million years ago, so Even if Oxygen availability was present about a billion years ago during the Great Oxidation event. What good is a breathable atmosphere if you die of skin cancer
LazyReader
<atmosphere><paleontology><biogeochemistry>
How far into the past would you have to go before you couldn't breathe the atmosphere?
Lord Nobody
# How far into the past would you have to go before you couldn't breathe the atmosphere?

(Asked by: username_0 on 2021-10-02)

Not counting temporary events, like mass extinctions.

**Tags:** atmosphere, paleontology, biogeochemistry

> **Comments:**
> **username_1**: The Ozone Layer didn't develop until around 600 million years ago, so Even if Oxygen availability was present about a billion years ago during the Great Oxidation event. What good is a breathable atmosphere if you die of skin cancer  

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## 2 answers

### Answer by username_2 on 2021-10-03. Score: 4

850 million years ago and earlier oxygen levels were below 6%, which is the minimum for human survival. But at oxygen levels below 14% we start to be significantly impaired, and we have evolved with current levels of 20%. 14% was attained about 500 million years ago, but there have been significant low oxygen events since, notably at the start of the Jurassic 200 million years ago when levels were 60% of current concentrations.


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### Answer by username_3 on 2021-10-03. Score: 3

Roughly 3,000,000,000 years ago Earth was a water world. This means terrestrial flora and fauna was 0%. Not much fun breathing in the atmosphere with limited photosynthesis from plants. And a nitrogen cycle that’s unrecognizable.

To be fair anything before the Cambrian explosion would have been a harsh, most likely inhospitable environment.

<https://www.username_3.org/content/article/earth-s-ancient-atmosphere-was-half-thick-it-today-rev2>

<https://www.nhm.ac.uk/discover/news/2018/february/plant-life-on-earth-is-much-older-than-we-thought.html>

<https://www.nationalgeographic.org/media/age-earth/>

> **Comments:**
> **username_0**: Thanks. I appreciate the answer.  

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