# How far could you go back in time and still breath?

**URL:** <https://boards.straightdope.com/t/how-far-could-you-go-back-in-time-and-still-breath/109350>\
**Category:** Factual Questions\
**Created:** [May 16, 2002, 7:36am UTC](https://boards.straightdope.com/t/how-far-could-you-go-back-in-time-and-still-breath/109350 "2002-05-16T07:36:06Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![gorillaboy](https://avatars.discourse-cdn.com/v4/letter/g/dc4da7/32.png) [@gorillaboy](https://boards.straightdope.com/u/gorillaboy)\
**Post date:** [May 16, 2002, 7:36am UTC](https://boards.straightdope.com/t/how-far-could-you-go-back-in-time-and-still-breath/109350/1 "2002-05-16T07:36:06Z")

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I’m thinking about a new roleplaying campaign that involves time travel, and was wondering…what’s the earliest point in Earth’s history in which the partial pressure of oxygen was sufficient for unaided breathing by a human being?

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**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\
**Post date:** [May 16, 2002, 7:51am UTC](https://boards.straightdope.com/t/how-far-could-you-go-back-in-time-and-still-breath/109350/2 "2002-05-16T07:51:16Z")

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The [Venidian](http://www.ucmp.berkeley.edu/vendian/vendian.html) era, some 650 to 544 million years ago, may be your best bet. It’s the era in which animals (as opposed to plants) started to arise, so oxygen-breathing must have been possible. Anything earlier remains unproven.

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**Author:** ![Mephisto](https://avatars.discourse-cdn.com/v4/letter/m/48db29/32.png) [@Mephisto](https://boards.straightdope.com/u/Mephisto)\
**Post date:** [May 16, 2002, 10:27am UTC](https://boards.straightdope.com/t/how-far-could-you-go-back-in-time-and-still-breath/109350/3 "2002-05-16T10:27:16Z")

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I read that the gas ratio of Earth’s atmosphere has fluctuated over time, with oxygen levels going up and down. This makes me wonder if the atmosphere would be continuously safe to breathe the whole time, or if there might have been “safe” and “unsafe” periods. I think oxygen levels really dropped during the Permian Period. I couldn’t say if the “Permian Plummet” reduced oxygen levels below that required for human life, but I thought **gorillaboy** should be aware of this before one of the people in his campaign used the wayback machine for a catastrophic trip back in time.

Sorry I don’t have a cite for this oxygen fluctuation. It was in an article some scientist wrote for either _Discover_ or _Scientific American_ magazine a couple years ago. The article was about why insects today (particularly dragonflies, IIRC) are a different size than many of their prehistoric relatives. I found one or two websites that touched on this subject, but they didn’t look like very good sources so I didn’t bother providing links. I will, though, if anybody asks.

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**Author:** ![Shalmanese](https://avatars.discourse-cdn.com/v4/letter/s/45deac/32.png) [@Shalmanese](https://boards.straightdope.com/u/Shalmanese)\
**Post date:** [May 16, 2002, 10:41am UTC](https://boards.straightdope.com/t/how-far-could-you-go-back-in-time-and-still-breath/109350/4 "2002-05-16T10:41:04Z")

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Yes, I remember also an article about the fact that dragonflies used to be 3 times bigger due a massive amount of oxygen in the air. I think it was 70% but that might just be my meory tricking me.

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**Author:** ![gorillaboy](https://avatars.discourse-cdn.com/v4/letter/g/dc4da7/32.png) [@gorillaboy](https://boards.straightdope.com/u/gorillaboy)\
**Post date:** [May 16, 2002, 4:58pm UTC](https://boards.straightdope.com/t/how-far-could-you-go-back-in-time-and-still-breath/109350/5 "2002-05-16T16:58:12Z")

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> [@](#):
>
> \*Originally posted by Bryan Ekers \*  
> \*\*The [Venidian](http://www.ucmp.berkeley.edu/vendian/vendian.html) era, some 650 to 544 million years ago, may be your best bet. It’s the era in which animals (as opposed to plants) started to arise, so oxygen-breathing must have been possible. Anything earlier remains unproven. \*\*

Interesting…that link led me to [this](http://www.ucmp.berkeley.edu/precambrian/proterostrat.html), which indicated that by 1.8 billion years ago, oxygen levels were greater than 15% of present levels. Assuming that the level continued to rise more-or-less evenly, then perhaps the atmosphere would be breathable at sea level starting around 1 billion years ago.