It's generally thought that the evolution of complex life was a rare, once-in-4.5-billion-years event. But new research suggests that conditions were right for complex cells to evolve and die off at least once - or perhaps several times - before our lineage even got started. The reason? New evidence that there was enough oxygen in Earth's atmosphere between 2.4 and 2 billion years ago before it dropped off again suddenly. This suggests that the ingredients for complex life were present before the first fossil evidence of complex life. Jeff and Anthony discuss the uses for oxygen, and propose some reasons why it may have fluctuated. GET BONUS EPISODES, VIDEO HANGOUTS AND MORE. VISIT: http://patreon.com/wehaveconcerns Get all your sweet We Have Concerns merch by swinging by http://wehaveconcerns.com/shop Hey! If you’re enjoying the show, please take a moment to rate/review it on whatever service you use to listen. Here’s the iTunes link: http://bit.ly/wehaveconcerns And here’s the Stitcher link: http://bit.ly/stitcherwhc Jeff on Twitter: http://twitter.com/jeffcannata Anthony on Twitter: http://twitter.com/acarboni Today’s story was sent in by Alik "The Golfer" Evans: http://www.sciencealert.com/complex-life-could-have-existed-on-earth-at-least-once-before If you’ve seen a story you think belongs on the show, send it to [email protected] or leave it on the subreddit:http://reddit.com/r/wehaveconcern Support the show and get bonus episodes, videos, Discord community access and more! http://patreon.com/wehaveconcernsJeff on Blue Sky: https://bsky.app/profile/jeffcannata.bsky.socialAnthony on Blue Sky: https://bsky.app/profile/acarboni.bsky.social