>A hypothetical human time traveler, however, wouldn't survive a hypothetical time travel back to [the Cambrian explosion].

I assume you are referencing about CO2 levels. Yes, around ~500 million years ago, CO2 levels reached 10,000ppm, which would be hazardous (but not fatal) to a human traveler. However if you pick a point between 300 million years ago and today, when CO2 levels were at 1000-2000ppm, a human traveler would have no problem surviving in that environment. Heck, it is even possible to hit those CO2 levels in a modern day warehouse or office that is poorly ventilated.

Increased CO2 levels are very survivable and do not mean the end of the human race.