How many millons of years ago could man have existed?

Question

I'm always hearing about how the Earths atmosphere was so different a billion years ago. Ok, so when was there enough oxygen and low enough quantities of poisonous gases that some guy could sit in a lawn chair, relax and read a magazine... and not die? How many millions or billions of years ago?

Answer 1

The correct answer is approximately 50 million years ago, when

"a sharp rise in oxygen about 50 million years ago gave mammals the evolutionary boost they needed to dominate the planet."  The Science Daily article is referenced below.

I realize mammals were here before 50 ma i.e. during the mesozoic, but they were much smaller and less complex, so based on when human lungs could take in a breath of suitable air, the answer is 50 ma.

Answer 2

A good benchmark would be from the time of the earliest dinosaurs: if a reptile can survive we could too.

This would be about 300 million years ago.

Answer 3

It was probably between 1 and 2 billion years ago.  There wouldn't have been anything to eat because all life was microscopic but the air was probably suitable.  It was certainly suitable by about 400 million years ago when life stated living on land.

Answer 4

We have pond scum to thank for sufficient levels of oxygen to sustain animal life.

No, really! It was the evolution and proliferation of blue-green algae (cyanobacteria)  two billion years ago that made the difference, scientists reckon.

So the next time someone calls you Scum, remind them that they owe their very life to pond scum.