October 31, 2014

Why Earth's animal life forms took so long to appear

It's a question that bugged scientists for a very long time: why did it take so long for animal life to appear on Earth? Was life so fragile? Did it necessarily take several billions of years to come into existence? And what, finally, did it take for animals to emerge via evolutionary processes?

Well, the answer is in: there wasn't enough oxygen on Earth prior to the time when animals first appeared (about 600-700 million years ago). They literally could not have existed prior to the moment when they first appeared in the fossil record.

Scientists have long believed that the oxygen level in the billion years that preceded the appearance of animal life on Earth was approximately 40% of the level we see today. You'd think some animals would pop up, given this relative abundance of oxygen. But a new experimental result shows that in the billion years that preceded animal life on Earth, there was only 0.01% of the oxygen we see today. That's the reason animal life held off for so long.

Six hundred to seven hundred million years ago, when there was finally enough oxygen for animals to exist and thrive, they appeared everywhere -- and some eventually evolved into us. Mystery solved. Pretty cool, huh?

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