article_snippet.sent: Life on Earth developed in Life on Earth oceans about a billion years after a planet formed.
article_snippet.sent: Of course, there is a difference between single-celled organisms -- which developed 3.8 billion years ago and remained the most sophisticated form of life for another billion years or so -- and mammals, which appeared about 200 million years ago.
article_snippet.sent: And then the humans, who have existed for only 200,000 years.