We are stardust, as they say.
A massive bombardment of meteorites billions of years ago could have brought in enough water and carbon dioxide to jump-start the chemistry that allowed the Earth to develop into the garden spot of our solar system. …
The incident at the heart of the study is known as the Late Heavy Bombardment, a time about 4 billion years ago, not long after the Earth was formed from the dust and debris swirling around the young sun. According to the scientists, the Late Heavy Bombardment lasted 20 million years and rained millions and millions of space rocks onto the surfaces of the Earth, moon and Mars.
Because of the Earth’s rapidly changing geology, evidence of this event was mostly lost, except for the leftover meteorites themselves. But the moon, with no atmosphere, has little erosion, allowing closer study of the bombardment period’s effects on the lunar surface. Court said that at least 6,000 craters greater than 14 miles across remain from this period. On Earth, the bombardment might have produced 22,000 of these huge craters, he said.
According to the scientists’ theory, the frictional heat of passing through the thin atmosphere that surrounded the Earth at that time would have been enough to strip the oxygen- and water-rich outer layers from the meteorites as they plunged toward the planet. That process would slowly have caused a buildup of oxygen and water in the atmosphere.
via Meteors may have brought building blocks of life to Earth billions of years ago – Los Angeles Times.
Other theories say that life didn’t have time to develop here and must have started elsewhere. One of my favorite theories is that we are aliens and that we were planted here by bacterial spores from another world. There is some surprising evidence for bacterial spores surviving in space.