Scientists believe that life first appeared on Earth nearly four billion years ago.
The possibility of life from space. So we're working with the hypothesis that if a part of a comet landed to the Earth intact, you'd have all of the ingredients essential to life. Comets are thought to contain a wealth of organic material, probably including amino acids. And yet scientists may be on the verge of determining whether amino acids in comets could survive the fiery trip through Earth's atmosphere. I'm Joel Block for Earth and Sky.