From the source:

The meteorites did this by providing building blocks with a slight preponderance of that handedness (known scientifically as chirality) that makes life possible.

"We know that all amino acids start mirror-image the same, but in living things they have this handedness," said Ronald Breslow, a Columbia University researcher who published recently on the topic. "This change doesn't happen spontaneously, and we've never been able to reproduce it in the laboratory" under conditions similar to early Earth.

"The answer to where it comes from looks increasingly like meteorites," he added, "from extraterrestrial bodies falling to Earth. It's a complex story, but we're beginning to understand it better."