There’s a reason we’ve sent a rover called "Curiosity" up to Mars: Astronomers want to prove the theory that microorganisms on an asteroid from Mars that crashed into the Earth billions of years ago may have started life here. We do know that fragments from distant planets might have been the "sprouts of life" on this one. Mars may now be dead (NOTE: Subscribers can still listen to this show), but we may live on as the progeny of that planet.
Under certain conditions there is a high probability that life came to Earth during the solar system’s infancy when Earth and its planetary neighbors orbiting other stars would have been close enough to each other to exchange lots of solid material. Microbes may have spread from Earth to OTHER planets as well–could this be an explanation for Visitor experiences? (NOTE: Subscribers can still listen to this incredible interview).
"Lithopanspermia" is the idea that basic life forms are distributed throughout the universe via meteorite-like planetary fragments cast forth by disruptions such as volcanic eruptions and collisions with other matter. Eventually, another planetary system’s gravity traps these roaming rocks, which can result in a mingling that transfers any living cargo. A slow-moving planetary fragment meanders into the outer edge of the gravitational pull, or weak stability boundary, of a planetary system. The system has only a loose grip on the fragment, meaning the fragment can escape and be propelled into space, drifting until it is pulled in by another planetary system. Thus a single fragment from a specific planet can populate MANY different planets, which may account for the fact that people see "Visitors" who appear to be human.