Answer: Meteoroids

First mentioned in writing all the way back in the 5th century BC by Greek philosopher Anaxagoras (although not given serious scientific form until well into the 19th century), panspermia is the hypothesis that states life exists throughout the universe and is spread by meteoroids and other objects (asteroids, comets, planetoids, and potentially by spacecraft) traveling through space.

The premise is that some organisms, known as extremophiles, could survive after a catastrophic impact shattered the planet they were living on and sent the chunks flying through space (or some such event). These organisms could land on other planets through subsequent impacts and serve as the evolutionary building blocks for more complex lifeforms.

While the hypothesis is taken seriously in the scientific community (acclaimed scientist Stephen Hawking has talked about how once we begin exploring space, we may find life that has been spread across the cosmos by panspermia), the greatest criticism for the hypothesis is not what it explains, but what it fails to explain. While it explains how life (potentially) moves around the universe, it doesn’t offer any insight into how the hitchhiking organisms came to be.