What If Ancient Life Escaped Earth?

In 7.5 billion years, the Sun will expand and engulf our planet, destroying all life as we know it. Of course, a mass extinction on Earth could happen much sooner than that. It might even be our fault. Sooner or later, humans might have to leave Earth to find a new home. But while that might sound scary, not to mention impossible, some say it's already been done before ... millions of years ago ...