The Ediacaran community (PDF), at first glance, appears to be about as close as we’re going to get for a while to extraterrestrial life, because, as far as can be told, it was totally wiped out at some point for reasons that remain controversial. None of the Ediacarans have left any obvious, or even non-obvious descendants; it’s as if a giant reset button was hit on macroscopic life. Did the Cambrian organisms arise and displace (or even just eat) the Ediacaran ones? Did ocean conditions change too much or too quickly for them? All of the above?