The fact that Opabinia has no current
descendants on Earth indicates that species can
die out as well as arise. Life on Earth -- and
we think probably life everywhere, by extension --
must be a series of biological failures as well
as successes.
Remember that our ancestor, Pikaia,
was a relatively unimpressive critter.
Some combination of the right physical
characteristics and probably a good deal
of luck let this guy evolve into us.
Fossil evidence as well as studies of current
populations and ecosystems inspired Charles
Darwin and Stephen Jay Gould (and many
others) to develop our moderm ideas of
evolution.