How did life end up using only L-amino acids?
Good question.
Amino acids in meteorites (we'll talk
about this later this semester) are
racemic. (You might think about what
this, and our later chirality arguments,
imply for panspermia.)
One possibility is that the early Earth
got a lot of polarizing UV light from the
young Sun.
Circularly polarized light is kind of like
a corkscrew. The light "goes around" in one
direction only. Potentially only molecules
with a certain handedness
could interact with this light. Perhaps molecules
with that handedness are broken apart (and the other
survive), or perhaps this special form of
light causes chemical reactions that will lead
to the surviving L-amino acids.
Polarizing light could easily have arisen from
light passing through dust between the Sun
and Earth, or dust in the Earth's atmosphere.
And we know there were lots of impacts and
volcanos throwing dust into the air ....