A great challenge I tried to quantify (and failed) is when lightning strikes the primordial soup and produces both left hand and right-hand amino acids in equal numbers and in the same space, somehow the left and the right had to be separated and isolated such that they didn't come in contact and annihilate each other. All proteins of life on Earth comprises left-hand amino acid molecules. Since DNA didn't exist at the time to provide instructions to form a cell membrane to separate the two, I can't imagine how any non-symmetric process separated the two such that they would remain isolated long enough to become the very base building blocks of all life on Earth.