Our left and correct sides are largely mirror images of every other. That makes us bilaterians, a massive group of animals that range from tiny flies to giant whales. But we and the whales share a significant difference with flies—where we place our nerve cord. For us, the spinal cord runs down on our back. For flies and other insects, the principal nerve cord is on the underside of the animal.
This would not be that surprising if the two nerve cords had distinct origins, but we’ve found that all the molecules that spot and pattern the nerve cord are related in insects and vertebrates. That suggests the nerve cord has constantly been in the exact same location rather, the rest of the body somehow ended up flipped relative to the nerves. Now, the genome of a strange animal called the acorn worm may support us sort out how that could have happened.
One feasible hypothesis for how the nerve cord flipped is that the ancestor of us vertebrates lived as a burrowing animal in a marine environment. There, such an ancestor might not want any prime/bottom axis and could have lost it. Once its descendants re-emerged from the mud, they re-established this axis but did it in the opposite orientation.