Which way’s up? Figuring out how our spinal cord ended up on our back

Proboscis to the right, collar in the middle, and freakiness throughout. (credit: John Gerhart)

Our left and right sides are largely mirror images of each other. That makes us bilaterians, a large group of animals that range from tiny flies to giant whales. But we and the whales share a significant difference with flies—where we put our nerve cord. For us, the spinal cord runs down on our back. For flies and other insects, the primary nerve cord is on the underside of the animal.

This wouldn't be that surprising if the two nerve cords had distinct origins, but we've discovered that all the molecules that place and pattern the nerve cord are similar in insects and vertebrates. That suggests the nerve cord has always been in the same place; instead, 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 help us sort out how that could have happened.

One possible 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 need any top/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.

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