How vertebrates evolved from sea to land

A new study has suggested that vertebrate locomotion has evolved from the simple left-right bending of the body exemplified by lampreys to the appearance of fins in bony fish to the movement of humans, with the complex nerve and muscle coordination necessary to move four limbs.

Neuroscientists Martha W. Bagnall and David L. McLean from Northwestern University have found that the spinal cord circuits that produce body bending in swimming fish are more complicated than previously thought.

Bagnall and McLean report that differential control of an animal's musculature- the basic template for controlling more complex limbs- is already in place in the spinal networks of simple fish. Neural circuits in zebrafish are completely segregated: individual neurons map to specific muscles.

Specifically, the neural circuits that drive muscle movement on the dorsal (or back) side are separate from the neural circuits activating muscles on the ventral (or front) side. This is in addition to the fish being able to separately control the left and right sides of its body.

"Evolution builds on pre-existing patterns, and this is a critical piece of the puzzle," McLean said. "Our data help clarify how the transition from water to land could have been accomplished by simple changes in the connections of spinal networks."

The researchers said separate control of dorsal and ventral muscles in the fish body is a possible predecessor to separate control of extensors and flexors in human limbs.

The findings will be published in the journal Science.