This movement, which at first just happened underwater, later helped some species make the transition from water to land.
Heather King of the University of Chicago and colleagues studied living lungfish to see how that transition might have happened.
"Lungfish are very closely related to the animals that were able to evolve and come out of the water and onto land, but that was so long ago that almost everything except the lungfish has gone extinct," she explained.
King and her team found that lungfish could, as their name suggests, blow up with air like a balloon, giving their body buoyancy. Their scrawny back paired appendages can then either sort of hop or actually walk by alternating the movement of these limbs.
Co-author Neil Shubin said, "This shows us -- pardon the pun -- the steps that are involved in the origin of walking."
Since those first steps from water to land were taken, some animals evolved four limbs for walking. Even for these animals, like dogs and cats, the limbs come in pairs. For that innovation, we can probably thank the unusual, long-extinct jaw-less fish Euphanerops.