The conventional wisdom about how our tetrapod ancestors invaded land (“tetrapods” are four-footed land animals that include birds, mammals, reptiles, and amphibians) was that the evolution of limbs with digits occurred about the same time as the the walking gait evolved, perhaps when a lobe-finned fish (“sarcopterygian”) like Tiktaalik began frequenting shallow waters. Those ancestors might have propped themselves up in the shallows, and eventually made forays onto land for food, creating selection pressures on both morphology and behavior to move about on the land. As this scenario goes, the typical land-animal leg with toes evolved along with the typical land-animal gait, which is an alternation of fore- and hindlimbs that push off the ground.
This scenario may have to be revised, though, in light of a new paper by Heather King and colleagues in the Proceedings of the National Academy of Sciences. What they found is that the closest living relative of tetrapods, the lungfish, seem to have a precursor of the alternating-limb gait and the ability to push off the ground, even though they don’t venture onto land. And this suggests that the common ancestor of lungfish and modern tetrapods, which lived about 400 million years ago, was “preadapted” or “exadapted” to walk. That is, that ancestor might have had its own adapted form of movement that could be co-opted for walking when its descendants invaded land. It’s a gait-first, limb-next hypothesis.
King and her colleagues, who work in the building right next to mine, studied the movement of one of the rare remaining lungfish, Protopterus annectens. It’s from Africa, and looks like this:
The lungfish were filmed from various angles as they moved about in water tanks and, surprisingly, the researchers found what for all the world looks like the precursor of a tetrapod-like gait. The lungfish “walk” along the substrate using their pelvic fins (the evolutionary source of hindlimbs) as the sole source of locomotion. (They didn’t “walk” on their front, or pectoral fins, which are the ancestors of forelimbs in tetrapods.) Moreover, they were capable of raising themselves above the bottom of the tank with their pelvic fins.
All this is shown in four movies in the paper’s supplementary material. I’ve provided the links below along with the author’s descriptions. Movies 3 and 4 (especially 4) clearly show the beast lifting itself off the substrate with its pelvic fins.
Movie 1: This movie shows the lungfish Protopterus annectens locomoting underwater in ventral view. Note that the pelvic fins alternate, and the pectoral fins do not move rhythmically. This movie corresponds to Figs. 1A and 2 A and C. Each square of the grid in this movie is 1 cm.
Movie 2: This movie shows the lungfish P. annectens locomoting underwater in ventral view. Note that the pelvic fins begin by alternating, then make a discrete transition to a synchronous gait. This movie corresponds to Figs. 1B and 2 B and D. Each square of the grid in this movie is 1 cm.
Movie 3: This movie shows the lungfish P. annectens locomoting underwater in simultaneous lateral and ventral views. In lateral view, the lifting of the body is evident, as is the range of motion of the pelvic fin, including movement in front of and above the articulation with the body. Each square of the grid in this movie is 1 cm.
Movie 4: This movie shows the lungfish P. annectens locomoting underwater in simultaneous lateral and ventral views. Here we show an example of theeffectiveness of the pelvic fins in lifting the body. Each square of the grid in this video is 1 cm.
While we don’t know whether other lungfish do this, nor whether this condition was present in the common ancestor of lungfish and tetrapods, it’s tempting to speculate that the alternating-gait was present in this ancestor and then became coopted for walking when its descendants developed sturdy limbs with digits. Or, as the authors note, “That P. annectens uses its paired appendages for substrate-associated locomotion provides evidence for this trait arising in sarcopterygians before the evolution of tetrapods, and before the evolution of digited limbs.”
This finding also casts doubt on some previous work: the discovery of fossil trackways from the Devonian—”footprints” that have been interpreted as some of the earliest evidence for tetrapod locomotion on land. Here’s one of them, the Valentia Island trackway from Ireland:
Since trackways like this lack unambiguous evidence for digits, the authors consider their status as evidence for tetrapod locomotion “now open to question.” They could have been made by lungfish/tetrapod ancestors, perhaps creatures like lungfish. Obviously, these conclusions are tentative and subject to revision when more fossils are found.
If you want an in-depth look at how these results fit into the historical context of work on the evolution of land-dwelling animals, Carl Zimmer has a nice piece at The Loom, “A long walk to land.” Zimmer, of course, has been writing about the water-land transition for a long time, most notably in his excellent book At the Water’s Edge : Fish with Fingers, Whales with Legs, and How Life Came Ashore but Then Went Back to Sea.
h/t: Matthew Cobb
King, H. M., N. H. Shubin, M. L. Coates and M. E. Hale. 2011. Behavioral evidence for the evoluton of walking and bounding before terrestriality in sarcopterygian fishes. Proc. Nat. Acad. Sci. USA. Published online, doi: 10.1073/pnas.1118669109.