Travel back far enough in your genealogy, and you will run into a fish.
Before about 370 million years ago, our ancestors were scaly creatures that lived in the sea, swimming with fins and using gills to get oxygen from the water. And then, over the course of millions of years, they began moving ashore, adapting to the terrestrial realm. They became tetrapods, a lineage that would eventually produce today’s amphibians, reptiles, birds, and mammals. As scientists have unearthed fossils from those early days, one lesson has come through ever more loud and clear: the transition was not a single leap. Instead, it was drawn out and piecemeal.
One of the most important of these fossils came to the world’s attention in 2006. Digging in the Arctic, a team of scientists found a 370-million-year-old creature they dubbed Tiktaalik. As I wrote at the time on the Loom, Tiktaalik belonged to a lineage of aquatic vertebrates called lobefins–a group that today includes lungfish and coelacanths. A number of anatomical features set lobefins apart from other fish, and show them to be more closely related to us and other tetrapods. They generally have stout fins that contain bones corresponding to the upper bones of our arms and legs. Some fossils of lobe fins don’t just have a bone corresponding to the humerus–the long bone attached to the shoulder–but the radius and ulna, too.
But even among lobefins, Tiktaalik was remarkably tetrapod-like. It had a distinct neck, for example, and its fins had additional limb-like bones. Along with bones corresponding to a humerus, radius, and ulna, it even had wrist-like bones that functioned as a joint, as they do in our hands. Without digits, Tiktaalik couldn’t grasp a branch with its fins. But it could do a decent push-up in the muddy shallows that it called home in the Devonian Period. (Neil Shubin, one of the discoverers of Tiktaalik, told the creature’s story in his 2009 book Your Inner Fish.)
The bones that Shubin and his colleagues described in 2006 came from the front half of Tiktaalik. Only now, eight years later, have Shubin and his colleagues unveiled the other half of this remarkable beast. And they’ve now stretched out the transition from fish to tetrapod even more.
An eight-year delay is hardly unheard of in the world of paleontology. Unearthing and analyzing fossils is a very slow business. When Shubin and his colleagues first discovered Tiktaalik in the Arctic in 2004, they didn’t try to extract the bones then and there. Instead, they hacked out a three-foot wide hunk of rock that contained the fossils, which they would then bring back to the University of Chicago. There, they could carefully extract the fossils in the comfort of a lab.
But before they could put the rock on a helicopter to start the journey home, they had to protect it by covering it in plaster. Unfortunately, they hadn’t expected to end up with such a massive boulder. Its immense weight would require a thick plaster jacket, and they didn’t have enough plaster at their camp for the job. Instead, Shubin and his fellow paleontologists realized, they’d need to split the rock in two and wrap the two hunks in thinner jackets.
The two rocks made it safely back to Chicago. The scientists began work on the rock that contained the skull and other bones from the front half of Tiktaalik. By the time they were done, they had isolated bones from three different individuals in the rock. Once they had analyzed the bones and written up detailed descriptions of Tiktaalik’s anatomy, they turned their attention in 2008 to the other rock, which had been sitting untouched for four years.
Chipping away, they started to come across bones. Some were fin rays from the pelvic fin. Some were ribs from the back half of the animal. And nestled in the rock was an especially valuable bone: a pelvis.
It was not what Shubin and his colleagues were expecting. The closest lobe-fin relatives of tetrapods had tiny pelvises, which only served to attach muscles that controlled the pelvic fin during swimming. Tiktaalik had a massive pelvis–as big as those of the earliest true tetrapods with legs and digits. And like us, it also had a massive scoop carved out of the side, where the ball of the femur could fit.
The discovery prompted Shubin and his colleagues to look back at the thousands of other fossil fragments they had found at the Tiktaalik site over the years, many of which remained puzzling to them. They compared the new Tiktaalik bone to those unclassified fossils and found that they had unwittingly found five other Tiktaalik pelvises. Until they knew what a Tiktaalik pelvis actually looked like, they didn’t know what they had.
All those hip bones have brought Tiktaalik into sharper focus. For one thing, they show that the creature could get big. The largest pelvis bones they’ve found suggest that Tiktaalik could grow up to nine feet long. Our ancient relatives, in other words, were the size of alligators.
Not only was its pelvis big, but its pelvic fin was big, too. Shubin and his colleagues envision Tiktaalik using massive muscles anchored to its pelvis to power its hind fins–not just to swim, but to walk underwater or push its way across muddy flats.
While Tiktaalik had hips that were tetrapod-like in size, they were still fish-like in anatomy. Our own hips are tightly fused to our spine. It would be catastrophic for them to be floating free in our bodies, because we wouldn’t be able to hold up our torsos against the force of gravity, nor could we transmit much of the force generated by our legs to the rest of our body. That is true of most other tetrapods, all of which are adapted for moving on dry land rather than being supported by water. By 360 million years ago, early tetrapods had evolved attachments from the pelvis to the spine.
But their forerunner Tiktaalik still had free-floating hips. IN other words, Tiktaalik shows that 370 million years ago the tetrapod body plan was still very much a work in progress–from head to tail.
(For a pre-Titaalik history of this research, see my book At the Water’s Edge.)
Reference: Neil H. Shubina, Edward B. Daeschler, and Farish A. Jenkins, Jr., “Pelvic girdle and fin of Tiktaalik roseae,” Proceedings of the National Academy of Sciences. 2014. http://www.pnas.org/cgi/doi/10.1073/pnas.1322559111