Some three hundred and eighty million years ago, the first creatures attempting to haul themselves toward dry ground carried necks that, for all their promise, hardly let them turn their heads. A team working across Penn State, Uppsala, the University of Washington, Ottawa and Carleton, and reporting in Nature, has overturned a long-held story that necks arose so early land-bound animals could look about them. Their models, built with help from a living sturgeon and the fossil Tiktaalik, suggest the neck was a side effect of limbs learning to carry weight, and that mobility arrived only later,
Early Necks Were Stiff: Fossil and Sturgeon Study Overturns Idea That Necks Evolved for Head Mobility
The first necks may have been built for walking, not for looking
Mark
So the headline is that early necks were stiff. Why does that matter?
Mimi
Because the standard story said necks evolved so animals could turn their heads. If the first necks barely allowed that, the reason they appeared has to be something else.
Mark
And the something else is walking?
Mimi
The proposal is that separating skull from shoulders came with changes that let the front limbs support and move the body. Flexibility came later.
Mark
How do you measure the stiffness of an animal that's 380 million years old?
Mimi
You borrow. Sturgeon have a similar notochord-based backbone, so the team measured those and fed the numbers into models of fossils like Tiktaalik.
Luke
Worth saying plainly: that's a model built on a distant relative, not a measurement of the fossils themselves. Sturgeon are only distantly related.
Mimi
Fair, and the researchers say as much. Still, the sturgeon backbone didn't seem terribly stiff, which made the result more surprising to them.
Luke
And the walking explanation? Is that demonstrated?
Mimi
It's what the findings point to. The source describes it as something the neck "may have" been, so it's a proposal, not a proven cause.
Mark
Could the stiffness be overestimated?
Luke
The team says the opposite risk. Muscles, skin and bones in the skin weren't modeled, and they could make necks stiffer still. How much, nobody knows yet.
The Pulse
- A decades-old explanation that necks evolved for head turning now faces a direct challenge from fossil-based modeling.
- Early tetrapods with separated skulls and shoulders appear to have gained almost no room to lift or twist their heads.
- Researchers borrowed stiffness measurements from living sturgeon and fed them into models of Tiktaalik and its kin.
- The team proposes that neck formation followed limb-supporting skeletal changes, with flexibility becoming valuable only afterward.
- The conclusions are considered conservative, since muscles, skin and bony plates could have stiffened the neck further, and future fossil and biomechanical work will test the reframing.
Some three hundred and eighty million years ago, the first creatures attempting to haul themselves toward dry ground carried necks that, for all their promise, hardly let them turn their heads. A team working across Penn State, Uppsala, the University of Washington, Ottawa and Carleton, and reporting in Nature, has overturned a long-held story that necks arose so early land-bound animals could look about them. Their models, built with help from a living sturgeon and the fossil Tiktaalik, suggest the neck was a side effect of limbs learning to carry weight, and that mobility arrived only later, once it could be put to use.
The first animals that crawled toward land about 380 million years ago had necks, but those necks barely let them move their heads. An international team reported this in Nature on October 14, and the finding unsettles an explanation that has held for decades: that necks evolved so these creatures could look down, turn, and track prey or threats without moving their whole bodies. The old reasoning was intuitive. Modern fish have no distinct neck, since their shoulders join straight to the skull, so separating the two seemed like an obvious route to freedom of movement.
Testing that idea proved difficult. The fish in this transition had backbones made mostly of soft tissue, which seldom fossilizes, dominated by a flexible notochord rather than a modern vertebral column. Tom Stewart of Penn State, a team member, explained that this made it hard to know how far such a neck could bend. The researchers turned to the sturgeon, a living fish sometimes called a living fossil. Though only distantly related, it shares a similar notochord structure. They measured its stiffness and built those values into models of fossil animals, including Tiktaalik, first described in 2006 from Ellesmere Island in the Canadian Arctic.
The results surprised the team. Even with head and shoulders anatomically separated, the backbone sharply limited movement. Dana Korneisel, the paper's first author, had set out to measure how much the new anatomy mattered, and found that head elevation was no better than in fish and sometimes worse. Shaking the head, as we do, was not feasible for any modeled animal. The estimates may even be generous, since muscles, skin and bony plates in some species could have stiffened the neck further.
If the neck did not serve the head, the team suggests it came from changes that let the front limbs support and move the body. The neck was a by-product of locomotion rather than an adaptation for sight. Tetrapods later gained real neck mobility, and that clearly proved advantageous, but the earliest ones did not have it. Stewart stressed that the method mattered as much as the result: linking fossils, living animals and mechanical analysis to infer what anatomy meant for behavior. Further fossil and biomechanical work will determine how well this reframing holds.
The animals that first crawled toward land roughly 380 million years ago had necks, but those necks barely let them move their heads. That is the finding of an international team whose paper appeared October 14 in the journal Nature, and it cuts against an explanation that has stood for decades: that necks evolved so early four-limbed animals could look down, turn side to side, and track prey and threats without shifting their whole bodies.
The logic of the old idea was easy to follow. Modern fish generally have no distinct neck, because their shoulder structures connect straight to the skull. To look elsewhere, a fish turns its body. Separating head from shoulders seemed an obvious step toward freedom of movement for an animal working its way into shallow water and then onto land.
Testing it was another matter. Tom Stewart, an assistant professor of biology at Penn State and a member of the team, explained that the fish going through these changes had a vertebral column made largely of soft tissue, which rarely fossilizes. Their backbones were dominated by a notochord, a long, flexible rod of tissue running through the column, with small vertebrae arranged around a much larger tube of soft tissue. That arrangement handles forces differently from a modern backbone, so researchers could not easily say how far such a neck could bend.
The team found an analogue in the sturgeon, a living fish sometimes called a living fossil. Sturgeon are only distantly related to the ancient animals, but their backbones have a similar notochord structure. The researchers measured the stiffness and mechanical properties of sturgeon notochords, then built those values into models of fossil animals, including early tetrapods such as Tiktaalik. Tiktaalik was first described in 2006 from remains found on Ellesmere Island in Nunavut, in the Canadian Arctic. It is one of the closest relatives of limbed vertebrates, and it helps show intermediate steps such as the move from suction feeding in water to biting on land, and from swimming to walking.
The models showed that even with head and shoulders anatomically separated, the backbone would have sharply limited movement. Dana Korneisel, a postdoctoral researcher at Uppsala University in Sweden and first author of the paper, said she set out to learn how big a difference the new anatomy made. Sturgeon backbones are built much like those of early tetrapods but do not seem terribly stiff, so the near-absence of head mobility in almost all the animals modeled was a huge surprise. Head elevation was no greater than in fish and sometimes less. The kind of head twisting we do when shaking our heads was not feasible in any of them.
The researchers added that their estimates may be generous. The models considered only the backbone, while muscles, skin, and even bones embedded in the skin of some early tetrapods could have stiffened the neck further.
So what were the first necks for? The team's proposal is that separation of skull and shoulders arose alongside skeletal changes that let the front limbs support and move the body. On that view the neck was a by-product of changes for locomotion, not an adaptation for vision or head movement. Korneisel noted that tetrapods did eventually gain greater neck mobility and that it clearly became advantageous, but it was not an advantage available to the earliest ones given their anatomy.
Stewart pointed to the method as much as the result: combining fossils, living animals, and a mechanical, functional view to infer what anatomical change meant for behavior. Backbones dominated by a notochord are uncommon today but were widespread through much of Earth's history, so living animals that retain them can help reconstruct how extinct ones moved.
The team also included Cassandra M. Donatelli of the University of Washington; Linfang Han, Marcos Martinez, Tetsuto Miyashita and Emily M. Standen of the University of Ottawa; and Andrew L. Traynor, Benjamin K. A. Otoo and Hillary C. Maddin of Carleton University. Funding came from the Society of Vertebrate Paleontology's Steven Cohen Award for Student Research and the Natural Sciences and Engineering Research Council of Canada. Further fossil and biomechanical work will test how far this reframing holds.
Notable Quotes
Head elevation in these animals was no more than what you would expect in fish and, sometimes, even less.— Dana Korneisel, Uppsala University, first author
Neck mobility was eventually advantageous, but not available to early tetrapods because of their anatomy.— Dana Korneisel (paraphrased)
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