The discovery challenges a decades-old explanation for why animals developed distinct necks in the first place. Rather than evolving to improve head movement, necks may have emerged as part of broader changes to the skeleton that allowed animals to use their front limbs for movement.

To investigate this possibility, an international team of researchers combined new fossil evidence with experiments on living fish and computer models of ancient anatomy. Their findings, published October 14 in the journal Nature, offer a new perspective on one of the most important evolutionary transitions in Earth's history.

"This is a really interesting time in Earth's history where some of the first forests are forming, and there's a really radical reorganization of the Earth," said Tom Stewart, an assistant professor of biology at Penn State and a member of the research team. "So, understanding exactly when our lineage made that transition from water to land, how it was achieved, and which anatomical systems were changing is an important part of that story. What's cool is that many of the changes that were occurring in the skeleton at this time, about 380 million years ago, are still reflected in our own bodies today."

Why Scientists Thought Animals Evolved Necks

For decades, the prevailing explanation was that necks evolved to give early tetrapods, the ancestors of four-limbed vertebrates, greater control over their heads as they adapted to life outside the water.

The reasoning seemed straightforward. An animal living in shallow water or moving onto land would benefit from looking downward or turning its head from side to side to locate prey, recognize threats, and explore its surroundings. A flexible neck would allow it to do this without repositioning its entire body.

Modern fish generally lack a distinct neck because their shoulder structures connect directly to their skulls. To look in another direction, they typically have to turn their bodies instead. Separating the head from the shoulders therefore appeared to be an important step toward greater mobility.

However, determining how these ancient animals actually moved has been difficult. Much of their backbone consisted of soft tissue that rarely survives long enough to become fossilized, leaving scientists with an incomplete picture of how their necks functioned.

"The fish that were undergoing these changes had a vertebral column that was largely made up of soft tissue, which, unlike bone, is rarely preserved in the fossil record," Stewart said. "In these cases, we can look to modern animals to serve as analogues for the past, like how living birds can teach us about now extinct dinosaurs."

Ancient Fish Fossils Reveal a Turning Point in Evolution

Some of the best evidence for the transition from water to land comes from fish that lived during the Devonian Period, approximately 420 to 360 million years ago.

These ancient animals possessed an unusual combination of features. While they retained many characteristics of aquatic fish, some had stronger, more substantial fins that may have helped support their weight. Such adaptations could have allowed them to lift themselves off the bottom or push through shallow water.

At the same time, certain species began showing a clear separation between the skull and shoulders, creating the first distinct necks.

One especially important example is Tiktaalik, a fossil fish first described in 2006. Its remains were discovered on Ellesmere Island in Nunavut, Canada, in the northern Arctic. Because Tiktaalik is closely related to the earliest vertebrates with limbs, its anatomy provides valuable clues about the evolutionary changes that eventually made life on land possible.

"Tiktaalik is one of these fossil fish, and it was first described in 2006," Stewart said. "It has been reasonably assumed for decades that this separation of head and shoulders must have allowed the head to move more freely, so the hypothesis that neck mobility was an evolutionary driver for this change in the tetrapod body plan has prevailed, at least until now."

Tiktaalik also helps researchers understand other major changes that occurred during this evolutionary transition, including the shift from drawing food into the mouth through suction underwater to biting on land, as well as the gradual transformation of swimming structures into limbs used for walking.

"The materials have been really important because Tiktaalik is one of the closest relatives to limbed vertebrates," Stewart said. "It helps us to reconstruct a lot of intermediary steps in the transition to life on land, such as the move from suction feeding in water to biting on land or from swimming to walking."

How Modern Sturgeon Helped Solve an Ancient Mystery

The key to testing the traditional explanation for neck evolution came from an unexpected source: the sturgeon, a modern fish sometimes described as a living fossil because it retains several ancient anatomical features.

Early tetrapods had backbones that were very different from those of most vertebrates living today. Instead of relying primarily on a series of large vertebrae connected by joints, their spinal columns contained a prominent structure called the notochord.

The notochord is a long, flexible rod of tissue that runs through the vertebral column and helps support the body. In these ancient animals, the individual vertebrae were relatively small and arranged around a much larger tube of soft tissue.

Because this arrangement responds to forces differently from a modern backbone, researchers had struggled to determine how much flexibility it would have allowed. That uncertainty prevented them from directly testing the idea that necks initially evolved to improve head movement.

Sturgeon provided an opportunity to overcome that problem. Although they are only distantly related to the ancient animals being studied, their backbones have a similar notochord structure.

The research team measured the stiffness and mechanical properties of the notochord in modern sturgeon. They then incorporated those measurements into models of fossil animals, including early tetrapods such as Tiktaalik, to estimate how far their necks could actually move.

The results were striking. Even when the head and shoulders were anatomically separated, the structure of the backbone would have severely restricted movement. The earliest necks appear to have been much less flexible than researchers had assumed.

"We know that tetrapods did eventually gain higher neck mobility," said Dana Korneisel, a postdoctoral researcher at Uppsala University in Sweden and first author of the paper. "Clearly it was advantageous eventually, but it was not an advantage available to these early tetrapods yet because of the limitations of their anatomy."

The First Necks Were Surprisingly Stiff

The researchers had expected the new models to clarify how much additional movement early necks provided. Instead, they found that most of the animals they examined would have gained little ability to reposition their heads.

"I undertook this study to find out how big of a difference this change in anatomy made for these animals," Korneisel said. "The vertebral columns of sturgeon are built a lot like those of early tetrapods, but they didn't seem terribly stiff. So, after doing the modeling, the lack of significant head mobility for almost all of these animals was a huge surprise."

The findings suggest that the first animals with distinct necks could not simply turn their heads to survey their surroundings the way many modern land animals can.

Their ability to lift their heads was also extremely limited, sometimes falling below what would be expected in fish.

"Head elevation in these animals was no more than what you would expect in fish and, sometimes, even less," Korneisel said. "We found that the kind of head twisting that we can do when we shake our heads was not feasible in any of these animals."

The researchers also cautioned that their estimates may actually overstate how much movement was possible.

Their models considered the mechanical properties of the backbone itself, but other structures would have added further restrictions. Muscles, skin, and even bones embedded within the skin of some early tetrapods may have made their necks still more rigid.

Necks May Have Evolved for Walking Before Looking

Together, the findings point to a different explanation for the origin of the neck.

Instead of first appearing because animals needed to turn their heads, the separation of the skull and shoulders may have developed alongside anatomical changes that allowed the front limbs to support and move the body.

In other words, the earliest necks may have been a consequence of adaptations for locomotion rather than an adaptation for improved vision or head movement.

Greater neck flexibility eventually evolved and became an important advantage for land vertebrates. But the new evidence suggests that this benefit came later, after the basic structure of the neck was already established.

The discovery also highlights the importance of the notochord in understanding animal evolution. Backbones dominated by this structure are uncommon among living vertebrates, but they were widespread during much of Earth's history. Studying modern animals that retain similar anatomy can therefore help scientists reconstruct how extinct creatures moved and behaved.

"We want to understand how these early tetrapods were living and how that is reflected in changes to their bodies," Stewart said. "Trying to reconstruct where an animal lived, how it moved, what its behavior was, is a really difficult thing. The cool part of this story is that it combines data from fossils and studies of living animals in a mechanical, functional perspective to reconstruct what the changes we see in the animal's anatomy mean for their behavior."

By bringing together fossil discoveries, experiments on living fish, and biomechanical modeling, the researchers have challenged a familiar explanation for an anatomical feature that remains central to the bodies of humans and many other vertebrates today.

Research Team and Funding

Along with Stewart and Korneisel, the international research team included Cassandra M. Donatelli of the University of Washington; Linfang Han, Marcos Martinez, Tetsuto Miyashita, and Emily M. Standen of the University of Ottawa in Canada; and Andrew L. Traynor, Benjamin K. A. Otoo, and Hillary C. Maddin of Carleton University in Canada.

The research received support from The Steven Cohen Award for Student Research, presented by the Society of Vertebrate Paleontology, and the Natural Sciences and Engineering Research Council of Canada (RGPIN-06442). The authors are solely responsible for the content, which does not necessarily reflect the views of the funding organizations.