The first animals to make the transition from water to land nearly 380 million years ago could barely turn their heads, Science Daily reported on October 14, 2026.
Challenging evolutionary assumptions
Scientists previously believed that the evolution of necks gave these pioneering creatures greater freedom to look around. New research published in the journal Nature suggests their necks were actually remarkably stiff, challenging a decades-old explanation for why animals developed distinct necks.
Instead of evolving to improve head rotation, necks might have developed as part of wider skeletal modifications that permitted animals to employ their front limbs for locomotion. These findings were reached by an international team of researchers who combined new fossil evidence with computer models of ancient anatomy and experiments on living fish.
“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,” Stewart said.
Reconstructing ancient anatomy
For decades, the prevailing explanation was that necks evolved to give early tetrapods greater control over their heads as they adapted to life outside the water. Modern fish generally lack a distinct neck because their shoulder structures connect directly to their skulls, requiring them to turn their entire bodies to look in another direction.
Determining how ancient animals moved has proven difficult because much of their backbone consisted of soft tissue that rarely survives long enough to become fossilized. “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.
Researchers looked to modern animals as analogues for the past, much like living birds teach researchers about extinct dinosaurs. Some of the best evidence comes from fish that lived during the Devonian Period, approximately 420 to 360 million years ago.
The role of Tiktaalik
One especially important example is Tiktaalik, a fossil fish first described in 2006 on Ellesmere Island in Nunavut, Canada. Because Tiktaalik shares a close evolutionary relationship with the earliest limbed vertebrates, its anatomy supplies vital insights into the structural shifts that enabled terrestrial life.
“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,” Stewart said.
Additionally, Tiktaalik assists scientists in comprehending the transition from drawing food into the mouth via aquatic suction to biting on land, alongside the conversion of swimming organs into walking appendages.
Testing sturgeon backbones
Early tetrapods had backbones containing a prominent notochord, which is a long, flexible rod of tissue running through the vertebral column. Because individual vertebrae were relatively small and arranged around this large tube of soft tissue, researchers struggled to determine how much flexibility it allowed.
To overcome this, the research team measured the stiffness and mechanical properties of the notochord in modern sturgeon, which retain several ancient anatomical features. They incorporated those measurements into models of fossil animals, including early tetrapods such as Tiktaalik.
The results indicated that even when the head and shoulders were anatomically separated, the structure of the backbone would have severely restricted movement. “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,” Korneisel said.
Unexpected mechanical limits
Most of the animals 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,” Korneisel said.
Head elevation in these animals fell below what would be expected in fish, and the kind of head twisting used to shake heads was not feasible. Muscles, skin, and bones embedded within the skin may have added further restrictions.
Locomotion before vision
The findings point to the separation of the skull and shoulders developing alongside anatomical changes that allowed front limbs to support and move the body. The earliest necks were likely a consequence of adaptations for locomotion rather than adaptations for improved vision or head movement.
Greater neck flexibility eventually evolved later after the basic structure of the neck was established. “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,” Stewart said.
