Early Animal Necks May Have Evolved for Limb Movement, Not Head Turning

A new study suggests that the long-held explanation for why animals developed necks may be wrong. According to ScienceDaily, researchers propose that necks may have appeared as part of wider changes to the skeleton that let animals use their front limbs for movement. The work was published in the journal Nature.

The team was international. It merged fresh fossil findings with tests on modern fish and simulations of how ancient bodies were built. Tom Stewart, who teaches biology at Penn State as an assistant professor and took part in the work, said the period studied, about 380 million years ago, was a time of major change on Earth, when some of the first forests were taking shape.

The Old Explanation

For many years, researchers held that the neck developed so that early tetrapods, forerunners of animals with four limbs, could steer their heads more precisely. Animals moving from water onto land would gain from looking down or turning their heads to find food, spot danger and explore.

Modern fish usually lack a distinct neck because their shoulders join directly to their skulls. To look elsewhere, they generally have to turn their whole bodies. Separating the head from the shoulders therefore seemed like an important step toward greater mobility.

Testing that idea has been hard. Much of the backbone in these early animals was soft tissue, which rarely survives as fossil. Stewart noted that scientists have had to rely on living animals as stand-ins for the past, in the same way living birds help researchers understand extinct dinosaurs.

Fossils and the Role of Sturgeon

Fish from the Devonian Period, roughly 420 to 360 million years ago, show a mix of traits. Some had stronger fins that may have helped them lift off the bottom or move through shallow water. Some species also began to show a clear gap between skull and shoulders.

Tiktaalik, first described in 2006, is one important example. Its remains were found on Ellesmere Island in Nunavut, Canada. Because it is closely related to the earliest limbed vertebrates, it helps researchers track changes such as the shift from suction feeding in water to biting on land.

The team turned to the sturgeon, a modern fish that keeps several ancient features. Early tetrapods had a large notochord, a flexible rod of soft tissue running through the spine, with comparatively small vertebrae around it. The researchers measured the stiffness of the notochord in sturgeon and used those figures in models of fossil animals, including Tiktaalik.

Surprisingly Stiff Necks

The models suggested that early necks were far less flexible than many scientists had assumed. Even with the head and shoulders separated, the backbone would have sharply limited movement. Dana Korneisel, who led writing of the paper and carries out postdoctoral work at Uppsala University in Sweden, said most of the animals studied would have gained little ability to reposition their heads.

Head lifting was also limited, in some cases no better than in fish. The researchers found that the head twisting people do when they shake their heads was not possible for any of the animals studied.

The team also warned that its estimates could be too generous. Muscles, skin and bones embedded in the skin of some early tetrapods may have made necks even more rigid.

A Different Origin

The authors suggest the neck may have come about through changes that helped front limbs support and move the body. Greater neck flexibility appeared later and became valuable for land animals, but it was not an early advantage.

The study also highlights the value of the notochord in understanding evolution. Such backbones are rare among living vertebrates but were widespread for much of Earth’s history.

Frequently asked questions

Sources

• ScienceDaily: Scientists discover that necks didn't evolve for the reason we thought

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