The first animals to venture out of the water and onto land wouldn't have been able to turn their heads and look around much, according to a new study on the evolution of necks.
A model developed in collaboration with Penn State scientists using new fossil evidence and biomechanical studies in a modern fish suggests the earliest land animals may have had stiff necks, not the flexible necks that scientists have long believed. The results call into question a long-standing hypothesis about the evolution of a distinct neck in animals that first moved onto land some 380 million years ago, the researchers explained. The neck did not evolve to allow animals to move their heads more freely, but rather it likely evolved due to changes in the anatomy of these animals related to using their front limbs for locomotion, according to the new study.
The study, by an international team of scientists including Penn State Assistant Professor of Biology Tom Stewart, was published last week (Oct. 14) in the journal Nature.
"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," Stewart said. "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."
Scientists have thought for decades that the necks of the earliest ancestors of four-limbed animals - tetrapods - became more mobile early on in their transition to land. The idea was that in shallow water or on land, these animals might need to be able to look down or from side-to-side to find food and avoid predators more than in deep water. However, Stewart said, reconstructing the function of the neck in these animals is difficult from the fossil record alone.
"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."
Fish don't have necks; instead, their shoulder blades attach directly to their skull. If they need to look to the right or left, they must turn their entire bodies. But some fossil fish from the Devonian Period, between about 420 and 360 million years ago, have been found with a mix of traits - some purely aquatic and some appearing more like animals that live on land. For example, some have more robust fins that could potentially bear the animal's weight and allow them to prop themselves up or push along the bottom through shallow water. These transitional fossil fish also, for the first time, start to have distinct necks.
"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."
Stewart explained that the fossils come from Ellesmere Island in Nunavut, Canada, which is the northern Arctic.
"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."
