Using computer simulations and a robot, researchers have recreated the likely gait of a 300-million-year-old animal considered to be among the planet’s earliest terrestrial walkers.
Hundreds of millions of years ago, aquatic animals began transitioning to land. But how did the world’s first quadrupedal vertebrates walk? The question sounds simple enough, but no one was around back then to watch this evolutionary development unfold, and analyses of fossils have only hinted at the process.
We don’t know, for example, if these early walkers, known as tetrapods, were capable of standing upright on their legs, or if their movements were co-ordinated, balanced, and energy-efficient. This would be good to know because the first walkers eventually evolved into reptiles and mammals. Skillful and sophisticated walking styles—as opposed to basic locomotion techniques such as rolling, dragging forward, slithering, or hopping— likely facilitated this important evolutionary transition.
To get a sense of how these early terrestrial pioneers walked around, a research team led by John Nyakatura from Humboldt University in Berlin and Kamilo Melo from École Polytechnique Fédérale de Lausanne in Switzerland, turned to computer simulations and robots. Their fascinating new paper, published today in Nature, shows that early walkers had adopted an advanced gait earlier than scientists expected.
The model for their experiment was an early four-legged, 35-inch-long tetrapod known as Orobates pabsti. This ancient lizard-like creature is a “stem amniote,” an early animal positioned between amphibians and reptiles in the evolutionary family tree.
“Orobates is an ideal candidate for understanding how land vertebrates evolved because it [represents] the lineage leading to modern amniotes—that is, animals that became largely independent from water, as they developed within eggs laid on land,” said Nyakatura in a statement.
Also, Orobates fossils are critically important for understanding vertebrate evolution; these creatures are a very close cousin to the last common ancestor of mammals, reptiles (including all extinct dinosaurs and pterosaurs), and birds. Finally, Orobates left behind an excellent fossil record of its time on Earth, along with fossilized trackways of its footprints—a double-whammy that allowed the researchers to conduct a quantitative physical analysis of this extinct creature.
Indeed, the first step of the process was to create a digital model of the Orobates skeleton. X-rays of Orobates fossils offered an important start, but to figure out how they moved their bodies through time and space, and to get a sense of their biomechanical capacities, the researchers looked to similar living creatures, including caimans (a small crocodile), salamanders, iguanas, and skinks. Like Orobates, these extant animals are quadrupedal sprawlers, with arms and legs extending out from the side, instead of straight down (think of how a crocodile stands in a sprawling position, as opposed to an elephant, with its columnar limbs). Using x-rays and force measurements, the scientists documented the way these animals prop their bodies upright, how their backbones move, and the degree to which they can bend their elbows and knees as they walk. This data was then used to power a kinematic computer simulation of Orobates’ likely walking style.
Not content to stop there, the researchers used a robot, dubbed OroBOT, to confirm or reject the computer’s suggestions. With a robot working in the physical world, the researchers were better able to calculate the physics involved—that is, the actual energy transferred—in the various walking styles proposed by the simulation.
“Our robotic model allowed us to test our hypotheses about the animal’s locomotion dynamics,” Melo said in a statement. “It factors in the real-world physics of its walk.”
OroBOT tested hundreds of different walking styles to determine the ones most likely used—and not used—by Orobates. In yet another act of due diligence, the researchers correlated the candidate walking styles against fossilized footprints made by Orobates. If the gait didn’t match the prints, it was given a lower score. The researchers even created an interactive website where the public and scientists can explore the possible gaits used by Orobates.
The most likely gait utilized by Orobates, according to this research, was a style very similar to one used by caimans. This ancient animal moved with an athletic gait and it could hold itself slightly upright on itself—something salamanders and skinks cannot do. This turned out to be a more advanced walking style than suspected.
“Our metrics indicate that Orobates exhibited more advanced locomotion than has previously been assumed for earlier tetrapods, which suggests that advanced terrestrial locomotion preceded the diversification of [modern] amniotes,” the researchers wrote in the study.
Excitingly, the researchers said their new multi-faceted methodology can be used to study other important evolutionary transitions, such as the origin of flight or the galloping gaits of mammals.
“This is a remarkable study that uses such a range of approaches it is difficult to find fault,” Emily M. Standen, an assistant professor at the University of Ottawa and a specialist in early animal biomechanics and comparative physiology, told Gizmodo. “By integrating fossil anatomy and trackways with a wide range of known sprawling postured animals and then applying the engineering and computer modeling tools to fit the data, the authors have taken a huge amount of information and dove-tailed it together to create a very realistic and hard to refute description of how an ancient animal may have walked.”
Standen found it interesting to learn that an upright walking gait appears to have predated the emergence and diversification of modern amniotes and their successors.
“Because locomotory performance is often selected for in an adaptive evolutionary context, this data suggests that the advantages of an upright gait—speed, efficiency, and agility—may have contributed to the diversification of [modern] amniotes,” she said. “That is exciting to think about.”
Standen was impressed with this finding, but it was the methodology used by the researchers that impressed her most. She said the work represents what science should be: open-minded, interdisciplinary, broadly focused, and accessible.
“This is really an outstanding effort of incredible quality and precision,” she said. “Not to mention that they have made these tools available online so that others can add information and fine tune resultant outcomes. It seems unbelievable and reminiscent of those late-night television commercials that would sell something amazing and then keep adding other ‘gifts’ at no additional cost,” to which she added: “The only thing this paper is missing is a 12 piece knife set.”