The first vertebrates to walk on our planet could have done so in the great ocean depths, millions of years before their later relatives passed to earth.
In 2018, scientists were surprised to find small fish (Leucoraja erinacea) and some basal sharks were able to walk on the ocean floor using many of the same neural circuits we use today to walk.
It is generally believed that vertebrates only learned to walk when they began fleeing the sea along the coast, about 380 million years ago. But other models based on small skateboarding fish (one of the most primitive animals with a spine) suggest a much deeper origin, possibly more than 400 million years ago.
Using published video data on the overflow dynamics of this benthic creature, mathematicians have developed a model to investigate how the first leg-like movements in the deep sea might have evolved.
The simple model they have created predicts the most efficient, controlled and balanced type of gait in a neutrally floating environment: the best result requires an alternating left-right-foot pattern very similar to the wadding of the small skate.
In addition, this type of trudging does not require any additional energy costs and could be reinforced over time through a simple learning scheme.
“In the context of our model, these results suggest that, despite the vast range of gear solutions, a left-right alternating bipedal control strategy can and will be discovered, which is the optimal solution for energy-efficient locomotion. “, write the authors of the study.
Finding a real example of this ancient organism is similar to the discovery of a “needle in a haystack,” the team admits, but they say only rudimentary legs would be needed to achieve this pattern of foot placement. After these foot-like fins evolved, the old creature would have needed to gain only minimal neuronal control over its new and improved limbs.
After four episodes of model learning, a one-leg locomotion strategy began to emerge. After 200 episodes, he took over a two-legged walking pattern. In chapter 600, the modeled creature began to alternate the left and right steps.
With approximately 50 learning cases over 5,000 episodes, including various learning parameters and rewards, the authors found that the best solution coincided with the small skate walk in 70 percent of all cases.
This simple control strategy suggests that walking in the deep sea is a robust and efficient behavior similar to passive walking, such as the sliding toy that “walks” down a slope without the need for complex control, only gravity.
The little skateboard, of course, is not a completely passive rancher. Their brain cells still control six muscles for movement, but the authors say this system exploits the same principles as a passive: “Sustained locomotion under a constant energy source without feedback control.”
The authors are unsure why the small skate developed a slow walk to the bottom of the sea, but suggest that it is more efficient and cost-effective than swimming at a similar pace. Additional metabolic studies on the deep sea creature will be needed to verify this idea.
Sometimes, in the wild, the little skater will use both legs at the same time to “point” forward and quickly start his left-right gait pattern. This type of movement was not found in the model, but the authors think it could be favored when faster acceleration is needed and energy efficiency is not as important. This unusual point requires a little more work.
“The combination of a reliable low-gravity environment and leg body morphology may have helped pave the way for bipedal marches before our aquatic ancestors landed,” says applied mathematician Lakshminarayanan Mahadevan of the Harvard University.
“As our ancient ancestors moved ashore, the control strategy is likely to become more complex. But in homogeneous and reliable environments, such as the seabed, perhaps just a simple strategy was needed.”
To complement this theoretical model, even researchers built a simple bipedal robot based on similar deep-sea conditions. In the end, the behavior of this robot showed striking similarities to the ideal walker of its model. Its regular step pattern does not require additional energy and ripples on either side of the body for stability.
The robot, however, tends to walk a little faster than what is seen on the small skate.
The authors admit that they may never know exactly how the first walking march came about, but their model helps refine some of the passive dynamics and neural circuits seen in living organisms.
“Understanding how the brain, body, and environment worked together in heterogeneous aquatic and terrestrial environments that should likely include proprioceptive feedback,” the authors suggest.
“But in reliably homogeneous environments, perhaps the simple strategy quantified here was where it all began.”
The study was published in Journal of the Royal Society Interface.