The first vertebrates to walk on our planet could have done so in the deep ocean, millions of years before their later relatives landed.
In 2018, scientists were shocked to find small skate fish (Leucoraja erinacea) and some basal sharks have been able to travel across the ocean floor using many of the same neural circuits we use today.
It is generally believed that vertebrates learned to walk only when they began to avoid the sea to shore, about 380 million years ago. But other models based on small skate fish – one of the most primitive vertebrate animals – suggest a much deeper origin, possibly more than 400 million years ago.
Using published video data on the dynamics of the shaking of this benthic creature, mathematicians developed a model to investigate how early foot movements could have evolved in the depths of the sea.
The simple model they created predicts the most efficient, controlled and balanced type of walking in a neutral environment: the best result requires an alternative left-foot-right model very similar to small patina hunting.
Moreover, this type of trudging does not require any additional energy costs and could be consolidated over time using a simple learning scheme.
“In the context of our model, these results suggest that, despite the vast space of the walking solution, an alternative left-right bipedal control strategy can and will be discovered and is the optimal solution for energy-efficient locomotion,” the authors write. study.
Finding a real-world example of this ancient organism is similar to finding a “needle in a haystack,” the team acknowledges, but says it would only take rudimentary feet to achieve this footing pattern. After those leg-like wings evolved, the old creature should then have gained only minimal neural control over their new and improved limbs.
After four episodes of model learning, a one-legged locomotion strategy began to emerge. After 200 episodes, a two-legged walking pattern took over. In the 600th episode, the modeled creature began to alternate between left and right steps.
Running about 50 instances of learning for 5,000 episodes, including various learning parameters and rewards, the authors found that the best solution fits the skating walk in 70% of all cases.
This simple control strategy suggests that deep walking is a robust and efficient behavior similar to passive walking, such as the slinky toy that “walks” on a slope without the need for complex control, only gravitational.
The small skate, of course, is not a completely passive passenger. Its brain cells control six more muscles for movement, but the authors say that this system exploits the same principles as a passive one: “Locomotion sustained under a constant energy source without feedback control.”
The authors are not sure why the little patina has developed a slow walk on the seabed, but suggest that it is more efficient and cost-effective than swimming at a similar pace. Several metabolic studies on the deep creature will have to verify this idea.
Sometimes, in the wild, the little patina will use both feet at the same time to “throw” forward and quickly start the left-right gait pattern. This type of movement was not found in the model, but the authors believe that 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 a morphology of the standing body could have helped pave the way for bipedal walking before our aquatic ancestors landed,” says applied mathematician Lakshminarayanan Mahadevan of Harvard University.
“As our ancient ancestors went ashore, the control strategy probably became more complex. But in reliable homogeneous environments, such as the seabed, perhaps only a simple strategy was needed.”
To complete this theoretical model, the researchers even built a simple bipedal robot based on similar depth conditions. Finally, the behavior of this robot showed striking resemblances to the ideal course of their model. Its regular step pattern does not require extra energy and curls on both sides of the body for stability.
The robot, however, tends to move slightly faster than what is seen in the small patina.
The authors acknowledge that they may never know exactly how the first walk 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 functioned together in heterogeneous aquatic and terrestrial environments, probably necessary to 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 Royal Society Interface Journal.