The very first vertebrates to roam our planet may have done so in the Pacific, millions of years before their later relatives moved to land.
In 2018, scientists were shocked to find the little skate fish (Leucoraja erinacea) and some basal sharks were able to make it across the ocean floor using many of the same neural circuits we use for walking today.
It is generally thought that vertebrates did not learn to walk until they began to avoid the sea off the coast, about 380 million years ago. But other models based on the lesser skating fish – one of the most primitive backbone animals – suggest a much deeper origin, possibly more than 400 million years ago.
Using published video data on the sunken dynamics of this benthic creature, mathematicians have developed a model to investigate how early bony movements may have evolved in the deep sea.
The simple model they created predicts the most efficient, controlled and balanced gait in a neutral buoyancy environment: the best result requires an alternating left foot-right foot pattern that is very similar to the little skate wobble.
Additionally, this type of trudging requires no additional energetic expense and can be reinforced over time using a simple learning scheme.
“In the context of our model, these results suggest that, despite the vast solution space of corridors, a left-right alternating bipedal control strategy can and will be discovered and is the optimal solution for energy efficient locomotion,” write the study authors.
Finding a realistic example of this ancient organism is akin to discovering a “needle in a haystack,” the team admits, but they say it only takes rudimentary legs to achieve this pattern of foot placement. After those foot-like fins evolved, the old creature had only needed minimal neuronal control over their new and improved limbs.
After four episodes of learning in the model, a one-legged locomotion strategy began to emerge. After 200 episodes, a two-legged gait took over. In the 600th episode, the modeled creature started to alternate between left and right steps.
With about 50 learning experiences for 5,000 episodes, including various learning parameters and rewards, the authors found that the best solution matched the little skate’s gait in 70 percent of all cases.
This simple control strategy suggests that deep sea walking is a robust and efficient behavior similar to passive walking, such as the slinky toy “walking” down a slope without the need for complex control, just gravity.
The little skate isn’t a completely passive slog, of course. His brain cells still control six muscles for movement, but the authors say this system uses the same principles as a passive system: “Sustained locomotion under a constant energy source without feedback control.”
The authors aren’t sure why the little skate developed a slow seafloor walk, but they suggest it is more efficient and cost-effective than swimming at a similar pace. Further metabolic studies of the deep-sea creatures will need to verify this idea.
Sometimes in the wild the little skate uses both legs at the same time to “punt” forward and quickly start his left-right gait. This kind of motion was not found in the model, but the authors think it is preferable when faster acceleration is needed and energy efficiency is not that important. This unusual punt requires a bit more work.
“The combination of a reliable, low-gravity environment and a body morphology with legs may have paved the way for two-legged gaits before our aquatic ancestors transitioned to solid ground,” says Harvard University applied mathematician Lakshminarayan Mahadevan.
“When our ancient ancestors moved to land, the control strategy probably became more complex. But in reliably homogeneous environments, such as the sea floor, perhaps a simple strategy was all it took.”
To complement this theoretical model, researchers even built a simple bipedal robot based on similar deep-sea conditions. Ultimately, this robot’s behavior showed striking similarities to their model’s ideal walker. The regular footstep pattern requires no extra energy and waves on both sides of the body for stability.
However, the robot tends to run a little faster than what you see in the small skate.
The authors admit that they may never know exactly how the first corridor came to be, but their model helps refine some of the passive dynamics and neural circuits seen in living organisms.
“Understanding how the brain, body and environment interacted in heterogeneous aquatic and terrestrial environments likely required proprioceptive feedback,” the authors suggest.
“But in reliably homogeneous environments, the simple strategy quantified here may have been where it all started.”
The study is published in the Journal of the Royal Society Interface.