'Walking Fish' Fossil Discovery Explains How First Humans Left the Ocean
Scientists have long known that humans evolved out of the water long ago, eventually developing the neural circuitry to resist gravity and learn to walk. What they haven't been able to figure out, though, is how it happened.
Now, a new study published this week in the Cell journal says that the ancient skate fish Leucoraja erinacea exhibited similar bipedal walking behaviors, offering a clue as to how all vertebrates with paired appendages followed in step.
"Walking is the predominant locomotor behavior expressed by land-dwelling vertebrates, but it is unknown when the neural circuits that are essential for limb control first appeared," explains the study. "Certain fish species display walking-like behaviors, raising the possibility that the underlying circuitry originated in primitive marine vertebrates. We show that the neural substrates of bipedalism are present in the little skate Leucoraja erinacea, whose common ancestor with tetrapods [animals with four limbs] existed ∼420 million years ago."
The researchers discovered thatLeucoraja shares other features of animals with four limbs, including the ability to move along the ocean floor by moving their hind fins left and right, similar to how we move our legs when walking around.
But the most remarkable finding was the presence of a neural network that "encodes peripheral connectivity modules that are distinct from those used in axial muscle-based swimming and has apparently been diminished in most modern fish." Discovering this network taught the researchers that these were the same neural circuits found in mice and other tetrapods. The 420 million-year-old skate fossil changes the discovery of this ability to well before the earliest fossil evidence for tetrapods, meaning that the neural circuits required for walking evolved many years before actual legs and feet.
"These findings could provide a foundation for defining the evolutionary origins of limb-based motor behaviors and resolving the underlying developmental programs that contribute to the assembly of locomotor circuits," the study concludes.
The past few months, meanwhile, have been rich with related evolutionary discoveries. A recent study explained just how and when our modern human brains evolved to determine that the changes in brain shape perfectly align with the point in history when humans developed the ability to build tools, think critically, and practice self-awareness. Meanwhile, while a human jawbone found in Israel rewrote the history of when our ancestors first left Africa, upsetting a decades-old theory about human migration.