This skate embryo has been stained to show where the sensory hair cells are located (the little black blobs on the image).Andrew Gillis
- Sharks and humans share a distant relative that possessed a sixth sense.
- This shark-like relative of humans and sharks could detect electrical fields under water.
- The lineage that led to humans lost this ability a long time ago, but sharks and other fish retained the skill.
The common ancestor of sharks and humans -- and all jawed animals with a backbone for that matter -- possessed a sixth sense: the ability to detect electrical fields under water.
The anatomical tools for this ability, called electroreceptors and electrosensory ampullary organs, arose from the same cell population in both cartilaginous fishes (such as sharks and skates) and bony fishes (such as sturgeons and paddlefish) along with some salamanders, concludes the study, published in the journal Development.
Humans, even aquatic wonders like swimmer Michael Phelps, unfortunately lost this sixth sense a long time ago.
"There are absolutely no remnants of electrosensory ampullary organs in humans," lead author Andrew Gillis told Discovery News. "In fact, human embryos no longer possess the embryonic structures that give rise to the ampullary organs of our fishy ancestors. … So we (meaning our overall ancestral line) have not possessed the ability to detect electric fields underw ater using ampullary organs for a very long time."
Gillis, a Dalhousie University biologist, and his colleagues made the determinations after using long-term cell tracing techniques to study live skate embryos as they developed over a period of up to 70 days. The investigation revealed that the basic electrosensory system of this fish represents a more elaborate form of what was present in the last common ancestor of jawed animals with a backbone, vertebrates.
Amazingly, scientists have a pretty good idea of what this ancient relative of both sharks and humans looked like.
"The last common ancestor of jawed vertebrates was a fish that lived approximately 450 million years ago, and we can infer certain aspects of its anatomy based on features that are present in both of its descendant lineages," Gillis said.
These lineages all "possess jaws, teeth and paired fins," he explained, features that aren't seen in living jawless vertebrates, such as lampreys and hagfishes. Researchers can therefore reasonably conclude that these same features were present in that key last common ancestor.
Prior research further suggests that this prehistoric marine dweller looked more like a shark than another type of fish.
The electrosensory abilities passed down from this ancestor allow marine predators to hunt live prey in water. When prey, such as a smaller fish, swim or move their gills, they create changes to the surrounding electrical field, which the predators can detect.
When our distant ancestors left the water millions of years ago, they evolved other ways of hunting, so this natural ability to detect electrical fields under water was lost over time.
Certain mammals, however, such as dolphins and the semi-aquatic platypus, later independently evolved modified nerve endings that provide them with their own way of detecting underwater electrical fields.
Still other underwater inhabitants, such as knifefishes and elephantnose fishes, which are freshwater creatures, independently evolved a structure known appropriately as the "electric organ." This organ is used to generate electric fields that the fishes utilize for communication, mate selection and locating things in water.
"Strongly electric fish, like electric eels and Torpedo rays, use their electric organs to stun prey or as defense against predators," Gillis added.
William Bemis, a professor of ecology and evolutionary biology at Cornell University and director of the Shoals Marine Laboratory, told Discovery News that the new study is an "important and noteworthy work, and corrects some misinterpretations in the (scientific) literature."
The research was funded by a Royal Society Newton International Postdoctoral Fellowship to Gillis and by a grant from the U.K. Biotechnology & Biological Sciences Research Council to co-author Clare Baker of the University of Cambridge.