Photo: Sina Schuldt/dpa (Photo by Sina Schuldt/picture alliance via Getty Images)

dpa/picture alliance via Getty Images

There is something deeply strange about looking at a shark embryo.

When you first look at it, it does not resemble the sleek predator most people imagine slicing through our ocean. Instead, it looks delicate, fragile and almost a bit alien. Tiny bulging eyes form long before the animal resembles a shark at all and its future face exists only as clusters of migrating cells, slowly organizing themselves into the structures that will eventually become jaws, cartilage and sensory organs. Yet hidden within that developing embryo is a story that stretches back more than 400 million years.

A new study led by Markéta Kaucká at the Max Planck Institute for Evolutionary Biology focused on the small-spotted catshark (Scyliorhinus canicula) is helping scientists better understand one of evolution’s most important cellular populations known as “neural crest cells.” These cells are unique to vertebrates and are often described as evolutionary game changers because they helped enable some of the defining features of vertebrate animals, including jaws, facial skeletons and advanced sensory systems. They emerge very early during embryonic development before migrating through the body and transforming into many different tissues: some become pigment cells, others help form parts of the nervous system. Cranial neural crest cells, the subset examined in this study, are especially important because they create much of the facial skeleton.

Without neural crest cells, your face would not exist. Neither would a shark’s.

Scientists have known for years that these cells are remarkably conserved across vertebrates. Mice, chickens, zebrafish and humans all rely on similar genetic programs to guide cranial neural crest cell development. But there has been a major missing piece in the evolutionary story. Enter cartilaginous fishes, such as sharks and rays. They matter because sharks sit near the base of the jawed vertebrate family tree, having split from the lineage leading to bony fishes and mammals hundreds of millions of years ago. If researchers want to understand what the earliest jawed vertebrates may have looked like developmentally? Sharks are essential. The problem is that sharks are not easy research animals. Their embryos develop slowly and many of the genetic tools scientists use in mice or zebrafish simply do not exist for sharks. Even obtaining enough embryos for detailed experiments can be difficult! But that slow development turned out to be one of the study’s greatest strengths. Because while a mouse embryo can race through major developmental changes in a matter of hours or days, catshark embryos develop over roughly 175 days, giving researchers an unusually detailed window into transitional stages that are normally difficult to capture in rapidly-developing mammals. Using a combination of single-cell RNA sequencing, advanced microscopy and synchrotron radiation micro-CT scanning, the team of researchers mapped how cranial neural crest cells move through the developing shark head and eventually form the facial skeleton.

What they found was both familiar… and surprising. On the molecular level, the shark neural crest cells looked remarkably similar to those seen in other vertebrates — the same core genes appeared again and again, the same developmental pathways were activated. In many ways, the ancient shark embryo was using a deeply familiar evolutionary toolkit. But the cells behaved differently. In bony vertebrates, including mammals, cranial neural crest cells rapidly migrate toward the front of the face. In the catshark, however, many of these cells first gathered around the eye region, creating what the researchers call a periocular ectomesenchyme. Basically, instead of building the face from the front outward, the shark embryo appears to organize parts of the face around the eyes first. A subtle shift, sure, but development is all about timing and positioning so small changes in where cells accumulate, how long they proliferate or when they activate specific genes can produce enormous anatomical differences over evolutionary time!

Think about the incredible diversity among sharks and rays alone. Hammerheads have wide flattened skulls. Sawfish develop elongated rostrums lined with teeth. Manta rays possess dramatic cephalic lobes near their mouths. Even closely related shark species can have noticeably different facial structures! So the study suggests that many of these differences may not come from these animals reinventing the genetic toolkit itself. Instead, evolution may be modifying how and where that toolkit is “deployed” during development. And that singular idea has broader implications beyond sharks and rays. For decades, evolutionary developmental biology has wrestled with a major question: how do animals produce such extraordinary diversity while sharing so many of the same genes? Research like this points toward an answer. Evolution is not create entirely new genes! It’s rearranging familiar pieces into new forms.

The researchers also identified intriguing lineage-specific signals. One protein called periostin appeared strongly in the shark notochord (a structure that helps organize embryonic development). Similar expression patterns were found in chickens and frogs but not in mice or zebrafish, raising fascinating questions about how different vertebrate groups may have modified ancient signaling pathways to generate their own distinctive anatomies. By comparing how development unfolds across different lineages, scientists can begin piecing together which traits are ancient and which are evolutionary innovations. Was the earliest vertebrate face organized more like a shark’s or more like a mammal’s? When did certain neural crest cell behaviors evolve? How many times did developmental strategies change independently? These are difficult questions because evolution rarely leaves direct answers behind, and instead you have scientists trying to read clues hidden inside living organisms.

There is something inspiring about realizing how much of vertebrate diversity comes down to the choreography of tiny migrating cells. The faces staring back at us across the animal kingdom may seem radically different, yet they emerge from surprisingly similar beginnings. And in a month where we celebrate a community of people that are often “other”-ed for being different, maybe that is the biggest takeaway from this research: we’re all more alike than we think.