How are sharks equally related to bony fish and us?

This is one of my favorite evolutionary riddles.

The Chondrichthyes, encompassing sharks, skates, and rays, look like fish. They have gills and swim in the ocean like fish. We call them fish. So they must be fish, right?

Well, It depends on how you define fish. If you assign fish-dom based on things they all share, i.e. floppy slimy creatures who live underwater, breathe through gills, and swim with pairs of fins instead of limbs, then sure, sharks are fish and humans aren’t. However, if you define fish like we do in evolutionary biology, humans, dogs, tortoises, parrots, and sharks are friends, not food right alongside Dory and Nemo.

The evolutionary classification system defines groups of creatures based on common ancestry, not common characteristics. We ignore phenotype: the specific traits that distinguish a group of species like having gills versus lungs, bones versus no bones, or fins versus limbs, and focus only on genotype: the direct lines of genetic inheritance linking one group to another. Sometimes genetic and phenotypic lines follow each other closely. In that case, we can measure evolutionary distance with relative accuracy by comparing unique and shared features. But as you’ll see, looks can be deceiving.

Figure 1

There’s a clear fish out of water on this phylogenetic tree depicting the genetic relationships between four groups of vertebrates (Fig 1). However, it’s crucial to note that these groups are gross oversimplifications. Not drawn on our branch are such diverse critters as kangaroos, gila monsters, woolly mammoths, and velociraptors. The bony fish group includes over 28,000 species of contemporary fresh and saltwater fishes, and the sharks, skates, and rays currently boast at least 1,000 species.

At the root of the tree lurks our common ancestor, the placoderm fishes, a long-extinct group represented here by Dunkleosteus, a 20 foot behemoth predator with dense bone plate armor. Of course, Dirk the Dunkleo wasn’t our direct great great great grandpappy. The notion of a common ancestor is mostly hypothetical: we know there was one fish that started it all, and we’ve narrowed our options to any one of the thousand-odd Placodermi.

At its core, the principle of common ancestry measures time: how many generations of creatures have elapsed since one group split into two. Paleontology and models of the rate of molecular evolution tells us that we bony humans and bony fish split from a common ancestor 400 million years ago. Since we share ancestry with bony fish more recently than we do with sharks, we are more closely related to tuna than we are to sharks.

But here’s the catch. Sharks are not more closely related to chicken of the sea than they are to chicken of the land. Sharks are equally related to all bony fishes and all land vertebrates including humans, despite the fact that they are indeed fish.

Solving this puzzle requires thinking on evolutionary timescales, which are incomprehensibly vast. The group of creatures leading to modern-day sharks split off from the rest of fish between 450 and 430 million years ago. Since then, sharks have followed an independent evolutionary trajectory with many twists and turns and truly wild trait experiments. Some phenotypes, like their dermal denticles (tiny interlocking placoid scales that look and feel like row upon row of jagged teeth) and flexible cartilaginous skeletons have stuck around. Others, like sporting jaws reminiscent of circle saws and growing as long as a school bus, sadly have not. Meanwhile, every creature on the other side of that split continued on and underwent some wacky experiments of their own over the last 400 million years, including us handy hairless apes.

In fact, we could draw the tree with humans and tuna swapped…

Figure 2

and still convey the same relationships (Fig 2).

It’s jarring to see humans sandwiched between great whites and tuna. With our polyester fins and clunky scuba gear, we look like an undercover spy in a truly laughable fish disguise. But remember, everything on one side of a split is equally related to everything on the other side. Being higher on the tree does not mean a group is ‘more evolved.’ Modern sharks aren’t dinosaurs because they still look similar to their prehistoric relations. And humans aren’t farther along because our group left the water long ago while our fishy cousins thought it felt fine.

Nothing like evolutionary biology to put us in our place. 😉

Something fishy? Leave a me question about fish or non-fish in the comments, and I’ll answer it in a future investigation! And check out my favorite website, the Tree of Life Web Project, to practice reading trees and learn more about our common ancestry.

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