Dear Sarah,

Tuna, salmon, mackerel. Cats like me are famously big fans of fish. But I’ve never looked at my afternoon sushi and wondered if it’s related to my human friends.

So, I asked one of those friends named Thomas Siek. He’s a biological anthropologist at Washington State University. He studies how humans evolved.

He told me that fish and humans are distant relatives. All vertebrates—animals with backbones—are related.

“Humans and fish aren’t close evolutionary cousins,” Siek said. “We’re part of the same phylum called Chordata. We share a common ancestor. Those in Chordata have backbones. So, this will also include birds, dogs, cats, cows and many other animals.”

This is Methuselah the Australian Lungfish. She lives at the California Academy of Sciences. At about 93 years old, she’s the oldest fish with human caregivers. She shares a common ancestor with humans, cats and all animals with backbones. Images: Gayle Laird © California Academy of Sciences

If two or more species trace their history back to the same ancient species, that’s their common ancestor. We can find common ancestors on diagrams scientists make called phylogenetic trees. They show how different groups of living things are related to each other and to living things that lived a long time ago.

It’s not the same as making a family tree, but the idea is similar. If I make a family tree, I might find an ancestor that’s the same for me, my littermates and my cousins. I might find an ancestor that’s the same for me and cats I’ve never met. That ancestor would be our common ancestor.

Phylogenetic trees don’t look at individuals like family trees do. They look at whole groups of living things. They don’t look back a few generations like we do. They look at evolutionary relationships over millions or billions of years.

That’s how we know that all animals with backbones share a common ancestor. That ancestor was a group called the lobe-finned fish, or sarcopterygians.

One kind of ancient lobe-finned fish is Tiktaalik. They’re extinct now, but scientists think they lived about 375 million years ago.

Tiktaalik looked a lot like today’s fish. They had gills, scales and fins. But they also had non-fish parts that today’s vertebrates have. They had shoulder, elbow and wrist bones. These ancient fish probably used those strong bones to prop themselves up or walk. They had primitive lungs, so they probably took gulps of air when they needed to.

Scientists call animals like Tiktaalik transitional fossils. They have parts like fish. They have parts like land animals. They have parts like a mix of both.

These are bones and a model of a Tiktaalik species called Tiktaalik roseae. These fish probably grew up to 9 feet long. They probably used their strong fins to swim and walk—like lungfish still do today. Image: Ryan Schwark/Wiki 

But it isn’t like one fish had a dream to leave the water and be where the people were. (People didn’t even exist yet.)

These fish and their relatives just used whatever body parts they had to live their best lives. Strong fin bones probably helped some of them survive long enough to have babies. So, their babies had strong fin bones, too. Over millions of years—and gazillions of fish having babies—some babies eventually had big bones that were more like legs than fins.

That’s how extinct fish can be a common ancestor for humans like you and cats like me—and for cool fish like today’s lungfish.

Remind me not to take tuna casserole to the family reunion.

Sincerely,

Dr. Universe