Living Fossils Like the Coelacanth Have Remained Unchanged for 400 Million Years

• The most famous living fossil lis a fish called the coelacanth. It was once thought to be extinct. However, the fish hasn't changed in over 400 million years.

• Charles Darwin coined the term "living fossil" in his book On the Origin of Species.

• The idea and classification of living fossils is hotly debated among the scientific community. Some researchers believe the label "living fossil" should be thrown out altogether.

A living fossil, at least to scientists, is not your crusty old uncle who can’t seem to get with the times. It’s simply a living organism that doesn’t appear to have changed when compared with its extinct relatives.

But when talking about living fossils, things get complicated. The term, as scientists use it, was coined by Charles Darwin in On the Origin of Species. Since then, there has been a lot of, shall we say, variation and modification in the concept.

A commonly cited example of a living fossil is the coelacanth, a fish once thought to be extinct, that has remained mostly unchanged for more than 400 million years. According to the Smithsonian National Museum of Natural History, the coelacanth is one of the most famous living fossils. 

Other organisms often said to be living fossils are ginkgo trees, horseshoe crabs, komodo dragons, and even cockroaches. 

Read More: Take a Tour of These Incredible Living Fossils

In Darwin’s time, people were intrigued with the seeming stability of the morphology, or physical form, of some organisms over long periods of evolutionary time, explains Alan Love, a philosopher of science at the University of Minnesota, whose work focuses on conceptual issues in developmental and molecular biology. 

Later, as we gained a better understanding of the fossil record and better scientific tools, scientists began to have a deeper insight into that stability. And thanks to these new tools, the number of “living fossils” has increased. The concept has been used to describe everything from molecules to megafauna.

On the other hand, when viewed at the molecular level, it appeared that some organisms that had been considered living fossils (because their morphology was stable) were not as stable as previously thought, risking their membership in the living fossil club. This caused some confusion about the concept. 

Love says that for some people, the thinking went, “Well, we originally thought this was a living fossil because these features were stable, but then we look at new features that we couldn't previously look at, and those are not stable. Should we still call it a living fossil or not?” And that, he says, led to an unhelpful tit-for-tat, with some saying, “Yes, it is,” while others say, “No, it isn’t.”

Ambiguous terminology is not uncommon in biology. Love points out, for example, as many as 20 different concepts of “species” are used by biologists. That’s because there are different things about species that are interesting to biologists. And that’s what’s really going on here. Scientists are not trying to decide what goes into a box labelled “Living Fossils.” They’re trying to explain patterns, in this case stability, Love says. And the concept of living fossils is a tool for investigating those patterns.

So why not just relegate the concept “living fossil” to history and come up with a less ambiguous one? That would not be helpful, says Love. The various ways of looking at the concept are useful depending on what you’re trying to learn. 

“We want to have the ability to move back and forth between the different things that we find interesting and worthy of study,” he says. “And if we invent new terminology, we might push people into spaces where they don't talk to each other anymore.” 

The fluidity of the concept is part of what makes it useful.

Still, it’s not much use if everyone is bickering about what is and what is not a living fossil. To help bring some organization to this chaos, Love and Scott Lidgard have proposed a framework for thinking about the concept. Lidgard is a paleontologist at the Field Museum who studies the conceptual and interdisciplinary aspects of the term living fossils.

In a paper published a few years ago, the pair argued that scientists should put less emphasis on defining the term and more on a conceptual framework based on how the concept can be used in answering research questions. 

This will, they say, make it easier for scientists from different disciplines to work together to answer shared questions. “Some concepts are used to categorize,” says Lidgard, “but there's another role for concepts, and that is the role as an agenda or an umbrella that links together lots of specific research questions.”

So, what is a living fossil? Well, it’s not so much a coelacanth or a ginkgo tree, but a concept — a tool for thinking about how organisms and parts of organisms change or do not change over long stretches of time. That’s not a pithy answer to the question, but it’s the kind of thinking that makes it possible for scientists to do the work they do.

Read More: Living Fossils Revealed: The Hidden Evolution of These 4 Ancient Species

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• Smithsonian National Museum of Natural History. Coelacanths

• Field Museum

• BioScience. Rethinking Living Fossils

Avery Hurt is a freelance science journalist. In addition to writing for Discover, she writes regularly for a variety of outlets, both print and online, including National Geographic, Science News Explores, Medscape, and WebMD. She’s the author of Bullet With Your Name on It: What You Will Probably Die From and What You Can Do About It, Clerisy Press 2007, as well as several books for young readers. Avery got her start in journalism while attending university, writing for the school newspaper and editing the student non-fiction magazine. Though she writes about all areas of science, she is particularly interested in neuroscience, the science of consciousness, and AI–interests she developed while earning a degree in philosophy.