Living fossils

Red pandas. Horseshoe crabs. Ginkgo trees. Crocodilians. They’ve all been called living fossils, species that appear to have changed very little since their now-fossilized ancestors lived, or that have no close living relatives. Such a wide range of plants and animals can be described as such that it’s hard to grasp why it might be useful, other than as a jumping off point for internet-ready lists of the most amazing living fossils in the world.

Close examination of the DNA of tadpole shrimp, coelacanths, and tuataras have revealed that they have actually changed significantly over the eons. This has led some scientists to suggest that we abandon the term. Not only are living fossils not really a thing, they argue, the concept deepens common misunderstandings about the way evolution works. So, living fossils—are they for real, or has our thinking on them evolved?

1: Times Charles Darwin mentions “living fossils” in On the Origin of Species

>20 years: Lifespan of a horseshoe crab

4: Days a horseshoe crab can survive out of the water

100 years: Time that elapsed between the discovery of the platypus and the realization that they lay eggs

13 hours: Time each day red pandas devote to finding and eating food, mostly bamboo leaves

18 inches (46 cm): Length of a red panda tail

2,575 ft (785 m): Underwater depth limit for chambered nautiluses; any deeper and their shells implode from the pressure

5,000: Ticks an adult possum can eat in a single summer

2: Vaginas a female possum has

The whole notion of living fossils is based on a passage from On the Origin of Species. In reference to some freshwater dwelling species including the platypus and the lungfish, Charles Darwin writes, “These anomalous forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition.”

From there the idea that some organisms change less than others over time—whether because they have reached the apex of their form or encounter fewer selective pressures—took hold. The problem is, there’s no real scientific definition or criteria that gives the concept real meaning. “Although the idea of living fossils flourished after Darwin introduces the idea, it was never formally defined and was used as a catch all for apparently any organism that has an interesting fossil record,” Mark Carnall writes in the Guardian.

“Crocodiles and snapping turtles are often called living fossils, more on the basis that they look ‘prehistoric’ than any scientific grounds.”

—Mark Carnall, writing in the Guardian

Evolution is a far more subtle concept than we generally imagine, and the notion of living fossils implies that some organisms simply don’t change. At all. Even if it’s not outwardly observable, species are constantly changing on the molecular and genetic level.

Species do not stop evolving once they’ve become “perfect,” and while some species may evolve over time to better thrive in their environment, the process of evolution is not a steady march of progress toward an endpoint. Scientific literacy is a necessary skill for fully engaging in civic life, whether you’re considering climate policy or where to invest your money, and there are long lists of basic scientific concepts with robust definitions that get misused in everyday life and in the media on a regular basis. Overly broad, non-specific concepts like living fossils can detract from our collective understanding of science and scientific language.

There are way too many definitions for the term to be scientifically meaningful. Here are a few ways to get living fossil status:

• Be very old. Organisms with long fossil-record histories like horseshoe crabs and coelacanths are often called living fossils.
• Don’t ever change. Organisms change at different rates, whether that’s on the molecular or morphological level, but when species look very much the same as their fossilized relatives, like tadpole shrimp and ginkgo biloba, they’re often called living fossils.
• You’re a loner. Red pandas, aardvarks, and opossums are all singular—they have no close living relatives and their closest ancestors died out a long time ago.
• You’re “primitive.” Animals like sharks, crocodiles, and horseshoe crabs that seem like they’re throwbacks to the prehistoric world may have long lines of ancestors, but don’t we all?

Often held up as examples of living fossils, horseshoe crabs have existed in some form for over 450 million years. But while they survived the Great Dying, they may not survive the Anthropocene era.

The blood of the horseshoe crab is extremely valuable. It’s used to make tests that detect bacterial toxins in pharmaceutical products and medical devices. Horseshoe crabs are harvested, transported to labs where about a third of their blood is drained, then dumped back in the ocean. As demand for their blood had grown, their habitat has shrunk.

Horseshoe crab blood works because it’s so old. Its immune system is primitive but efficient, and its simplicity is a feature—rather than fight an infection, it immediately clots, producing an “instantaneous, visible reaction to endotoxins.” And what’s worked so well to preserve the creature is now a liability for it.

Creationists hold living fossils up as “evidence” that evolution is not real, and that the world was recently created.

If you’re looking for something to obsess over rather than the opaque category of living fossils, try EDGE species (Evolutionary Distinct and Globally Endangered), like the Plains-wanderer, the Cuban solenodon, the largetooth sawfish, and the Roti Island snake-necked turtle.