1. Where Did the Name "Living Fossil" Come From?
One of the best-known early uses is in Charles Darwin's On the Origin of Species (1859). Darwin wrote about odd animals living in fresh water, among them the platypus (Ornithorhynchus) and a South American lungfish (Lepidosiren). He wrote that such forms "may almost be called living fossils" [1].
Notice the words "almost" and "may." Darwin was being careful. His idea was that animals like these survived because they lived in a small area where competition was less severe [1].
The label was later used for many others, and today it has no strict definition. Roughly, it points to a living group that looks much like relatives known from the fossil record [4]. Let's test the idea on three examples, starting with the fish that made it famous.
2. How Was a Fish Thought Extinct Found Alive?
Coelacanths are lobe-finned fish, with fleshy fins that stick out from the body like short limbs. Their fossils go back more than 400 million years, to the Devonian period. For a long time, scientists believed the group had died out around the end of the Cretaceous, about 66 million years ago [2].
On 22 December 1938, near the Chalumna River on the east coast of South Africa, a museum curator named Marjorie Courtenay-Latimer spotted an odd fish in a local fisherman's catch. She sent drawings to J. L. B. Smith, a fish scientist at Rhodes University. His telegram back read, "Most Important Preserve Skeleton and Gills = Fish Described" [2]. In other words, he wanted her to preserve the fish so he could identify it.
A second living kind turned up in 1997, when a scientist recognized one at a fish market in Manado, on the Indonesian island of Sulawesi. It was later described as a separate species. So two species of coelacanth are alive today [2]. A group that vanished from the fossil record for tens of millions of years and then reappeared is sometimes called a "Lazarus taxon," after the man in the Bible who came back from the dead [4].
That is the story that gave "living fossil" its thrill. But a fish can look old without being old in every way, as the genetics will show in section 5. First, an animal and a tree with very different stories.
3. Is a Horseshoe Crab a Crab?
No. Horseshoe crabs are chelicerates, the group that includes spiders and scorpions. They are more closely related to those animals than to true crabs or shrimp [3]. Only four species survive: one along the Atlantic coast of North America, and three in South and East Asia [3].
Their fossil record is long. Sources give the age of the oldest relatives as about 445 or about 480 million years, and a relative called Lunataspis, around 445 million years old, already looked a lot like today's horseshoe crabs. The line that leads to the living kinds appeared much later, about 250 million years ago [3]. Their blood is blue because they carry oxygen with a copper-based protein, hemocyanin, instead of the iron-based hemoglobin we have [3].
In 2014, researchers described a fossil horseshoe crab from Poland that lived around 148 million years ago. They found no important shape differences between it and young American horseshoe crabs today. The authors suggested a new word for this long-lasting shape: "stabilomorphism," meaning a body form that stays the same for a very long time. They offered it as a way to say what the fossils show without the idea that evolution stopped [5].
4. How Did a Tree Get a Name Before Anyone Knew It Was Alive?
The dawn redwood, Metasequoia, is a tall conifer that, unlike most conifers, drops its leaves in autumn [6]. It is the reverse of the coelacanth story: the tree's fossils had a name before anyone knew that the same kind of tree was still living.
In 1941, the Japanese paleobotanist Shigeru Miki described the fossil genus Metasequoia from fossil leaves and cones [6]. In China, meanwhile, a large leafless tree had been noticed in Hubei province in the winter of 1941. Specimens were collected in the mid-1940s, and botanists realized that the tree was a living Metasequoia [7]. The exact years differ a little between sources, so treat the early-1940s dates as approximate. Seeds reached the Arnold Arboretum near Boston in January and February 1948, and packets were then sent to gardens and universities around the world [7].
Today this one species is the only living member of its genus, and it is listed as endangered in the wild [6]. Planted in parks and along streets, it is also among the easiest living fossils to look at yourself.
5. Does "Living Fossil" Mean a Species Never Changed?
No, and this is where scientists push back on the name. In 2013, researchers read the genome of the African coelacanth. They found that lungfish, not coelacanths, are the closest living fish relatives of animals with four limbs. They also found that coelacanth protein-coding genes have changed more slowly than those of four-limbed animals [8]. So even for the most famous example, the picture is mixed: slower change in some genes, but not "no change."
A body that looks the same does not mean the same species, either. Changes can build up in genes from one generation to the next, often by chance, so a living species is not necessarily the same species as a fossil relative it resembles [4]. Looking alike over a long time can also happen when natural selection keeps a body shape that already works, which is itself a kind of evolution [4]. Because of these problems, many paleontologists and evolutionary biologists avoid the term today [4].
That brings us back to the question in the title. The name is fair if you mean "this looks like its ancient relatives." It is not fair if you hear "nothing has changed inside."
6. How Can You Look for Living Fossils Yourself?
In autumn, look for dawn redwoods in a park or along a street, with an adult if you go out. Compare them with pines and spruces: this conifer turns color and drops its needles. Many aquariums and museums also show horseshoe crabs. When a sign says "living fossil," ask which parts match the old fossils, and which parts nobody can check from a fossil at all, like genes and behavior.
You could also read the free abstract of the 2014 horseshoe crab paper and see how its authors chose their words.