“Living fossils”—organisms from horseshoe crabs and coelocanths to ginkgos and horsetails—look much like their prehistoric ancestors. They have been praised on both sides of the ongoing evolution argument by Charles Darwin (who coined the term) as glimpses into the ancient world and by creationists as evidence that not all species change over time. But the concept is often misunderstood and of limited use, say Hampden-Sydney College biologists Alexander J. Werth and William A. Shear. Their argument, supported by many examples, was published in the November-December 2014 issue of American Scientist.
What are “living fossils”? No universal definition exists, and none could cover all of the species to which the moniker has been applied. Werth and Shear espouse their classification as organisms that “superficially resemble their predecessors as members of ancient genetic lines that have not been fully extinguished.” They are not the same species as their predecessors, and are unlikely to be truly identical. Most are now relatively rare, and form taxonomic groups with low diversity and no close living relatives, although their ancestors may have been diverse and abundant. This excludes an array of ubiquitous and little-changed organisms such as jellyfish, earthworms, and many bacteria.
Even this definition belies the limits of our ability to learn just how much any organism differs from its extinct ancestors. Fossilization preserves anatomy, sometimes quite complete, but rarely allows access to DNA (known to mutate at a steady rate in living lineages, often with no apparent effect), biochemistry, or behavior. As Werth and Shear note, this highlights the general paleontological difficulty in distinguishing one species from another; the ability to reproduce only with conspecifics—the “biological species concept” by which living species are usually defined—can be limited by many factors not detectable in fossils. And every species displays a mix of newer and older traits, so that “although no organism is a complete living fossil, all are to some extent living fossils,” with human researchers deciding where to draw the line.
Still, some traits have clearly lingered exceptionally long in certain species, often through great environmental change. Identifying such characters in “living fossils” can inspire investigations into the reasons for their longevity. Some such survivors are generalists, able to live in diverse environments, but others specialize in using resources for which they haven’t yet been out-competed. Natural selection always favors what works in the present, but doesn’t always bring out the best on its own. Some species could have unused potential for bursts of diversification triggered by events such as mutation of a mutation-suppressing gene (observed in fruit flies) or evolution of a trait that opens many more possibilities for specialization, such as spiders’ sticky webs. Artificial though their designation is, “living fossils” can greatly aid our understanding of evolution’s complexity.