Living Fossils

The term "living fossil" is a bit of a misleading one. It sounds like it is referring to a species previously known only from the fossil record which has somehow survived into the present, a species unaffected by evolution or extinction, but instead remaining in a kind of stasis for thousands or even millions of years. This, anyways, seems to be the popular understanding of the term. The truth, however, is that while this definition of a living fossil may be excellent fodder for adventure novels and horror movies, it doesn't describe any real organisms. "Actual fossils," as noted by biologists Didier Casane and Patrick Laurenti (2013), "are dead" and what we call living fossils today are not somehow immune to evolution. 

Providing a more accurate definition of a living fossil is a bit tricky, but I would argue that in general, the term refers to organisms that are (1) the only living representatives of a mostly extinct taxon and (2) have significant morphological similarities to their long extinct relatives. Probably the best-known examples are the coelacanths, an order of lobe-finned fish which first appeared around 410 million years ago, and which were thought to have gone extinct with the dinosaurs until two living species were discovered* in 1938 and 1997 respectively. Modern coelacanths don't actually appear to be quite as morphologically similar to their extinct relatives as was once thought, but they are certainly the only living examples of what was once a much more diverse order, and so I think that they can still be considered living fossils. 

A West Indian Ocean coelacanth (Latimeria chalumnae), one of only two surviving coelacanth species (1).

For living fossils that do retain significant morphological similarities to their extinct relatives, it's important to remember that this is not because they all belong to one species that has just survived unchanged for millions of years. Over such large time scales, genetic drift - that is, changes in allele frequencies due to chance events - would still cause significant genomic changes, enough for one species to phase into another over the generations. Nor is it the case that living fossils are somehow not being acted upon by natural selection; rather, the selection pressures that exist are just reenforcing morphological stasis. It's also worth noting that just because a living fossil appears morphologically similar to its ancient relatives doesn't mean that those extinct species were not very different in other ways. Many things, like organs and behavior, do not fossilize well and these could be very different between living and extinct organisms.

With these qualifications in mind, living fossils are still very interesting and can sometimes even give us limited insights into the lives of their extinct relatives and the story of life on earth more generally. Coelacanths, for example, seem to be less closely related than lung fish to the lineage of lobe-finned fish that first evolved to live and walk on land, but their morphology and genetic code can still help to inform theories about what certain steps along that journey may have looked like (Amemiya et al. 2013). On the more poetic side of things, living fossils stir the imagination, prodding us to consider the shuffling history of life and how every species is successful until the moment it isn't.

Below, I have provided some details about the natural histories of two living fossils that can be found in New England. Both are pretty common, the kind of thing that you can find by your front stoop or by the side of a highway or some train tracks. Look around for them, imagine the ancient worlds their extinct relatives once dominated, and be blown away by the sheer mass of all the years that came before us, and which will pass by when we are gone.

Ginkgo (Ginkgo biloba):

Unlike the coelacanth, the gingko tree fulfills both of the requirements I listed above for being a living fossil. It is the only surviving member, not only of its genus, but of the whole order Ginkgoales, which first emerged in the Permian period over 250 million years ago. The genus ginkgo itself emerged during the Jurassic, when species very similar to the modern ginkgo thrived alongside the dinosaurs (Cohn 2013). 

The unique, fan-shaped leaves of a ginkgo tree.

Ginkgoes are gymnosperms, meaning that despite their broad leaves, they are actually more closely related to conifers than to flowering plants (angiosperms) such as maples and oaks. Like all gymnosperms, ginkgoes do not produce flowers; instead, males produce pollen cones and females produce pairs of ovules on stalks which grow at the base of the leaves in early spring (Little et al. 1980; Moore and Wilson). Fertilization occurs via wind pollination and the resulting seeds are surrounded by a three-layer seed coat. The resulting structure looks like a fruit, but technically isn't one - fruits are only formed by angiosperms and are actually the plant's ovaries. Gymnosperms (literally, "naked seed") do not produce seeds surrounded by ovaries (Glasgow and Hooper 2020). The ginkgo's seeds are reportedly quite tasty (though I've never had one personally) and are commonly used in Chinese cuisine. The "fruit," on the other hand, is a bit of a different story, as the outer most layer of the seed coat is filled with butyric acid, causing it to smell like rancid butter or vomit upon falling to the ground and starting to rot. In order to avoid the smell, horticulturalists will often plant only male or only female ginkgoes in the same area, so that they aren't able to fertilize and produce fruit. 

One of the gingko trees growing in my parent's neighborhood outside New Haven, CT.

In spite of its ancient lineage, the story of the modern gingko is one which is intimately tied to humanity. As the rest of the Gingko genus declined and went extinct, the modern gingko managed to find a small refuge in eastern China. There, it became a sacred tree in both the Confucian and Buddhist traditions and was planted widely across East Asia as a result, possible saving the species from extinction (Little et al. 1980). Today, gingkoes are planted all over the world, especially in urban areas, to which they are uniquely well suited due to their high tolerance for smoke, dust, and pollution (Moore and Wilson; Little et al. 1980). Yet in spite of its now global distribution, the ginkgo is still listed by the IUCN as endangered because there are so few known wild populations remaining. Unless these populations can be built up, it seems that the fate of not only Gingko biloba, but its whole order may well be bound to that of humanity. 

Field Horsetail (Equisetum arvense):

Ginkgoes may be ancient, but they are not nearly so old as the horsetails, the only surviving member of the once diverse class Equisetopsida. Horsetails first began to emerge in the late Devonian, around 350 million years ago, and were an important part of the swampy, forest ecosystems of that period and of the later Carboniferous (Elgorriaga et al. 2018). During this time, horsetails ranged in size from small, herbaceous plants to species as big as modern trees (Wagner et al. 2016), and actually make up much of the ancient dead plant matter that was eventually compressed beneath the earth's surface into coal (Del Trici 2020). Today, only one genus of horsetails (Equisetum) survives, the 15 members of which are very similar to their extinct relatives from the Jurassic, the period during which the genus first appeared. New England is home to a couple of different species of horsetails, but by far the most common is the field horsetail.

An infertile, vegetative horsetail stalk (left), alongside a fertile sporophyte (right).

Like ferns, horsetails have a very complicated, very interesting life cycle. If you visit a patch of horsetails in the early spring, you will see two kinds of structures coming up from the ground. The first, and more numerous of the two, will be the infertile, vegetative stalks, which will basically look like any other herbaceous plant with green leaves that perform photosynthesis. The second will be a fertile stalk with a pale cone on the top. This is called a sporophyte and, as the name suggests, it produces spores. A spore has the same dispersal function as a seed, but is produced asexually, meaning that it is a genetic clone of the parent. Horsetail spores are dispersed by the wind and, if they land in a suitable spot, they will sprout into yet another, near microscopic structure called a gametophyte. Again, as the name suggests, gametophytes produce gametes, meaning sperm and eggs. When it rains, the sperm produced by a gametophyte will swim through the film of water that forms on the ground and, if they are lucky, will find another gametophyte with eggs to fertilize. These fertilized eggs will, in turn, grown into a new, vegetative, sporophyte producing horsetail. Thus, horsetails reproduce both sexually through the process just described and asexually by extending their rhizomes through the soil and producing vegetative clones (Wagner et al. 2016; Del Tredici 2020).

Field horsetail growing along a well-worn trail near a wetland in northwestern CT.

Field horsetails evolved to live in herbaceous wetlands and meadows, but like ginkgoes, they also do surprisingly well in human-dominated environments such as poorly drained roadsides and in the ballast of railroad tracks. Even when these areas are sprayed with herbicides, horsetails often remain where over plants die back because they are resistant to many chemicals designed to kill flowering plants. Field horsetails are also known for their ability to concentrate silica and various heavy metals in their tissues. This means that they can be planted on contaminated sites as part of phytoremediation schemes in which plants are used to suck contaminates out of the soil and are then harvested for disposal elsewhere (Del Tredici 2020). Unlike the gingko, wild field horsetails are pretty abundant and likely won't be going anywhere anytime soon. 

*When I talk about a species' "discovery" on this blog, what I am typically referring to is the first time that a species is noted by western taxonomists and fit into the system of Linnean classification. This involves a scientist finding the species; killing, photographing, or otherwise documenting a type specimen; and publishing a formal description in a peer reviewed journal. It does not mean that this event was the first time that anybody ever lay eyes on the relevant species. The West Indian Ocean coelacanth (Latimeria chalumnae), for example, had been known by local people in South Africa and the Comoros Islands for a long time before it was "discovered" by western scientists at a fish market. 

Sources:

Amemiya, C. T., Alföldi, J., Lee, A. P., Fan, S., Philippe, H., MacCallum, I., ... & Lindblad-Toh, K. (2013). The African coelacanth genome provides insights into tetrapod evolution. Nature, 496(7445), 311-316.

Casane, D., & Laurenti, P. (2013). Why coelacanths are not ‘living fossils’ A review of molecular and morphological data. Bioessays, 35(4), 332-338.

Cohn, R., (2013). The life story of the oldest tree on earth. YaleEnvironment360. https://e360.yale.edu/features/peter_crane_history_of_ginkgo_earths_oldest_tree

Del Tredici, P. (2020). Wild urban plants of the northeast: a field guide. Cornnell University Press, 34-35.

Elgorriaga, A., Escapa, I. H., Rothwell, G. W., Tomescu, A. M., & Rubén Cúneo, N. (2018). Origin of Equisetum: Evolution of horsetails (Equisetales) within the major euphyllophyte clade Sphenopsida. American Journal of Botany, 105(8), 1286-1303.

Glasgow, T., & Hooper, J. (2020). Do ginkgo trees produce fruit? NC Cooperative Extension. https://craven.ces.ncsu.edu/2020/12/do-ginkgo-trees-produce-fruit/

Little, E.L., Bullaty, S., Lomeo, A., Rayfield, S. and Buehl, O. (1980). National Audubon Society field guide to trees, eastern region. Alfred A. Knopf, 271-272. 

Moore, L. M., & Wilson, J. D. W. (N.A.). Gingko plant guide. United States Department of Agriculture. https://view.officeapps.live.com/op/view.aspx?src=https%3A%2F%2Fplants.usda.gov%2FDocumentLibrary%2Fplantguide%2Fdoc%2Fpg_ginkg.docx&wdOrigin=BROWSELINK 

Wagner, W. H., Gifford, E. M., & Mickel, J. T. (2016). Equisetopsida. Encyclopedia Britannica. https://www.britannica.com/plant/Equisetidae 

Photo Credits:

(1) Coelacanth off Pumula on the KwaZulu-Natal South Coast, South Africa, on 22 November 2019 - by Bruce A. S. Henderson CC BY 4.0