A living fossil is a term used for a species that still exists today and looks similar to related species that are only known from fossils. However, scientists no longer use this term because it can be misleading. For a species to be called a living fossil, the fossil species it resembles must be much older than the time when the living species first appeared. Living fossils are often from groups with few species, but this is not always the case. Although the body structure of a living fossil may look similar to its ancient relatives, it is not the same species because changes in genes over time would have altered its chromosomes.
Living fossils show little change in their appearance over very long periods of time. Some people incorrectly say that living fossils have not changed much since they first appeared in the fossil record, claiming they have not evolved much in their physical traits or genetic makeup. However, scientific research has shown these claims to be false.
Small changes in the appearance of living fossils are sometimes mistaken as a lack of evolution. In reality, these changes are examples of stabilizing selection, a type of evolution that helps keep certain traits unchanged over time.
Scientists today no longer use the term "living fossil" because it is considered outdated.
Characteristics
Living fossils have two main traits, though some may have a third. The first two traits are necessary to recognize a species as a living fossil. Some scientists also consider the third trait, while others only mention it as a common feature.
These traits are not clearly defined or measured, but modern methods can show how little change happens in some species over time. Species that stay the same for very short periods are not called living fossils. However, the time period over which a species must remain unchanged to be considered a living fossil is not clearly defined.
The term "living fossil" is often misunderstood, especially in popular media, where it is sometimes used without clear meaning. In scientific writing, the term is rarely used and must be applied carefully. It has also been used in different ways by different scientists.
A concept that might be confused with "living fossil" is "Lazarus taxon." A Lazarus taxon is a species or group of species that suddenly appears again in the fossil record or in nature, as if it had "come back to life." In contrast, a living fossil is a species or lineage that has changed very little over a long time, even though it is still alive today. Because of genetic changes over time, the DNA of modern species is different from that of their ancient, similar-looking ancestors. These species likely cannot reproduce with their ancestors and are not the same species.
The average time a species exists before disappearing varies among different groups of organisms but is usually about 2 to 3 million years. A species once thought to be extinct may be called a Lazarus taxon if it is later found to still exist. An example is the Coelacanth, a group of fish that disappeared from the fossil record about 80 million years ago. When a living species of Coelacanth was discovered in 1938, it was called a Lazarus taxon. Some scientists later debated whether it should also be called a living fossil.
Coelacanths were not seen in the fossil record for 80 million years, but they are now known to still exist. If they show little change in their physical features, they can be called living fossils. This definition is based on fossil evidence and does not depend on whether the species was affected by natural selection, which all living species experience.
The lack of change in physical features can cause confusion. Fossils usually only preserve the general shape of an organism, not details about its body functions. Even species that seem unchanged over time have likely changed in ways that are not visible in fossils. Noncoding DNA, which does not affect an organism's traits, also changes over time. So, a species that appears unchanged in fossils may still have changed in ways not visible in the fossil record. This does not mean the species stopped evolving or that natural selection stopped acting on it.
Some living fossils were first known from fossils before living examples were found. Examples include species that were originally described from fossils but are now known to have living relatives. Other living fossils are single species with no close living relatives, but they are the last surviving members of large groups that were common in the past. These species were first described from fossils and later found alive.
Being a living fossil does not mean a species must have all the original features of its ancient ancestors. While living fossils are often described as showing little change in physical traits, this does not mean they are exactly the same as their ancestors, even distant ones.
Some living fossils are the last survivors of once-diverse groups of species. However, not all species from ancient groups are called living fossils. For example, oxpeckers are unique birds that appear to be the only surviving members of an ancient group related to starlings and mockingbirds.
Evolution and living fossils
The term "living fossil" is often used to describe species or groups of related species that show very little change in their physical appearance over long periods of time. Scientists have proposed several possible reasons for this lack of change. Early studies focused on how long a species could survive rather than how quickly it changed. More recent research looks at how traits change over time, but most studies have examined groups that rapidly diversified into many new species rather than those considered living fossils. As a result, little is known about the processes that lead to living fossils or how often they occur. Some recent studies have found that certain groups show very slow changes in their traits and environments, even though they create many new species. This is called a "non-adaptive radiation," which means species diversify without adapting to different environments. Staying adapted to a single type of environment is often given as a reason for physical traits remaining unchanged. However, research on very slow evolutionary rates has received less attention than research on fast rates.
Living fossils are not expected to have very slow changes in their genetic material, and some studies confirm this. For example, in the case of tadpole shrimp (Triops), one study states, "Our work shows that organisms with simple body plans are constantly changing and adapting to new conditions… I would prefer to stop using the term 'living fossil,' as it can be misleading." Some scientists suggest using the term "stabilomorph" instead, which refers to a situation where organisms with limited genetic differences successfully adapt to their environment without needing many different physical forms. This adaptation reduces the need for many different physical traits and allows species to survive for long periods.
Recent studies have questioned whether coelacanths, a group of fish, are truly physically conservative, as their ancient fossils do not support this idea. Additionally, research showing that coelacanths evolve slowly at the genetic level may be biased by the assumption that they are living fossils. Recent findings show that the genomes of two living coelacanth species contain genetic changes that happened after they split into separate species, suggesting their DNA has not remained completely unchanged. These findings challenge the idea that coelacanth DNA has remained stable over time but do not affect the idea that their physical traits change very slowly.
History
The term "living fossil" was first used by Charles Darwin in his book On the Origin of Species from 1859. He discussed this idea while studying the platypus (Ornithorhynchus) and the South American lungfish (Lepidosiren):
Freshwater areas, when combined, cover a small region compared to the ocean or land. Because of this, competition among freshwater species has been less intense than in other environments. As a result, new species have formed more slowly, and older species have been replaced more slowly. In freshwater, we find seven types of Ganoid fish, which are leftovers from a once-dominant group. Also, in freshwater, we find unusual species like the platypus and lungfish. These species connect groups of animals that are now very different. These unusual species are sometimes called "living fossils" because they have survived for a long time. They have endured because they live in limited areas and face less competition.
— On the Origin of Species, 1859
A living taxon that has existed for a large part of Earth's history.
The Australian lungfish (Neoceratodus fosteri), also called the Queensland lungfish, is an example of this. Fossils that look like modern Queensland lungfish are over 100 million years old. Today’s lungfish have existed as a species for nearly 30 million years. The nurse shark has existed for more than 112 million years, making it one of the oldest, if not the oldest, living vertebrate species.
A living taxon that has remained physically and/or biologically similar to a fossil taxon for a long time (called morphological stasis).
A living taxon with many traits that are considered primitive. This is a neutral definition but does not clearly state whether the taxon is truly ancient or simply has many basic features. Some living fossils may have many advanced traits instead of being primitive.
Any of the above definitions, but the species also lives in a limited area that has remained unchanged over time.
Some scientists believe that species with wide distributions, like Triops cancriformis, are not true living fossils. Triops cancriformis has lived since the Triassic period and still exists today. However, Triassic fossils of this species had fewer body parts, mostly just a shell, and have not been studied thoroughly since 1938.
Any of the first three definitions, but the group also has few species (low diversity).
Oxpeckers look somewhat like starlings because they share basic traits. However, they are uniquely adapted to feed on parasites and blood from large mammals. This has made it hard to understand their relationships. Oxpeckers belong to a group that includes starlings and mockingbirds but are the oldest of these groups. Geography suggests oxpeckers originated in eastern Asia and later moved to Africa, where they now live in a limited area.
The two living species of oxpeckers seem to represent a group that is now extinct and very old. This is likely because the oxpecker lineage never lived in areas where small bird bones could be preserved as fossils. However, fossils of ancient oxpeckers might one day be found, allowing scientists to test this idea.
In 2017, scientists proposed a specific way to define "living fossils." Under this definition, a living fossil is a group that evolves slowly and has traits near the middle of the range of physical differences among related species. However, some scientists have criticized the accuracy of the methods used to classify tuatara as living fossils under this definition. The original researchers responded to these criticisms.