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Scientists are looking to end how we categorize living fossils, and in doing so, give the phrase new life.

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Sources:

https://www.sciencedirect.com/science/article/pii/S0098847219305490#bib0040
https://palaeo-electronica.org/content/2018/2194-quantifying-the-living-fossil
https://www.smithsonianmag.com/smart-news/theres-no-such-thing-as-a-living-fossil-14360996/
https://www.nytimes.com/2019/06/03/science/crocodiles-dinosaurs-metabolism.html
https://www.nationalgeographic.com/science/article/crocodiles-are-not-living-fossils
https://www.nature.com/articles/s42003-020-01561-5
https://www.sciencedaily.com/releases/2021/01/210107083751.htm
http://www.sci-news.com/biology/coelacanth-genes-09341.html
https://www.eurekalert.org/pub_releases/2021-02/uot-nal020321.php
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC23015/
https://www.bbc.com/news/science-environment-38057741
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https://www.iucnredlist.org/search?query=coelacanth&searchType=species
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https://theconversation.com/we-scanned-one-of-our-closest-cousins-the-coelacanth-to-learn-how-its-brain-grows-115147
https://www.nature.com/articles/s41467-019-12133-5
https://www.iucnredlist.org/species/32353/9700472


Images:
https://www.istockphoto.com/photo/croc-gm1057125426-282521010

https://www.istockphoto.com/photo/horseshoe-crab-sitting-on-the-beach-gm158589282-22493028
https://commons.wikimedia.org/wiki/File:Deinosuchus_hatcheri_-_Natural_History_Museum_of_Utah_-_DSC07251.JPG
https://en.wikipedia.org/wiki/File:Fossil_horseshoe_crab_dead_in_its_tracks.jpg
https://www.istockphoto.com/photo/crocodile-skull-gm173017618-7176651
https://www.istockphoto.com/photo/american-alligator-swimming-in-the-spring-swamp-gm1171368832-324498924
https://www.istockphoto.com/photo/great-white-shark-smile-gm533130811-56157656
https://www.istockphoto.com/photo/close-up-of-ginkgo-biloba-leaves-back-lit-gm97119026-6981768
https://www.eurekalert.org/multimedia/pub/255382.php
https://www.istockphoto.com/photo/coelacanth-gm1307878517-398020225
https://www.istockphoto.com/photo/horseshoe-crab-on-the-beach-gm518653826-90151855
https://www.eurekalert.org/multimedia/pub/112655.php
https://www.istockphoto.com/photo/horseshoe-crab-and-sand-gm1150525195-311479706
https://www.istockphoto.com/photo/saltwater-crocodile-leap-out-of-the-water-gm533923648-94636473
Thanks to Brilliant for supporting this episode of SciShow.

Go to Brilliant.org/SciShow to learn about their Scientific Thinking course and get 20% off an annual premium subscription. [ intro ]. Back when Charles Darwin was working on his theory of evolution, he noticed that some species seemed to change very little over time.

Creatures like crocodiles and horseshoe crabs have looked basically the same for millions of years. So he called these types of organisms living fossils. And for better or worse, this term has stuck.

Over the years though, scientists have argued that looks alone can’t really tell you how much or little an organism has actually changed over millions of years. And the idea of the “living fossil” became kind of contentious. Recently though some scientists started suggesting that more rigorous ways of identifying so-called living fossils could actually be really useful.

But not so much to learn about the creatures of the past, as to reframe the way we talk about those of the present. And in doing so, we might be able to give “living fossils” a new life. Now, even Darwin admitted that “living fossil” was a sort of fanciful term when he coined it.

But in his eyes, the useful thing about it was that it identified organisms that could give scientists a glimpse into the past. He thought that researchers could study these organisms and form a pretty good idea of what their ancient counterparts had been like. The problem is, that was only partly true.

We know today that, while some creatures look pretty much the same, their genomes have changed plenty. Because all creatures are changing all the time. No one gets to sidestep evolution—not even modern-day humans.

Now it’s not always obvious, because not all evolutionary changes lead to visible differences. Take, for example, the horseshoe crab. This organism has long been called a living fossil because it looks almost identical to its fossilized ancestors.

But when researchers analyzed their genetics, they found that these little guys have gone through a similar amount of genetic change to many species alive today. It just hasn’t changed their shape. Similarly, today’s crocodilians, including species like crocodiles and alligators, haven’t changed much in looks in the last two hundred million years.

But their genomes have changed a lot over time as the climate has gone through different phases. So they’re not as fossil-like as we once thought either. In fact, thanks to advancements in genetic testing, this type of revelation has happened over and over and over again, with many so-called “living fossils.” And this is because scientists can now follow creatures’ genetic trail through evolutionary time, instead of drawing conclusions based on looks alone.

And this doesn’t just call into question a bunch of organisms’ status as living fossils. It shows that there’s a problem with the entire concept of treating organisms as time capsules from the ancient evolutionary past. But, as problematic as the label is, some scientists do think it’s worth fighting for.

Not as a label for species that are virtually unchanged since the ancient past because we now know those don’t exist. Instead, at least one research group has proposed using it as a way to identify and protect organisms from ancient lineages. Some researchers believe that these species add diversity to the tree of life— which overall is a good thing for our planet’s ecosystems.~ And on top of that, they believe there’s a lot to learn from these organisms, especially in the field of genetics.

The argument goes like this: even though these creatures have changed plenty over the years, they’re still directly related to ancient organisms. That means they can be studied to answer questions like how quickly the lineage evolved. And if researchers can pin down the timing of these changes, that can even offer clues about how the environment was changing at the time.

So some scientists think that organisms with direct ties to the ancient past could give us a better picture of both evolutionary and geologic history. The problem is, as useful as a label like this might be, there’s currently no standard set of criteria that can tell you whether or not a given species or group qualifies as a living fossil in the first place. Scientists have basically just… used their own judgment.

And the term appears willy-nilly throughout hundreds of scientific publications. So, to be useful as a diagnostic tool, the term also needs to be standardized. With that in mind, a couple of research groups around the world have approached this issue in a few different ways.

One group of researchers in the UK has developed a methodical approach that uses what’s called the Evolutionary Performance Index, or EPI. This is meant to identify living organisms with relatively close ties to ancient lineages. To create this index, the team reviewed tons of scientific literature so they could narrow down in on some of the most common ways scientists have used the term “living fossil” in the past.

This gave them a few key metrics. The first is age: Older taxonomic groups are usually considered more fossil-like. As organisms diversify, they don’t fit into the same groups they once did..

Then there’s the number of species in the group:. More species signifies more change to a particular group over time. So this group of organisms has changed a bunch from its early ancestors and from each other, and then scientists will have classified them as different species and they will be less living-fossil-like.

And finally, how long has it been since the group’s branch of the evolutionary tree first appeared. A branch with fewer forks over time is likely to be more fossil-like. The researchers combined these three different elements into a single equation that would spit out an EPI value for any given species.

And then they ran it… on 24,000 groups of organisms across the tree of life. They were able to then generate a ranked list of animals and plants, showing which ones came the closest to matching their standards for living fossils. And the result was a bit of a shakeup.

On the one hand, crocodilians did keep their status despite not being perfect replicas of their ancient counterparts. On the other hand, creatures like sharks and rays which have long been viewed as living fossils have changed too much over time to make the cut. However, their parent group, which includes all cartilaginous fish, is pretty living-fossil-like compared to other fish lineages.

Now aside from stirring up drama in the world of living fossils, these new standards are also meant to help scientists use the label in a more consistent and useful way. A second group of researchers in Spain has taken a slightly different approach to the problem, focusing specifically on living fossils in danger of going extinct. In a 2020 paper, they outlined a process for identifying what they call endangered living fossils, or ELFs.

For an organism to qualify as an ELF, it has to meet three criteria. First, it has to be considered endangered or critically endangered, according to the International Union for Conservation of Nature. Second, the organism needs to be evolutionarily distinct— meaning it’s not closely related to any other living species.

And third, it needs to come from an ancient lineage, which they defined as 5.3 million years or older. Now so far, they’ve only applied this terminology to plants they were already working with on the Iberian Peninsula and around the Mediterranean regions of Europe. But they propose that it could one day be applied more broadly— and to animals as well.

And the organisms that qualify, they say, would be especially important to protect. For instance, the ginkgo is the only remaining species of a once-diverse lineage of trees. Its ancestors survived major climate fluctuations, including multiple glaciation periods.

And the species we know today even survived the nuclear fallout of the atomic bomb in. Hiroshima. But today, in spite of all it’s survived the ginkgo is now endangered.

And since it’s the sole survivor of an ancient lineage, it’s a species that might fit within the researchers’ criteria for an endangered living fossil. If the ELF concept were expanded to animals, it would likely also apply to two species of lobe-finned fish called Coelacanths. These, along with the lungfishes, are the sole surviving species of an ancient line of lobe-finned fish.

In fact, up until the discovery of the first species in 1938, researchers believed that the coelacanth lineage had gone extinct seventy million years ago. And although some are still around, the two surviving species of coelacanths are either vulnerable or endangered. So far, the label “endangered living fossil” has only been used by this one Spanish research team.

But the researchers hope that eventually it could help scientists and environmentalists prioritize certain species in biodiversity research and conservation programs. Now the main difference with these more modern designations is that they’re not meant to imply that a species hasn’t changed at all—because every species has. Like, even though the ginkgo’s looks haven’t changed much over millions of years its genome has grown a ton.

It’s expanded to include all sorts of genes that provide the ginkgo with a wide range of defenses. And that’s why it’s such a survivor! Today, the genome of the ginkgo is gigantic— more than triple the size of the human genome.

And it’s basically an arsenal of defenses against all kinds of threats, like fungi, bacteria, insects, even atomic radiation. So it’s well understood that these trees aren’t just perfect replicas of ancient ginkgos. And have changed a bunch too.

Research shows they’ve acquired sixty-two new genes over the past ten million years. In particular, the gene sequences researchers have uncovered suggest that these fish have been exchanging genetic material with several other species over time. It all comes in the same old package, but on the level of its DNA, the coelacanth is not the same creature at all.

But that’s okay, because a label like ELF has the goal of identifying an organism as a candidate for special protection, not attempting to identify it as a time capsule from the past. These efforts by researchers to redefine the term living fossil could end up being valuable tools for the scientific community in the future. Like every living fossil, scientists’ vocabulary also has to evolve sometimes to fit the needs of the times.

By standardizing the term living fossil, scientists could remove the contention around the terminology. And application of the ELF label could one day bring about greater protections for the organisms that need it. And that way, maybe living fossils can stick around even longer— long live living fossils!

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