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Apr 27

Why living fossils need to die

I’m the proud owner of a Madagascan cycad. He or she (but never “it”, for cycads have two sexes) gets an annual decking with baubles at Christmas, but spends most of the rest of the year getting in the way of the television.

Cycas thouarsii, Christmas 2011 [CC-BY-SA-3.0 Steve Cook]

Deck the cycad with Pantone baubles, fa-la-la-la-lah la-la-la-lah

Cycads look a lot like palms, but rather than producing flowers and fruits, they make cones and bear fruitless naked seeds. This hints at their origins. They are much more closely related to pine trees and other gymnosperms, than to flowering plants like palms.

Encephalartos ferox [cc-by-sa-3.0 Steve Cook]

Fierce cycad, with cones

Cycads are a beautiful and enigmatic group, and it is a great pity that if you’ve ever been introduced to these lovely plants at all, it will probably be in the clichéd context of the title of this post: “cycads are living fossils that were eaten by dinosaurs.”

The latter is quite true: cycad seeds, like those of many plants, are eaten and distributed by dinosaurs.

Blue-tit feeding at bird-feeder, London 2012 [cc-by-sa-3.0 Steve Cook]

I’m quite aware this is neither a good photo, nor is the blue-tit feeding on cycad seeds, but even my patience has its limits

It is also quite true that the common ancestor of all cycads lived an exceedingly long time ago (maybe 260 million years ago), pre-dating the common ancestor of the blurry feathered dinosaur above and the dinosaur poster-boy Tyrannosaurus rex by as much as 70 million years.

But that doesn’t mean that cycads are living fossils.

A paper published in Science last year attempted to shed light on this. Researchers at Kew Gardens took DNA from a representative sample of those 300 living species of cycads, many of which they have growing in the Palm House (botany nerds amongst you may wish to play “palm or cycad?” if you visit it). DNA tends to accumulate changes over time at a predictable rate in particular genes in particular groups of living things. This allows you to use the number of differences in the DNA sequences from two different species to estimate how long ago those two species diverged from their common ancestor.

The two species shown above (Encephalartos ferox and Cycas thouarsii), are about as distantly related as two cycads can be. They appear to have diverged from a common ancestor that lived around 180 million years ago: about the same time that the common ancestor of blue-tits and Tyrannosaurus was strutting the Jurassic plains.

Interestingly, the six largest genera of cycads (Cycas, Encephalartos, Zamia, Macrozamia, Ceratozamia and Dioon) all seem to have rapidly diversified in the last 12 million years or so. The authors consider that “living fossil” is therefore an inappropriate term for these plants. As ever, the science was various misreported: spot the obvious mistake in this report compared to the simplified summary of the paper’s findings below.

Cycad evolution [cc-by-sa-3.0 Steve Cook, based on Nagalingum et al. 2011, DOI: 10.1126/science.1209926]

The six larger genera of cycads have all diversified rapidly in the past 12 million years, but share a much more ancient common ancestor, around 180 million years ago.

Personally though, I don’t think the paper goes far enough.

Cycads are not “living fossils”, but the reason is more fundamental than their recent spurt of diversification. “Living fossil” is – at best – an almost meaningless term, and – at worst – a thoroughly misleading one.

What exactly do people mean when they call something a “living fossil”?

“Living fossil” seems to be used to mean “a living species that appears to be the same as a species known only from fossils, and is the sole surviving representative of an archaic lineage“.

Biologists almost universally agree that all living things are descended from a population of organisms that existed some 3500 million years ago. Every bacterium is descended from this Last Universal Common Ancestor (LUCA); every human is descended from that same LUCA; the great-great-great-ur-grandmother of every cycad, dinosaur, toadstool and eyelash-mite was that same LUCA.

When cycads, or coelacanths, or horseshoe crabs are called “living fossils”, there is an implication that these are very “ancient” organisms. But this is obvious bunkum: all living organisms are the descendents of LUCA; all living organisms have been evolving for at least 3500 million years; and all living organisms are therefore the venerable relics of an archaic lineage that stretches back to a time when the Earth was young and buxom and care-free. Being a “surviving representative of an archaic lineage” is something that all living things have in common; it’s not  something specific to “living fossils”.

It’s also very difficult to judge whether a living organism is genuinely similar to a fossil form. Most of the ‘cycads’ found in the fossil record have actually turned out to belong to a quite different group of plants called the Bennettitaleans. Bennettitalean leaves look very similar to those of cycads, but their cones were embedded amongst their leaf-bases rather than borne in the whorl of leaves at the top. There are fossils that are thought to be more closely related to cycads than to any other living plants, but even these ‘true’ cycad fossils (e.g. Baenia/Nilssonia) often have dangly open cones quite different from the tight upright cones of living cycads. It’s almost as if someone deliberately chose to ignore the differences in those features that didn’t fit the nice story about cycads being decrepit relics of a bygone age.

Compare this with the damp blue-tit above. The bone structure of blue-tits (and other birds) is very similar to some theropod dinosaurs, so why don’t we refer to birds as “living fossils”? The similarity of fossil cycad and living cycad leaves is enough to get cycads branded as “living fossils” despite significant differences in their cones. So why shouldn’t similarity in the bones of fossil dinosaurs and living dinosaurs get birds branded in the same way, despite significant differences in the structure of their feathers? The choice of which features to fixate on seems quite arbitrary to me.

Even if a fossil cycad’s morphology were absolutely identical in every conceivable way to a living cycad’s, there is much that does not fossilise easily (or at all): soft tissues, behaviour, favourite TV show. Even if these invisible features were somehow recorded, it would be inconceivable that their DNA sequences would have remained unchanged for millions of years. In fact, this is very easily disproved in most cases, including the case of the cycads, as we’ve seen. Larry Moran makes the argument much more eloquently than I: evolution is not just about obvious changes in the conveniently fossilisable bits of living things, and the term “living fossil” gives an unhelpful impression of how evolution works.

So, if a “living fossil” is just “a living species that appears to be the same similar in some rather arbitrary ways (but not in others) to a species known only from fossils, and is the sole surviving representative of an archaic lineage“, then the only thing that seems to distinguishes a “living fossil” from any organism we’ve arbitrarily decided looks like a fossil from a certain angle is that a “living fossil” is somewhat alone in the world, with no close living relatives.

The 300 species of cycad (which frankly stretches “sole” to breaking point) may not share a common ancestor with any other group of plants younger than 250 million years or more. But “having no close living relatives” does not mean that cycads (or coelacanths, or horseshoe crabs…) are evolution-shy slackers, so “living fossil” is a curiously inappropriate term. If it were, there are only two things we’d need to do to make human beings a “living fossil” within the mammals. Firstly, wipe out every other species of primate on Earth (a goal that we appear to be making substantial progress towards. Go team Homo! Make Chicxulub proud!) and secondly ignore the fact that humans hold their cones upright – sorry – hold themselves upright rather than walk on all fours, like most fossil primates.

Of the 300 living species of cycads, about a third are endangered; some critically so. In addition to being pretty meaningless, “living fossil” subtly suggests that these plants have been doddering around for millions of years, simply waiting for the curtain to fall. This is hogwash. Cycads are going extinct because of unscrupulous collecting and habitat destruction, not because of some grinding evolutionary inevitability. Using terms like “living fossil” gives us an unwarranted excuse to ignore our complicity in their loss.

“Living fossil” is an arbitrary and belittling term, and it needs to die.

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