“Living fossils” abound in popular science writing. The phrase refers to modern species that are uncannily similar to extinct ones. Their bodies seem to have gone unchanged over millions of years, as if evolution took its foot off the pedal and allowed them to coast. These species are painted as either relics desperately clinging onto existence, or great survivors triumphing against the odds. They range from the famous coelacanth, to the horseshoe crab, to a new eel discovered just months ago.

But one classic example – a group of plants called the cycads – shows just how slippery the concept of the “living fossil” can be.

Cycads look superficially like palm trees, but they belong to a very different group. They first appeared on the planet around 280 million years ago, but they really hit their stride in the Jurassic and Cretaceous period, between 200 and 65 million years ago. But their time would soon be over. Out-competed by flowering plants, and suffering from the decline of their dinosaur polliantors, the cycads started to disappear.

Today, the cycads are a mere shadow of their former glory. There are just 300 species, commonly thought to have endured since their heyday in the dinosaur era. But Nathalie Nagalingum from the Arnold Arboretum of Harvard University has found evidence that this narrative is a fiction. The cycads are indeed an ancient group, but the living species aren’t much older than 12 million years. They would never have been nibbled by dinosaur teeth. Living? Yes. Fossils? Hardly.

Naganlingum sequenced a gene called PHYP from 199 living cycads and used it to build a family tree for the group. The tree showed that the modern cycad lineages have “long fuses” – most of them arose during the Cretaceous period. But these major groups only diversified into today’s species during a recent five million year window. The family tree looks like a set of rakes, with long poles representing the deep ancestry of the groups, and several tiny prongs at the end, representing the youth of the individual species.  And Naganlingum found the same pattern when she looked at different genes.

Other scientists have used genetic analyses to suggest that modern cycads are evolutionarily young, but they sampled far fewer species than Naganlingum has now done. There were other clues too. Several cycad species have very low genetic diversity, as do the insects that pollinate them, like weevils. As the cycads started branching out into new species, so did the weevils and their genes bear the legacy of these recent radiations.

This perfectly illustrates why the term “living fossil” is so tricky. The term has a rich history: it was coined by Darwin himself, referring to the platypus and the South American lungfish; Richard Dawkins uses it in his books; and working scientists use it in their papers. But the term is misleading. Modern cycads have the appearance of being ancient, but they’re recent arrivals. They retain the basic shape and form of their long-extinct ancestors, but they weren’t around at the same time.

In fact, the cycads aren’t so much living fossils as comeback kings. Greatly diminished from their Cretaceous prime, they enjoyed a second bout of expansion around 12 million years ago. They did so simultaneously across four continents – Australia, Africa, Asia and central America – as if they were racing to a common starting pistol. Naganlingum thinks that they were responding to a changing climate.

Whatever the cause, the cycad comeback was short-lived. No new species have emerged in the last two million years and two thirds of the group are now endangered. It would be wrong to call them living fossils because the “fossils” bit is wrong. Soon, the “living” part may be inaccurate too.

Reference: Nagalingum, Marshall, Quental, Rai, Little & Mathews. 2011. Recent Synchronous Radiation of a Living Fossil. Science http://dx.doi.org/10.1126/science.1209926

More on cycads: Ancient plants manipulate insects for hot, smelly sex

Photos by Nathalie Nagalingum

October 20th, 2011 by Ed Yong in Evolution, Plants | 3 comments | RSS feed | Trackback >