Thu October 20, 2011
'Living Fossils' Just A Branch On Cycad Family Tree
Although dinosaurs died out 65 million years ago, there are still thought to be a few species left over from those days. Plants called cycads are among these rare "living fossils" — they have remained pretty much unchanged for more than 300 million years, but a study in Science magazine suggests that glamorous title may not be deserved.
There's no time machine in Washington, D.C., but Harvard botanist Sarah Mathews leads me to what's arguably the next best thing — a room made of glass in the U.S. Botanic Garden, just downhill from the U.S. Capitol.
The sign says "The Garden Primeval — The First Land Plants."
Right away we see something that looks like a fern growing out of the top of a palm trunk. But it's not a fern or a palm. In fact, it's more closely related to a pine tree. Cycads produce seeds but not flowers. They evolved along with dinosaurs, which presumably munched them for lunch. So they've earned the title living fossil.
But "that assumption began to break down as we began sequencing DNA," Mathews says.
She and her colleagues — notably Nathalie Nagalingum from the Royal Botanic Gardens in Sydney — have used that DNA to reconstruct the "family tree" of cycads. They find that the "trunk" of the family tree may reach back 300 million years, but the "branches," today's 300 species, actually burst onto the scene about 12 million years ago.
"And then it looks like around the world on multiple continents, cycads became more species-rich," Mathews says.
What caused that sudden burst of new species?
"That's the really fun puzzle of course," she says.
It's probably not a coincidence that other plants also put forth a burst of new species around that time, including cacti, ice-plants and agave. Mathews suspects climate change played a role.
"There was drying out and cooling going on, globally," she says.
This research is part of a broader effort to understand how all plants — most notably flowering plants — evolved. That story is gradually taking shape as scientists study more and more of the DNA from plants.
Of course, you might argue this research has some broader philosophical repercussions as well. By finding that these species of cycads are just 12 million years old — and so were not survivors from the days of the dinosaurs — has Mathew's team demoted these species from their lofty status as living fossils?
She says not.
"I think that we've actually found some interesting patterns for people who didn't think much about cycads before," she says.
What about people who think a lot about cycads? Bart Schutzman edits the Cycad Society's journal (global circulation: 500 copies). He's attracted to these plants because he feels a primal bond with this ancient species. And he says the news does not rock his world. Today's cycads still predate human species, and by a lot.
"What's the difference between old, older and very old, and very, very old? I mean they're all still very old," he says with a chuckle.
As for the moniker, living fossil?
"It won't stop people from glamorizing the cycads as the living fossils because their lineage extends so far back," Schutzman says.
So here's a little good news from Washington: A walk through the "Garden Primeval" greenhouse still offers a reasonable glimpse of foliage from the days of the dinosaurs, though the species themselves don't have quite the same bragging rights.