Probing the History of the Planet Through the Genes of Shy Creatures

Creep-out! A harmless, but ancient (in form) daddy longlegs from Brazil.

They are in your basement, or in your yard, hiding in the fallen leaves at the foot of your trees. They are living relics, walking the earth virtually unchanged since they first appeared 400 million years ago-- about twice as long ago as the first dinosaurs. They are hiding in plain sight, but in their genes they hold a record of the deep history of the planet and its landmasses.

And they are also the reason National Geographic helped send UNC Charlotte bioinformatics postdoctoral researcher Ronald Clouse to the Philippines this summer, and asked him to blog about the experience.

These unusual animals are Opiliones, otherwise known as daddy-long-legs or harvestmen. Though most of us confuse them with spiders (there are some varieties of spider that are called “daddy-long-legs”) they are actually a significantly different group of arachnids more closely related to scorpions, from which they diverged a little over 400 million years ago, shortly after scorpion ancestors first came on land. Contrary to their popular name, not all daddy-long-legs have long legs – those Clouse will be studying (Cyphophthalmi, or “mite harvestmen”) look like tiny short-legged spiders and live in humid leaf litter, where they tear up and eat plant matter and even tinier insects with the miniscule claws they have in place of spider fangs.

One of Clouse's litter-dwelling "daddy longlegs," greatly enlarged.

They are obscure animals, and though widespread and probably present nearly everywhere on the planet, there is a lot we don’t know about them. Some of the things that we do know about them, however, make them important to science, Clouse points out. “We think they are exciting animals, we think they give us a lot of great information about deep, deep history,” he said.

 First of all, they have passed through a very long stretch of time virtually unchanged. “Daddy-long-legs evolved from scorpions – they are sister groups – and scorpions started to come out on land about 425 million years ago and at 400 million years ago there is a beautiful fossil from Scotland of a daddy-long-legs that looks almost exactly like the ones in your basement today,” he said.

What may just seem to you like a garden spider is, in fact, a living fossil
that lived on this land before your ancestors were mammals!

Clouse notes that Opiliones seem to have quickly found land niches everywhere and then remained happily in them. “We have a fossil of the southeast Asian ones from 100 million years ago in amber,” he said. “It looks like they first showed up from 425 to 400 million years ago and they quickly evolved into these really elaborate morphologies that we have today. And it was another 200 million years before the dinosaurs showed up, and the dinosaurs blink out after a little more than a hundred million years. These guys have been hanging tough the whole time.”

Second of all, it turns out that daddy-long-legs have perfected a lifestyle that makes them extreme homebodies.

“The reason we like this group of animals is they don’t go anywhere in their lives and when we find them in the forest floor in the leaf litter, even if we find a bunch of them, there will be a completely different species a few kilometers over. They are very highly local,” he said.

“What makes them so localized is that they really require humid leaf litter. They don’t like leaf litter that gets too wet or too dry. And so they like pristine forest, deep leaf litter where there is the right kind of layer for them.

“So why didn’t they just evolve the ability to not need that humid leaf litter?” Clouse asked. “Because they also have behavioral issues where they just don’t go anywhere. Even super long-legged daddy longlegs that have the ability to move around a lot, they also don’t go anywhere. During the day they are under a log and at night they walk out a couple of meters and they will sit there all night.”

Clouse knows that Opiliones are confirmed stay-at-home types thanks to bioinformatics. “When we sequence their DNA we find that all the ones in this forest, even though some groups may be spread out over wider areas, when you look at it population by population, there is almost no gene flow,” he noted. “Everyone here has one set of sequences, everyone there has a completely different set of sequences.”

And the small, leaf litter dwelling types turn out to be even more localized.

“For these little guys – we were down in Florida sequencing and we found that just a few meters away the sequences were different. When we find them in the forest floor in the leaf litter, even if we find a bunch of them, there will be a completely different species a few kilometers over. They are very highly local. And the entire group, which is found around the world, exhibits the same the same high-need microniche requirements and the same behavior.”

This old, established pattern of localization has some important implications that go far beyond invertebrate biology. You can find markers for the ancient history of the earth in a daddy-long-legs’ genes, Clouse explained.

“The end result of this is that their current distribution around the world is due to the movement of continental landmasses. So, when we reconstruct their history from their DNA, we get a nice match to the history of the landmasses on which they live,” he said.

“There one is South Carolina on Sassafras Mountain – its genes show its closest relative is in West Africa. These species split apart about 200 million years ago. And guess what? Geologists say the Atlantic Ocean opened up about 200 million years ago.”

Continental drift and the movement of landmasses. If you stay
in one place long enough, you get moved around.

According to the geology and the theory of continental drift, evidence indicates that all the planet’s landmasses have not always been where they now are, but have drifted around the planet, colliding and joining with each other and then separating -- again and again. Around the time Opiliones first established themselves on land, all landmasses were shoved together in a supercontinent geologists refer to as Pangea, and geologists know that what is now North America was continuous land with what is now Europe and Africa.

But on other parts of the globe, the evidence for exactly what happened is not quite as clear. Just like we might think daddy-long-legs are spiders, we might assume that the Philippine Islands all came from the same place… but again we would be wrong.

“It turns out that the geologists don’t always have a very good handle on what is going on,” Clouse notes. “The Philippines is one of those places where they really have a very poor view of what happened in the past. All those islands are coming from different directions. So the Philippine Opiliones project kind of got legs because these animals are a nice model for tracking origins of the land.”

Though the animals are small, this is big science -- which is perhaps why National Geographic, one of the premier public providers of scientific content on the internet, has asked Clouse to blog about his work in the field, though the animals he studies are not as charismatic as jungle birds and butterflies.

“I was apologetic to national Geographic about the possible lack of action in our field work and said most of it consists of us sitting on a log, looking at a pan of leaf litter, examining it for daddy long legs. These guys will curl up in a ball and look like a piece of dirt.”

“They just try to wait until they think you’re gone. That’s why they are so obscure, so poorly known for centuries and why people have overlooked them. Now that we really look for them, we find that they are everywhere.”

“They said ‘Oh, don’t worry. That gives us new ideas – in fact you could take a picture of your pan of leaf litter in high resolution and we could put it up and ask people if they can find the daddy-long-legs.’”

In a sense, Clouse said, it is a chance for daddy-long-legs to get their “close-up” in the spotlight of science.

“The animals I want for the science but it’s also a chance to put these animals up on a stage because there are a lot of myths about them – people think they are dangerous or poisonous, and we know that they are not but that they are really, really interesting. In fact some of them are right here in South Carolina tell deep, deep history about the southern US. I think that’s very exciting.”

So, if you run into a daddy-long-legs in the garden or in your basement, be respectful. Chances are, its ancestors lived in this place a lot longer than your kind has.

And oh, the stories they can tell.

You can follow Dr. Ronald Clouse blogs from the field at: http://www.nationalgeographic.com/explorers/projects/ or from a link at his author page: http://newswatch.nationalgeographic.com/author/ronaldclouse/