The New York Times, August 28, 2007Link
Few people have heard of the mite harvestman, and fewer still would recognize it at close range. The animal is a relative of the far more familiar daddy longlegs. But its legs are stubby rather than long, and its body is only as big as a sesame seed.
To find mite harvestmen, scientists go to dark, humid forests and sift through the leaf litter. The animals respond by turning motionless, making them impossible for even a trained eye to pick out. “They look like grains of dirt,” said Gonzalo Giribet, an invertebrate biologist at Harvard.
As frustrating as mite harvestmen may be, Dr. Giribet and his colleagues have spent six years searching for them on five continents. The animals have an extraordinary story to tell: they carry a record of hundreds of millions of years of geological history, chronicling the journeys that continents have made around the Earth.
The Earth’s land masses have slowly collided and broken apart again several times, carrying animals and plants with them. These species have provided clues to the continents’ paths.
The notion of continent drift originally came from such clues. In 1911, the German scientist Alfred Wegner was struck by the fact that fossils of similar animals and plants could be found on either side of the Atlantic. The ocean was too far for the species to have traveled themselves. Wegner speculated — correctly, as it turned out — that the surrounding continents had originally been welded together in a single landmass, which he called Pangea.
Continental drift, or plate tectonics as it is scientifically known, helped move species around the world. Armadillos and their relatives are found in South America and Africa today because their ancestors evolved when the continents were joined. When South America and North America connected a few million years ago, armadillos spread north, too.
Biogeographers can learn clues about this history by comparing related species. To do so, they must also recognize cases where species crossed great stretches of water.
The island of Hawaii, for example, was home to a giant flightless goose that has become extinct. Studies on DNA extracted from its bones show that it evolved from the Canada goose.
Based on its DNA, scientists estimated that the giant goose branched off from Canada geese half a million years ago. That is also when geologists estimate that Hawaii emerged from the Pacific.
When species jump around the planet, their histories blur. It is difficult to say much about where cockroaches evolved, for example, because they can move quickly from continent to continent. This process, known as dispersal, limits many studies.
“Most of them tend to concentrate on particular parts of the world,” Dr. Giribet said. “I wanted to find a new system for studying biogeography at a global scale.”
Dr. Giribet realized that mite harvestman might be that system. The 5,000 or so mite harvestmen species can be found on every continent except Antarctica. Unlike animals found around the world like cockroaches, mite harvestmen cannot disperse well. The typical harvestman species has a range of less than 50 miles. Harvestman are not found on young islands like Hawaii.
“It’s really hard to find a group of species that is distributed all over the world but that also don’t disperse very far,” said Sarah Boyer, a former student of Dr. Giribet, now an assistant professor at Macalester College in St. Paul.
What mite harvestmen lack in mobility, they make up in age. Their ancestors were among the first land animals, and daddy longlegs fossils have been found in 400 million-year-old rocks. Mite harvestmen evolved long before Pangea broke up and have been carried along by continental drift ever since.
“They’ve managed to get themselves around the world only because they’ve been around for hundreds of millions of years,” Dr. Boyer said.
Dr. Boyer, Dr. Giribet and their colleagues set off around the world to collect a diverse group of mite harvestmen. They ended up gathering thousands of animals from which they extracted DNA.
The gene variations helped the scientists build an evolutionary tree. By calculating how quickly the DNA mutated, the scientists could estimate when lineages branched off. They then compared the harvestmen evolution to the movements of the continents.
“The patterns are remarkably clear,” Dr. Boyer said.
She, Dr. Giribet and colleagues are publishing their results papers to appear in The Journal of Biogeography, Cladistics and other journals.
The scientists found that they could trace mite harvestmen from their ancestors on Pangea. One lineage includes species in Chile, South Africa, Sri Lanka and other places separated by thousands of miles of ocean. But 150 million years ago, all those sites were in the same region of southern Pangea, Gondwana.
The harvestmen preserve smaller patterns of continental drift, as well as bigger ones. After analyzing the DNA of a Florida harvestman, Metasiro americanus, the scientists found that it was not related to other North American species. Its closest relatives live in West Africa.
“It was a big surprise,” Dr. Giribet said.
Dr. Boyer then began investigating the geological history of Florida and found recent research to explain the mystery. Florida started out welded to West Africa near Senegal. North America then collided into them as Pangea was forming.
About 170 million years ago, North America ripped away from West Africa, taking Florida with it. The African ancestors of Florida’s harvestmen came along for the ride.
Dr. Giribet is now searching for other groups of animals that do an equally good job of recording geological history. By comparing dozens or hundreds of animals, he hopes to find clues about the plate tectonics that a single animal could not show.
“This is the beginning of a quest,” he said.
Copyright 2007 The New York Times Company. Reprinted with permission.