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.