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They might be giants, or dwarfs

Evolution often takes unexpected, and sometimes bizarre, directions on islands

Their lives could have been the plot of a monster movie. Three-foot-tall humans recently discovered to have lived on an Indonesian island had to dodge giant lizards and rats the size of dogs. They were so tiny it appears they couldn't even overcome adult dwarf elephants, forced instead to hunt the animals' young.

But their adult size -- comparable to a modern 4-year-old -- had an upside, too, apparently allowing them to survive in isolation for tens of thousands of years. The existence of these little people, reported in the journal Nature last month, provides new scientific fodder for a mysterious evolutionary phenomenon that can radically shrink or balloon a species' size when it becomes isolated on islands.

"When species get to an island, you get evolution taking unexpected directions," said James H. Brown, distinguished professor of biology at the University of Mexico who has studied the phenomenon. "And some of it is bizarre."

Ever since the days of Charles Darwin, who did much of his research on the Galapagos Islands off the coast of Ecuador, scientists have wondered about these island species and what lessons they hold about evolution's flexibility, speed and durability. Islands are natural laboratories to examine evolution without the clutter of too many species and too much competition.

The little human species found on Flores Island was the first sign that people were shaped by this powerful island force. But the world's islands have been -- and remain -- filled with dwarfs and giants of folklore proportions: Enormous 9-foot birds called moa used to roam the forests of New Zealand, a pygmy mammoth used to inhabit California's Channel Islands, and the fearsome Komodo dragon still stalks Indonesian island goats. Even field mice on tiny Muskeget Island off Nantucket exhibit this "gigantism" -- growing 20 percent bigger than mice found on the mainland.

These extreme creatures may also add a shade of truth to stories long dismissed as fairy tales. Indonesians have talked for centuries about the Ebu Gogo, tiny, hairy people who were said to eat anything they could. If little people were real, so, too, perhaps was the giant that inspired Jack's battle atop a beanstalk.

No one really knows what makes some island species grow and others shrink -- although scientists expect the answers hinge on the availability of food, the competition for that food, and the number of predators on an island. Some researchers even suggest that the size of the island may play some role in shaping a species' size.

One broad rule of thumb seems to hold true: Mammals that start out bigger than a rabbit evolve smaller on islands, and those that start tinier than a bunny get bigger.

"This is a pretty dramatic and regular phenomenon that suggests something fundamental underlies it," said V. Louise Roth, Duke University associate professor who studies mammalian evolution and body size. "There are many questions as well as some exceptions."

Many island species evolved from mainland relatives that found themselves on islands by swimming, floating or flying there. Once there, the plants, insects, or humans found themselves having to eat different foods and escape different predators.

Being large can help creatures avoid being eaten, but it also takes a lot of energy. Placed on an island with no enemies, it makes sense for a species to evolve to a less energetically demanding size. A small creature that may have remained tiny on the mainland to wriggle into crevices their enemies couldn't reach may grow a bit bigger on islands with fewer predators.

While big animals like hippos and elephants tend to get markedly smaller on islands, little mammals inexplicably don't get too big. "The giant rats aren't the size of Volkswagens," said John Damuth, research biologist at the University of California at Santa Barbara.

But just to add to the confusion, what we think of as a gigantic island species today may actually be a dwarf version of a far larger, extinct species. Some researchers hypothesize that the Komodo dragon is a small survivor of a much larger species, left to evolve alone while its mainland family was wiped out. Ditto for the famous Galapagos tortoises that can grow to 4 feet in length and weigh 450 pounds or more, said James Gibbs of the State University of New York College of Environmental Science and Forestry.

Researchers are not sure yet whether they will be able to figure out why the Flores people evolved to be so diminutive from what its discoverers believe were its Homo erectus ancestors. It might be because there were fewer predators on the island and less need for size. But it might also be that the humans who were by chance smaller had a greater likelihood of surviving and reproducing. It appears that the species also had long arms for their size, perhaps allowing them to climb trees to escape predators.

Australian and Indonesian researchers believe the dwarf humans were able to thrive on Flores from about 95,000 years ago until 12,000 years ago, before they and the dwarf elephants were wiped out by a volcanic eruption. The discovery upended theories that modern-day people have been the sole human species on the planet for tens of thousands of years, and raises questions about whether the two groups ever met.

The discovery was also humbling.

"We always think we are a special case," said Dirk Van Vuren, professor of wildlife biology at the University of California at Davis who was not involved in the research. "But humans follow the same rules."

Beth Daley can be reached by email at bdaley@globe.com

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