No one really knows why giant tortoises took up residence in a steamy group of islands millions of years ago, but scientists say without efforts to preserve the species the Galapagos tortoise could be extinct.

Theory holds that giant tortoises may have come in pairs, dozens or even more in ancient voyages along the equator that put 600 miles of ocean between them and their nearest predators.Scientists surmise the huge creatures, unaccustomed to life at sea, may have drifted either by natural buoyancy or atop floating mainland vegetation to reach this destination, a tiny Pacific archipelago that now bears their name.

Galapagos is Spanish for tortoises.

Indeed, there was a time when these islands were covered with millions of tortoises, and pirates told tales of having to traverse from one point to another by walking on the shells of the indolent beasts.

But scientists now are attempting to reverse a course that has put the tortoises on an unmistakable slide toward extinction. Three of 14 subspecies that once populated these islands are now extinct, and without human intervention, the remaining tortoises could face the same fate.

Approximately 4,500 giant tortoises are left in the Galapagos Islands, the last natural refuge for descendants of an extinct prehistoric species that existed millions of years ago, possibly before the Andes mountains rose.

Scientists say events that devastated populations of tortoises by the millions were set in motion when hungry 16th century sailors first dropped anchor here.

Buccaneers stewed tortoises in a gruel called sea pie, while rats and pigs that escaped from the ships dined on countless tortoise eggs tucked in nests throughout the archipelago.

The constant supply of food led to an explosion in the population of feral animals and rats, a problem now controlled on some islands but not others.

Upwards of 300,000 giant tortoises may have been taken as food in the 16th and 17th centuries, with thousands more slaughtered through the mid-20th century by poachers, who would crack open the shells and strip away fat for oil. The reptiles weigh up to 600 pounds, have life spans exceeding 150 years, and can exist without food and water for a year or more.

Scientists at the Charles Darwin Research Station on the island of Santa Cruz are supporting efforts by the Ecuadoran government to eto block extinction by poisoning rats and shooting feral animals.

"No one really knows why the tortoises came here," said Daniel Gerzon, who lectures at the Darwin Station. "Some people think that they came thousands of years ago to escape predators on the mainland.

"To do that, they had to float (600 miles)," he said. "We're just trying to bring them back from the brink of extinction and eliminate the animals that threaten their lives."

Since the late 1960s, an international team of scientists and Ecuadorian national park rangers have participated in a Darwin Station program to save tortoise hatchlings on the islands. More than 1,000 tortoises have been raised in this program.

Project workers search for tortoise nests on islands, such as Pinzon, where rats are a problem. Any eggs they find are brought to the station, a compound of bungalows and tortoise quarters, to ensure their safety from rodents.

"When the baby tortoises are about four or five years old, they are ready to live on their own," Gerzon said.