In the year 1551 a strange male animal was put on public display in Augsburg, Germany. He had humanlike fingers on his hands and feet, observers noted, and a "cheerful nature," although he also had a tendency to turn his backside to viewers. Based on an illustration of the creature, biologists think it was most likely a drill (Mandrillus leucophaeus), a baboonlike primate. Even today, more than 450 years later, drills are studied so infrequently in the wild that when a small team of biologists recently spotted a troop of them on Equatorial Guinea's Bioko Island, they collectively gasped, then sat down on the rain forest floor to watch.

The drills, the largest primates on Bioko, were climbing and feeding in a fig tree at the floor of the island's 7,000-foot-high Gran Caldera. Earlier that morning the scientists had spotted troops (each five to thirty strong) of chattering monkeys: red-eared, black colobus, and red colobus, the latter one of the most threatened of all primates.

Biologists regard Bioko Island as a living laboratory for studying how plants and animals evolve in isolation. It lies in the Gulf of Guinea, 20 miles off the west coast of Africa, one of four islands in an archipelago. The three others—São Tomé, Príncipe, and Annobón—are deepwater isles formed tens of millions of years ago and colonized by plants and animals from Africa that arrived on their shores by chance.

Bioko, however, was connected to the African mainland during each ice age, most recently about 12,000 years ago. Like an exclusive ark, the island shelters an isolated set of subspecies evolved separately from those on the mainland. There are seven species of monkeys, including the drills; four galagos (bush babies); two small antelopes (duikers); one species of porcupine; one species of tree hyrax; one species of pouched rat; and three species of scaly-tailed squirrels. There are catlike linsangs (but no lions or leopards). The roster once included forest buffalo, but they were hunted to extinction a century ago.

Add orchids, land snails, freshwater fish, amphibians, spiders, and insects—all evolving apart from their mainland relatives. In the island's interior, woodlands, grasslands, and rain forest remain much as they were when the first Portuguese explorers stepped ashore in the 15th century: largely untouched and beautiful.