As the little band of explorers approached a distant reddish plateau that loomed above the South American jungle, they saw a huge grey bird flap slowly up from the ground and glide away. “To the best of my belief, a pterodactyl,” Professor Challenger pronounced.

This first portent of The Lost World, Conan Doyle’s fictional story of 1912, has a curious parallel with the moment when Kris Helgen, an American mammalogist, stepped out of a helicopter in New Guinea last December and noticed a brightly coloured bird flitting among the trees. It stopped him in his tracks.

To Helgen’s expert eye, the creature seemed as incredible as a pterodactyl. Decorated with a bright orange face-patch and a pendant wattle under each eye, it was an unknown species of honeyeater, and the first new bird to be discovered on the Pacific island since the second world war. “It was an amazing clue that we were on to something big,” Helgen says.

Over the next 15 days, the 20-strong expedition from Conservation International rubbed their eyes in disbelief as dozens of new species came to light in the Foja forest. Fearlessly, normally shy forest wallabies hopped around the jungle camp. Above, a bird of paradise of almost mythological rarity performed a mating dance, while a golden-fronted bowerbird displayed on its sculpture of twigs.

The exciting possibility was that because of the area’s isolation in the Indonesian half of New Guinea, some distinct species had evolved separately, like Charles Darwin’s finches on the Galapagos Islands. “Lots of the plants and animals we found seemed to be unique to the area,” Helgen says.

Others were relicts of a lost Eden. “It showed a glimpse of what much of the world would have looked like before human influence spread,” Helgen judges.

Last week the expedition’s tally of finds — 40 new species of mammal, four new species of butterfly and innumerable insects — captivated the world. But to Shane Winser, who has advised explorers for the past 30 years at the Royal Geographical Society (RGS), the hoard was not so very remarkable.

“It’s really not that difficult to discover a new species, because the biodiversity of the world is so huge that comparatively few plants and animals have been named,” she says. She has no wish to play down the achievements of the American-Australian-Indonesian expedition: her point is that this cornucopia gives the lie to assumptions that the world is so thoroughly mapped and surveilled by satellites that there are no mysteries left to discover.

In the proud tradition of Sir Richard Burton and John Speke, who competed to discover the source of the Nile, Britain remains in the forefront of exploration. The RGS alone sponsors up to 50 expeditions a year on quests as diverse as tracking the last Simien wolves in Ethiopia and studying woolly monkeys in Colombia.

The most promising target areas for new discoveries are in the three great blocks of surviving tropical forest in South America, central and west Africa, and southeast Asia. The inaccessible and inhospitable rivulets of the Amazon conceal myriad undiscovered species, not to mention uncontacted tribes whom everyone supposed would be extinct by the 21st century.

New varieties of large primates are believed to lurk in the Congo basin, where exploration is confounded by human conflicts. Last year a puzzling new species of mammal — a carnivore about the size of a fox — was photographed in the jungles of Borneo, which are thought to contain the orang pendek, a manlike ape.

Even peninsular Malaysia is yielding up surprises. Little remains of the pristine forest of which Sir George Maxwell wrote in 1907: “Virgin in the days of which we cannot guess the morn, virgin it will remain in the days of generations yet unborn.” But in the past few months there have been reports of hairy “apemen” in the jungles of Johor, last seen in the 1950s.

Or take the island of Madagascar, separated from the African mainland 160m years ago. A new study identifies 10 new species of lemur and classifies nine others as separate species.