Orangutans have been named as the world's most intelligent animal in a study that places them above chimpanzees and gorillas, the species traditionally considered closest to humans.
The study found that out of 25 species of primate, orang-utans had developed the greatest power to learn and to solve problems.
The controversial findings challenge the belief that chimpanzees are the closest to humans in brainpower. They also suggest that the ancestry of orang-utans and humans may be more closely entwined than had been thought.
Orang-utans are now endangered as never before. Once widespread throughout the forests of Asia, they are confined to just two islands - Sumatra and Borneo - and are highly endangered as a result of habitat loss and poaching.
James Lee, the Harvard University psychologist behind the research, collated a series of separate studies into the intelligence of different primate species. However, his research first had to overcome a much greater hurdle: would it be possible to compare different species of primates at all?
Mr Lee found that in primates, at least, different rules seem to apply - the development of one set of mental skills seems to prompt the primate brain to develop other mental abilities as well.
"A primate genus with a high rank in an experiment testing particular mental abilities appears to have high ranks in all of them," Mr Lee said.
He also found that the single most important factor in deciding a species' intelligence was simply the size of its brain: "The correlation of brain size with mental ability found in humans appears to extend throughout the primate order."
This "remarkable finding" suggests, he said, that all primate brains work in much the same way. But they have evolved, allowing comparisons between species.
Mr Lee's research threw up some other surprises, too. Gorillas, for example, emerged as less intelligent than spider monkeys, while baboons were ranked only 14th.
Recent field work by Carel van Schaik, a Dutch primatologist at Duke University, North Carolina, appears to bear out Mr Lee's findings.
Studying orang-utans in Borneo, he found them capable of tasks well beyond chimpanzees' abilities - such as using leaves to make rain hats and leakproof roofs over their sleeping nests. He also found that in some food-rich areas, the creatures had developed a complex culture in which adults would teach youngsters how to make tools and find food.
He and Mr Lee both suggest that the key factor in such developments is the orang-utans' lifestyle, spent mostly in the tops of trees where there is little risk from predators. This has allowed them to establish long and settled lives similar to humans' and also to develop culture and intelligence.