By Helen Briggs
BBC News science reporter
A previously unknown monkey species has been found in the mountains of southern Tanzania.
There may only be a thousand individuals left
The animal is believed to be a critically endangered species, with no more than perhaps a thousand individuals remaining.
The highland mangabey, as it is called, lives in the trees and is thought to be closely related to the baboon family.
Full details of the discovery were revealed on Thursday in the journal Science.
Tom Butynski was one of the first scientists to track down the secretive monkey. He said it was a great thrill to be only the second or third primatologist to see the animal.
"Your mouth drops open and a big smile appears on your face," he told the BBC News Website. "You say 'wow'!"
Dr Butynski was part of a US research team visiting the Ndundulu Forest Reserve to investigate sightings of the Sanje mangabey, a rare "cousin" of the new species.
Unbeknown to them, a different team of researchers working a few hundred kilometres away in the southern highlands of Tanzania was also on the animal's trail.
Researchers have good photographic evidence
"These monkeys have probably been there for hundreds of thousands of years," said Dr Butynski. "What are the chances of two independent projects finding the animal within a 10-month period?"
The other research group, led by Tim Davenport of the New-York-based Wildlife Conservation Society, had been searching the area for many months after hunters spoke of seeing a strange animal known locally as the Kipunji. They finally spotted the monkey last year.
"This discovery proves that there is still so much to learn about the more remote and less well-known areas of Tanzania, and Africa as a whole," said Dr Davenport.
The existence of such a large mammal living out of sight for so long has astounded biologists. It is more than two decades since the last discovery of a new African monkey.
Russell Mittermeier, of IUCN - The World Conservation Union, said it demonstrated how little we knew about our closest living relatives, the non-human primates.
The monkey is brown with a black face
"A large, striking monkey in a country of considerable wildlife research over the last century has been hidden right under our noses," he said.
The highland mangabey (Lophocebus kipinji) is brown with a distinctive soft "honk-bark" call. It lives as high as 8,000ft (2,450m) above sea level and has long fur to withstand the cold.
Scientists believe there are two or more populations living in forests near the Udzungwa Mountains National Park. But with so few individuals left, they fear its future is in jeopardy.
Some of the monkey's habitat has been destroyed by illegal logging, prompting calls for the area to be better protected by expanding the boundaries of the national park.