The monkey is found in two high-altitude remote locations in Tanzania: the Rungwe-Livingstone forest in the Southern Highlands and the Ndundulu Forest in the Udzungwa Mountains.

Known locally as Kipunji, it stands at about 90cm (3ft) tall, is grey-brown in colour with off-white fur on its stomach and on the tip of its long curly tail, and has a crest of long hair on the top of its head. Adults have a distinctive call, described as a "honk-bark".

Better tests

When scientists spotted the animals in 2005, they originally placed them in the Lophocebus genus, commonly known as managabeys, but they were only able to study them from photographs.

However, the discovery of a dead Kipunji in a farmer's trap meant more extensive genetic and morphological tests could take place.

Tim Davenport, lead author of the paper, who is from the Wildlife Conservation Society and is based in Tanzania, said: "We first came across the monkey a couple of years ago - the realisation that it was a new species was really exciting.

"Since then we knew it would only be a matter of time before we got hold of a dead animal - because they are hunted - and once we had and we started looking at it more closely, we realised it was a new genus. That was just incredible - it is something that really doesn't happen that often."

Bill Stanley, an author on the paper, and mammal collections manager at the Field Museum, Chicago, US, said hearing the news that the monkey belonged to a new genus "sent shivers down my spine".

"Simply put, the genetics said that it was closely related to baboons, but the skull wasn't anything like a baboon. The conclusions we drew from the genetic and morphological data meant that it had to be named as a new genus."

An enigmatic monkey

Mr Stanley said one of the reasons why the monkey had until recently remained a mystery to science was because of its reclusive nature.

"They live in trees for the most part, they rarely come to the ground - and when they are in the trees they remain relatively hidden. This coupled with the fact that the places where the Kipunji are known are infrequently visited by outsiders is what probably led to them being unknown for so long."

But although the enigmatic Kipunji has just been described, it is already under threat, say the authors.

"At the moment we are doing a census, but the Kipunji will almost certainly number less than 1,000 in total," Tim Davenport told the BBC News website.

"There is a very small population in Ndundulu, but that is only two or three groups. In Mount Rungwe, where there are more, the forest is heavily disturbed. It is logged and it isn't managed. That couples with the fact that the monkey is hunted - they raid crops - and people set traps to protect their crops."

Bill Stanley agreed: "The bottom line is that they are living in a small area of forest that is increasing being utilised for human needs, and the ramifications of that human utilisation could have a serious effect on the remaining population."

The new genus is now being considered for the IUCN Red List of endangered species.

Evolutionary steps

Jonathan Kingdon, a biological anthropologist from Oxford University, commented: "The geneticists have shown that the closest relative of this rather slender, mainly tree-dwelling monkey is the hefty, mainly ground-dwelling baboon. Indeed of all the primates known it is the baboon's closest relative.

"The evolution of this unique monkey from a baboon and not a finely tuned lineage that was already 'monkey' offers us a unique opportunity to understand the evolution of monkeys in Africa.

"And the most likely reason for baboon and not monkey ancestry is that the Southern Highlands were separated from the great primate communities of central Africa by Lakes Tanganyika and Rukwa."

But Professor Colin Groves, a biological anthropologist from Australian National University, Canberra, was more cautious about the research.

"I'm not certain if this is a new genus. I'm unsure of the molecular analysis - when I look at the phylogenetic tree (a diagram of the evolutionary relationships of a group of organisms) there are aspects of it that are quite different to those that other people have generated. I would like to see them explore their DNA tree much much more."