Have a good look at the photos and drawings. Look at the faces. Take in the unusual names. They may not be around much longer. These are humanity's vanishing relatives.

Today a group of the world's leading zoologists reveals the 25 most endangered members of the primates – the biological order which contains monkeys, tarsiers, lemurs, gibbons and the great apes, including, of course, humans.

We may be doing fine, at least in terms of numbers: at 7pm last night, the human population of the world had reached 6,803,362,494. It hit 6 billion in 1999 and will hit 7 billion possibly as soon as next year. But our primate cousins are in a very different position.

There are just over 630 species in total, and incredible as it may seem, more than 300 are now threatened with extinction, from developments such as the destruction of tropical forests, the illegal wildlife trade and commercial hunting for bushmeat. This morning, the dangers facing the "top 25", the species really living on the edge, will be highlighted at a conference in Bristol Zoo.

The list includes five primate species from Madagascar, six from Africa, 11 from Asia, and three from Central and South America, all of which are now in need of urgent help to survive.

Conservationists want to highlight the plight of species such as the golden headed langur, which is found only on the island of Cat Ba in the Gulf of Tonkin, north-eastern Vietnam, where just 60 to 70 individuals remain.

Similarly, there are thought to be fewer than 100 individual northern sportive lemurs left in Madagascar, and about just 110 eastern black crested gibbons in north-eastern Vietnam.

"The purpose of our Top 25 list is to highlight those that are most at risk, to attract the attention of the public, to stimulate national governments to do more, and especially to find the resources to implement desperately needed conservation measures," said Dr Russell Mittermeier, chairman of the Primate Specialist Group of the International Union for the Conservation of Nature (IUCN).

"In particular, we want to encourage governments to commit to desperately needed biodiversity conservation measures. We have the resources to address this crisis, but so far, we have failed to act. The results from the most recent IUCN assessment of the world's mammals indicate that the primates are among the most endangered vertebrate groups."

The report gives a fascinating insight into some of the animals which, although they may share a distant common ancestor with us, are hardly known by most of us at all.

Madagascar is home, for example, to the stunning silky sifaka, a wonderful white lemur which is now one of the rarest mammals on earth, whose numbers may be down to no more than 100 because of forest destruction from slash-and-burn agriculture, illegal logging and firewood collection.

Africa holds creatures as remarkable as the rondo dwarf galago, with a tiny frame, huge ears and huge eyes, and the roloway guenon, a strikingly attractive dark-and-white treetop monkey with yellow thighs and a white beard – both shrinking drastically in numbers.

Asia's vanishing primates include such creatures as the pig-tailed snub-nosed langur – if you can't remember that, it's also usefully called the simakobu monkey – which is down to perhaps 3,300 individuals on its Indonesian islands, and also, sad to relate, an animal which is very familiar to us, the Sumatran orang-utan.

On Sumatra the "old man of the woods" has had a very rapid recent decline because of deforestation and its population is now thought to be below 7,000.

Three of the primates on the top 25 come from Central and South America and include the cotton top tamarin, found only in Colombia, with a fantastic white head of hair, and critically endangered.

However, despite the gloomy assessment, conservationists point to the success in helping targeted species recover. In Brazil, the black lion tamarin (Leontopithecus chrysopygus) was downlisted to Endangered from Critically Endangered on the IUCN Red List, as was the golden lion tamarin (Leontopithecus rosalia) in 2003, as a result of three decades of conservation efforts involving numerous institutions, many of which were zoos.

Populations of both animals are now well-protected but remain very small, indicating an urgent need for reforestation to provide new habitat for their long-term survival.