Leading article: Protect our nearest relatives

They are far closer to us than many people realise. They make and use tools; they employ plants for self-medication. In a rich and complex social life, they clearly experience a range of emotions, including joy and grief. Even more strikingly, they show the beginnings of morality, in the way that excessive harassment of a subordinate by a dominant animal will evoke expression of unease by other group members. "The great apes," said the UN treaty signed to protect them at the weekend, after much effort by conservationists, "form a unique bridge linking humans to the natural world."

Many reasons can be put forward for preserving different parts of the earth's wildlife. There are economic reasons (human communities can benefit directly), ecological reasons (other ecosystems are dependent on particular animals or plants), even aesthetic reasons (the world would be a poorer place without nightingales and primroses). All of these reasons apply to the four species of great apes, the gorilla, the chimpanzee and the bonobo or pygmy chimpanzee from Africa, and their Asian counterpart, the orangutan; but it is surely their closeness to us in so many ways - we come from a common ancestor and share more than 98 per cent of our DNA with them - that makes the case for preserving them special, even overwhelming. They are our nearest relatives.

That is why the treaty signed at the weekend in Kinshasa in the Democratic Republic of the Congo is of such importance. Today, all the great apes are mortally at risk. Illegal logging and mining, and legal deforestation, are destroying their habitat over enormous areas; the so-called "bushmeat" trade is decimating their numbers directly. Scientists have no doubt that they are heading directly for extinction and, within one or perhaps two generations, they may be gone in the wild.

Yet most of the 23 countries which hold great ape populations are among the poorest in the world, and for them, wildlife protection is sometimes an unaffordable luxury. The Kinshasa treaty recognises this by stressing the economic value of ape populations for wildlife tourism, and putting the conservation plans in the context of national strategies for poverty reduction. It also recognises that the rich West will have to offer considerable help in funding the global strategy for the survival of the great apes which has now been agreed.

Britain has taken an honourable lead in this. As the coming century progresses, the widespread extinction of animal and plant species will be the biggest environmental problem facing the planet after climate change. The great apes are the iconic examples, and there can be no more welcome development than the taking of concrete steps to preserve them.