Our closest relative, the chimpanzee - a master of communication. Credit: Punchstock

Four decades ago, scientists Jane Goodall and Toshisada Nishida began the groundbreaking research that made the world realize how similar humans are to their closest living relative, the chimpanzee. Tomorrow, Goodall, Nishida and 18 other leading scientists will gather in San Francisco, California, for a symposium convened by the Leakey Foundation, which supports and conducts research into human origins. Goodall and Nishida will each receive a US$25,000 prize from the foundation to honour their work; the wider symposium will discuss the key research challenges that primate researchers hope to tackle in the next 40 years. Nature asked the participants what five big questions are likely to emerge from the discussion.

Why are primates different from each other?

Primate species have myriad lifestyles: gorillas live in troops consisting of multiple females and a dominant 'silverback' male, for example, whereas gibbons live in nuclear families. And different species have very different temperaments, says Richard Wrangham of Harvard University in Cambridge, Massachusetts. Bonobos, humans and chimps, for instance, are closely related, but bonobos are famed for using sex to keep the peace, whereas chimps engage in warfare and even kill infants of their own species. "If you're interested in human evolution," Wrangham says, "having a real understanding of how the chimpanzee-bonobo split has led to these strikingly contrasting species, each of which has similarities to humans in their behaviour, is a very big and very exciting question."

How do primates think?

Captive apes are amazingly intelligent: they can learn sign language and use new technologies. But other animals also show signs of intelligence, so primates' relatively large brains can't explain everything. Husband-and-wife team Dorothy Cheney and Robert Seyfarth of the University of Pennsylvania in Philadelphia ask, "What's so special about primates?"

One possible answer is that primates need intelligence to manage their complex social lives - for example, male chimps trade meat for help in fights and support allies who have helped them, points out John Mitani of the University of Michigan in Ann Arbor. But scientists don't really understand how wild chimps experience and remember these interactions: "We know very little about the cognitive mechanisms that chimps employ to keep track of who cooperates with whom and in what situations," Mitani says.

Why do primates cooperate?

When chimps trade meat and band together to launch raids, are they motivated purely by reciprocity or the fact that helping closely related individuals boosts one's own genetic cause? Or, as Joan Silk of the University of California in Los Angeles asks, "Do other primates share human preferences for fairness, capacity for empathy, and concern for the welfare of others?"

Studying these questions in male primates has been especially difficult, because it's hard to tell how they are related to others in promiscuous societies. But non-invasive DNA sampling techniques are now helping sort this out. Such techniques are allowing scientists to study primate fatherhood for the first time: "It's only recently that we can ask who the fathers are, do offspring know their fathers, and whether they have special relationships with them," says Ann Pusey of the University of Minnesota in St Paul.

Where did primates come from?

Over the past 40 years, fossil and genetic studies of human ancestors and living primates have shown that humans evolved in Africa and are most closely related to chimps and bonobos. So what drove humans to lose their body hair, develop complex language skills and adopt other supposedly advanced traits? Theories abound – we needed big brains to form more complex social groups, for example; or, as Lynne Isbell of the University of California, Davis, speculates, that we evolved our brainpower and social behaviour along with binocular vision, learning, fear and memory as an adaptation to help avoid poisonous snakes. Scientists now hope to combine different techniques to sift the good theories from the bad. "This will be interdisciplinary, involving molecular, palaeontological, and comparative studies to test the various hypotheses about primate origins," Isbell says.

Which primates will still exist in the wild in 40 years?

Ask almost any scientist who studies primates – especially Goodall - and he or she will cite this as one of the field's top concerns. A major survey revealed this August that half of wild primate species are in danger of disappearing in the next 10 years (see ""Almost half of primate species face extinction":http:// http://www.nature.com/news/2008/080805/full/news.2008.1013.html"). "Obviously this question involves big political, economic and social issues that primatologists can't resolve," points out David Watts of Yale University in New Haven, Connecticut, "but it behooves us to think about what confluence of ideas and actions could resolve them and do all that we can to promote those ideas and try to make the actions happen."