By Colin Barras 18 May 2017 http://www.bbc.com/earth/story/2017...e-missing-link-between-us-and-apes?ocid=fbert The average missing person's inquiry begins with a few vital facts. Investigators often know when and where the missing party was last seen. They might have photographs that tell them what the missing person looks like, and they usually have a name to put to that face. Now imagine beginning a similar sort of inquiry with none of this information. About 150 years ago, when Charles Darwin published his theory of evolution through natural selection, scientists began to accept that humans – for all our sophisticated behaviour – belong to the same family tree as all other animals. The idea led to two inescapable conclusions. First, our species is not an only child. Somewhere out there in the natural world, there is at least one species of animal that is more closely related to humans than any other – what biologists would come to call humanity's "sister species". Secondly, and as importantly, our species has a long-lost parent. It stands to reason that if humanity has one or more sisters, then these siblings must have shared the same parent species at some point in prehistory. Evolutionary biologists call this species the "last common ancestor" (LCA). Most people know it by a non-scientific name: the "missing link". Scientists have been on the trail of the LCA for decades, and they still have not found it. But many are convinced that they have established enough information to make the hunt a lot easier. They think they know roughly when and where the LCA lived. They even have a reasonable idea of what it looked like and how it behaved. A chimpanzee (Pan troglodytes) (Credit: Anup Shah/naturepl.com) Even before Darwin formalised the idea of evolution through natural selection, it was clear that humans were primates – although earlier scientists did not think this categorisation had any evolutionary implications. Apes in general represented evolutionary staging posts on the road to humanity Darwin himself was initially reluctant to directly address human evolution. He barely mentioned the subject in his famous book On the Origin of Species. Darwin's colleague, Thomas Henry Huxley, was perhaps the first to try to identify humanity's roots using well-reasoned evolutionary thinking. In his 1863 book Evidence as to Man's Place in Nature, Huxley said it was "quite certain", anatomically speaking, that humans are most similar to gorillas and chimpanzees. One of these two must be humanity's sister species, although Huxley was not sure which. Huxley's ideas had a significant impact on 19th and early 20th Century evolutionary biologists. Many enthusiastically embraced the idea that chimps or gorillas – or even both – were our sister species. But they went further. To these biologists, it seemed that apes in general represented evolutionary staging posts on the road to humanity. Gibbons are more distantly related to us (Credit: Anna Yu/Alamy) "Lesser" apes like the gibbons offered a window into the anatomy of our earliest ape ancestors. Meanwhile the "great" apes – gorillas, chimpanzees and orangutans – showed the anatomical features our ancestors possessed at the moment they split away from the other apes and began to develop a uniquely human appearance. Gorillas and chimps were not simply our sister species: they were also a lot like the LCA. "The post-Darwinian 'paradigm' adopted living chimpanzees as stand-ins for the LCA," says Tim White, a palaeoanthropologist at the University of California, Berkeley. This led to some very particular ideas about how the LCA looked and behaved. Primates in general (particularly monkeys) are often relatively small-bodied, and they scamper around in forest canopies by running along branches. But apes are unusual primates. Most have big bodies with extraordinarily long arms. They often get around by swinging below branches rather than running along the top of them – a form of locomotion called "brachiation". According to many of these early researchers, the LCA was a large-bodied, long-armed, brachiating ape. Are chimpanzees (Pan troglodytes) our closest relatives? (Credit: Florian Möllers/naturepl.com) By the late 1960s, researchers were fleshing out the LCA even further. An anthropologist called Sherwood Washburn pointed out that chimpanzees, and particularly gorillas, actually spend significant amounts of time moving around on all fours on the forest floor. Humans just are not particularly "evolved" Both apes use their arms in an idiosyncratic way when they walk: they flex their fingers so that their weight bears down on the knuckles. To Washburn it made sense that the LCA "knuckle-walked" too. The behaviour could even be seen as a stepping-stone on the way to walking upright on two legs, he wrote. But it would be wrong to think that everyone was on board with these ideas of a brachiating, knuckle-walking, chimp-like LCA. In fact, almost from the moment that Huxley first put pen to paper, a minority of scientists were arguing that the earliest human ancestors – and the LCA – was decidedly not chimp-like. For instance, just a decade after Huxley's book, biologist St George Mivart argued that humans shared many features in common with monkeys or even lemurs. Meanwhile, from 1918 onwards an anatomist called Frederic Wood Jones argued that humans had a lot more in common with tarsiers than with chimpanzees or gorillas. Lemurs, tarsiers and monkeys are primates, but they have been evolving independently of the apes for tens of millions of years. How could anyone argue that humans are closely related to these groups? There is a simple and astonishing explanation, wrote anatomist William Straus in the 1940s. Humans just are not particularly "evolved".