SURVEY: HUMAN EVOLUTION
Meet the relatives
Dec 20th 2005
From The Economist print edition
A large and diverse family
WHEN Homo sapiens emerged as a species, he was not alone. The world he entered was already peopled by giants and dwarfs, elves, trolls and pixies—in other words, creatures that looked humanlike, but were not the genuine article. Or, at least, not as genuine as Homo sapiens has come to believe himself to be.
Like the story of Homo sapiens himself, the story of the whole human family begins in Africa. About 4.5m years ago, probably in response to a drying of the climate that caused forest cover in that continent to shrink, one species of great ape found itself pushed out into the savannah, an ecological niche not normally occupied by apes. Over the next 300,000 years these apes evolved an upright stance. No one know for sure why, but one plausible explanation, advanced by Peter Wheeler of John Moores University in Liverpool, is that standing upright reduces exposure to sunlight. To an animal adapted to the forest's shade, the remorseless noonday sun of the savannah would have been a threat. Dr Wheeler's calculations suggest that walking upright decreases exposure at noon by a third compared with going on all fours, since less of the body's surface faces the overhead sun. Humanity, in the form of Australopithecus anamensis, had arrived.
Australopithecines of various species lasted for over 3m years. But half-way through that period something interesting happened. One of them begat a species known to science variously as Homo rudolfensis and Homo habilis. All modern great apes make tools out of sticks and leaves to help them earn their living, and there is no reason to believe that this was not true of australopithecines. But, aided by hands that no longer needed to double as part-time feet, Homo habilis began to exploit a new and potent material that needs both precision and strength to work—stone. This provided its immediate descendants with a powerful technology, but also gave its distant descendants in human palaeontology laboratories an additional way of tracing their ancestry, for stone tools often survive where bones do not.
Homo habilis's successor species, Homo erectus, did not bestride the globe in the way that his eventual descendant Homo sapiens did, but he certainly stuck his nose out of Africa. Indeed, the first fossil erectus discovered was in Java, in 1891, and the second one, several decades later, turned up in China, near Beijing. It was not until 1960 that erectus bones were found in Africa.
Homo erectus is a frustrating species. His tools are found all over the southern half of Eurasia, as well as in Africa. But China and Java aside, his bones are scarce outside Africa. There are two skullcaps from Georgia and half of one from India. He did, however, leave lots of descendants.
Naming fossils is a game that beautifully illustrates Henry Kissinger's witticism about academic disputes being so bitter because the stakes are so low. The best definition of a species that biologists have been able to come up with is “a group of creatures capable of fertile interbreeding, given the chance”, which clearly makes it hard to determine what species a particular fossil belongs to. Researchers therefore have to fall back on the physical characteristics of the bones they find. That allows endless scope for argument between so-called splitters, who seem to want to give a new name to every skull discovered, and lumpers, who like to be as inclusive as possible.
Some splitters, for example, argue that the African version of Homo erectus should be called Homo ergaster. Whatever the niceties, it is clear that by 500,000 years ago, if not before, Homo erectus was breaking up into anatomically different populations. Splitters would like to turn the Georgia fossils, an early twig of the erectus tree, into Homo georgicus. There is also Homo rhodesiensis, found in southern Africa, Homo heidelbergensis from Europe, and a whole drawer's-worth of specimens known to some as Homo helmei and to others as archaic Homo sapiens.
How little is really known, though, was thrown into sharp relief by the announcement just over a year ago that yet another species, Homo floresiensis, had been found. It was discovered on Java's nearish neighbour island, Flores. Finding a new species of human is always exciting, but what is particularly intriguing about Homo floresiensis is how small it was—barely a metre tall when fully grown. Perhaps inevitably, though to the disgust of its discoverers, Homo floresiensis became known to journalists as the hobbit, after J.R.R. Tolkien's fictional humanoid. Homo neanderthalensis, the descendant of Homo heidelbergensis, by contrast, was if not a giant then at least a troll. Though he stood five or ten centimetres shorter than a modern European Homo sapiens, the thickness of his bones suggests he was a lot heavier.
Both Homo neanderthalensis and Homo floresiensis were certainly around when Homo sapiens left Africa—whichever version of that story turns out to be the correct one. There may also have been some lingering populations of other hominid species. That raises the intriguing question of what happened when these residents met the sapiens wave.
Some researchers believe there was interbreeding, echoing the ideas of an older school of palaeoanthropology called multiregionalism. The multiregionalists thought either that pre-sapiens hominids were all a vast, interbreeding species that gradually evolved into sapiens everywhere, or, against all Darwinian logic, that Homo sapiens arose independently in several places by some unknown process of parallel evolution.
As recently as 2002, Alan Templeton, then at the University of Washington at St Louis, claimed to have found a number of genetic trees whose roots were 400,000-800,000 years old, and yet which included non-Africans. That, if confirmed, would support multiregionalism. Meanwhile, John Relethford, of the State University of New York's campus at Oneonta, has criticised the conclusions of studies on mitochondrial DNA extracted from the bones of Neanderthals. This does not resemble DNA from any known modern humans, which led the authors of the work to conclude there was no interbreeding. Dr Relethford points out that Neanderthal DNA brought into the sapiens population by interbreeding could subsequently have been lost by chance in the lottery of who does and who does not reproduce. Similar losses are known to have happened in Australia, where mitochondrial DNA from human fossils is absent from modern Australians.
Most students of the field, though, think there was no interbreeding, full stop. Either Homo sapiens persecuted his cousins into extinction or, with his superior technology, he outhunted, outgathered and outbred them. The next question is where that technology—or, rather, the brainpower to invent and make it—came from.