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Human Origins Tim White HUMAN ORIGINS Tim White INSTANT EXPERT 5 101106_IE_Human origins.indd 25 21/10/10 14:13:28 EVOLVinG seaRch FOR OUR ORIGINS, FROM DARWIN TO TODAY technoLOGY NOT FROM CHIMPS Technology has transformed our search for human Nineteenth-century sceptics origins. The majority of methods used to date the illustrated what many people saw rocks that hold fossilised bones and artefacts are as the implausibility of human based on radioactive decay. For example, radioisotopic evolution with a cartoon depicting dating of the layers of volcanic ash sandwiching the Darwin’s head atop the body of a remains of “Ardi”, the partial skeleton of Ardipithecus knuckle-walking chimpanzee. ramidus (see below right), show that the sediments Even though Darwin was clear in which the skeleton was found were laid down from the start that we had not 4.4 million years ago. evolved from living chimpanzees, Using micro–computed tomography (micro-CT), similar ideas, and the “missing we can peer inside fossils without damaging them. link” concept, have stuck with us. In the case of Ardi, 5000 micro-CT “slices” through the Darwin’s champion, Thomas fragments of her squashed and scattered skull allowed Huxley, concluded from his own a team at the University of Tokyo in Japan to assemble anatomical studies of African apes that they were our closest living a virtual model and then “print” the skull on a 3D printer. relatives, a conclusion vindicated when molecular studies Other technologies that have had a huge impact showed – and continue to show – how genetically close these include differential GPS to map our finds with sub-metre animals are to us. Ironically, Darwin was almost alone accuracy and to pinpoint the location of ancient stone in calling for restraint in the use of modern tool quarries, satellite imagery to identify surface primates as “stand-in”, proxy ancestors. outcrops of ancient sediments and image-stabilised The recent discovery of human ancestors binoculars to examine those outcrops from afar. that were quite unlike chimps, dating from We use mass spectrometers to examine the soil soon after the two lines split, has shown around any animals we find and also measure the that his caution was well founded, and isotopic composition of their tooth enamel. This how living chimps have evolved a great helps us determine their environment and diet. We deal in relation to the common ancestor use digitisers to capture and analyse the shape of that we once shared with them. fossils. We can even match the chemical fingerprints of rocks thousands of kilometres apart. For example, we have matched volcanic ash from the Middle Awash, our study area in Ethiopia’s Afar Depression, to ash outcrops in other sites in Africa and to volcanic layers in deep-sea cores from the Gulf of Aden. The archaeopalaeontology tool kit has come a long way from little hammers and brushes. WHAT’S IN A NAME? L de U ra Ever since Darwin, all non-human Ga primates more closely related to humans nes G than to our closest living relatives, the Y / A chimpanzees, have been placed in the brar Li zoological family Hominidae. The finding rt A N that humans and African apes are ema G genetically very similar has met with calls rid B to change this classification, grouping he / T apes and humans into a single family. ion This means that “hominids” would then ect LL O include chimpanzees and gorillas, while C ate humans and their ancestors would be V ri classified at the subfamily or tribal level ’ / P et as “hominins”. orn H Whatever arbitrary name we choose he ‘T to apply to our branch of the primate om fr tree, the branch itself dates back to G, tan around 7 million years ago, when a G OU species of ape whose fossils we have ran O not yet found split into two branches. E L Because of this, I prefer the stability Gen Suwa, who scanned nerab and clarity of continuing to classify all Reconstruction of the skull Ve and restored “Ardi” at the / A the members of the human clade (on our of Ardipithecus ramidus University of Tokyo side of the last common ancestor we white tim shared with chimps) as “hominid”. ii | NewScientist 101106_IE_Human origins.indd 18 22/10/10 15:29:03 SEARCH FOR OUR ORIGINS, FROM DARWIN TO TODAY Charles Darwin’s only remark about human evolution in his seminal work On The Origin of Species was that “light will be thrown on the origin of man and his history”. In his autobiography, Darwin justifies his brevity: “It would have been useless and injurious… to have paraded, without giving any evidence, my conviction with respect to his origin.” His boldest statement was in The Descent of Man, where he concluded: “It is somewhat more probable that our early progenitors lived on the African continent than elsewhere.” Today, thanks to a range of discoveries and technologies, we can tell in amazing detail the story that Darwin only guessed at. the BIG PICTURE Twelve million years ago, Earth was chimpanzees. By 6 million years a planet of the apes. Fossil evidence ago, a daughter genus had evolved shows there were many ape species primitive bipedality and smaller spread across the Old World, from canines. Some 2 million years later, Namibia to Germany to China. About its descendants had extended their 7 million years ago, a long-gone range across Africa. After another African species whose fossils have million years, one of the species in yet to be found was the last common the genus Australopithecus sparked ancestor shared by humans and our a technological revolution based on closest living relatives, the stone tool manufacture that helped to push later hominids beyond Africa and across Europe and Asia. The genus Homo is the group of species that includes modern people as well as the first hominids to have left Africa. The first species ARDIPITHECUS: of the genus to do this, Homo erectus, THE WOODLAND HOMINID rapidly spread from Africa into Eurasia by 1.8 million years ago, reaching We still lack enough fossils to say much about the very deposits dated at 4.4 million years Indonesia and Spain, though this earliest hominids. The key features of the fossils that upset all of those expectations was still long before the ice ages have been found suggest that they walked on two because it is so different from even began. Many cycles of cold and legs. We know their social system was different to that the most primitive Australopithecus. nearly a million years later, another of any other living or fossil ape because the canines of The partial skeleton, nicknamed African descendent of Homo males were much smaller and blunter than those of “Ardi”, suggests that our last erectus – one that would eventually non-human apes, and so did not function as weapons. common ancestor with chimpanzees vaingloriously name itself Homo African fossils of these earliest hominids from was not a halfway-house between a sapiens – again ventured beyond about 6 million years ago have been given different chimpanzee and a human, but rather the continent. It has now reached names: Sahelanthropus tchadensis, found in Chad; a creature that lacked many of the the moon, and perhaps soon, will Orrorin tugenensis from Kenya; and Ardipithecus specialisations seen in our closest stand on a neighbouring planet. kadabba from Ethiopia. None resembles modern cousins, such as knuckle-walking, Not bad for a two-legged primate. apes, and all share anatomical features only with a fruit-based diet, male-male combat later Australopithecus. and climbing. A. ramidus was a mosaic Before these fossils were found, many organism: partly bipedal, omnivorous researchers had predicted that we would keep finding with small canines, relatively little Australopithecus-like hominids all the way back to difference between the sexes and the fork between hominids and the evolutionary line a preference for woodland habitats. leading to modern chimpanzees. The recent discovery Ardi represents the first phase of of a skeleton of Ardipithecus ramidus from Ethiopian hominid evolution. NewScientist | iii 101106_IE_Human origins.indd 19 22/10/10 15:29:32 CACTUS OR BUSH? The late American palaeontologist Steven Jay Homo Homo Homo Gould wrote a classic essay in 1977 in which floresiensis sapiens neanderthalensis he predicted that the hominid family tree Homo heidelbergensis would prove to be “bushy”. Today, it is common Homo rhodesiensis to see lists of more than 25 different hominid species, and Gould’s prediction is often declared to be fulfilled. 1 Homo erectus Not so fast. Many of these species are “Sinanthropus” “chronospecies”, which evolve from one to the other, such as the earliest two Australopithecus boisei Australopithecus species, A. afarensis and A. anamensis. These names are merely Homo ergaster arbitrary divisions of a single evolving lineage. Australopithecus robustus A modern biologist addressing the question 2 Australopithecus crassidens of species diversity counts the number of Homo habilis Australopithecus sediba related species existing at any one time. When we do the same thing across the hominid fossil record, what we get is not a bush but Australopithecus Stone tool from Gona, Australopithecus aethiopicus Ethiopia, made about something like a saguaro cactus, with only a garhi 2.6 million years ago few branches or species lineages. Indeed, the greatest diversity among hominid species 3 Australopithecus africanus appears to be at around 2 million years ago, when as many as four different lineages “briefly” co-existed in Africa. Australopithecus afarensis TECHNOLOGICAL The key question turns out to be not how AGO YEARS OF MILLIONS many species there were per se, but rather PriMate why species diversity has been so limited on our branch of the evolutionary tree compared 4 Hominids are frustratingly rare in with other mammals like fruit bats or South Australopithecus anamensis the fossil record, but at some time American monkeys? The reason is probably around 2.6 million years ago they that our ancestors’ niche kept broadening, Ardipithecus ramidus began to leave calling cards, in the as a woodland omnivore 6 million years ago form of stone artefacts.
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