Читать книгу The Rational Optimist: How Prosperity Evolves - Matt Ridley, Matt Ridley - Страница 23

Barter was the trick that changed the world. To paraphrase H.G. Wells, ‘We had struck our camp forever, and were out upon the roads.’ Having conquered much of Africa by about 80,000 years ago, the modern people did not stop there. Genes tell an almost incredible story. The pattern of variation in the DNA of both mitochondrial and Y chromosomes in all people of non-African origin attests that some time around 65,000 years ago, or not much later, a group of people, numbering just a few hundred in all, left Africa. They probably crossed the narrow southern end of the Red Sea, a channel much narrower then than it is now. They then spread along the south coast of Arabia, hopping over a largely dry Persian Gulf, skirting round India and a then-connected Sri Lanka, moving gradually down through Burma, Malaya and along the coast of a landmass called Sunda in which most of the Indonesian islands were then embedded, until they came to a strait somewhere near Bali. But they did not stop there either. They paddled across at least eight straits, the largest at least forty miles wide, presumably on canoes or rafts, working their way through an archipelago to land, probably around 45,000 years ago, on the continent of Sahul, in which Australia and New Guinea were conjoined.

This great movement from Africa to Australia was not a migration, but an expansion. As bands of people feasted on the coconuts, clams, turtles, fish and birds on one part of the coast and grew fat and numerous, so they would send out pioneers (or exile troublemakers?) to the east in search of new camp sites. Sometimes these emigrants would have to leapfrog others already in possession of the coast by trekking inland or taking to canoes.

Along the way they left tribes of hunter-gatherer descendants, a few of whom survive to this day genetically unmixed with other races. On the Malay Peninsula, forest hunter-gatherers called the Orang Asli (‘original people’) look ‘negrito’ in appearance and prove to have mitochondrial genes that branched off from the African tree about 60,000 years ago. In New Guinea and Australia, too, the genetics tell an unambiguous story of almost complete isolation since the first migration. Most remarkable of all, the native people of the Andaman islands, black-skinned, curly-haired and speaking a language unrelated to any other, have Y-chromosome and mitochondrial genes that diverged from the common ancestor with the rest of humankind 65,000 years ago. At least this is true of the Jarawa tribe on Great Andaman. The North Sentinelese, on the nearby island of North Sentinel, have not volunteered to give blood – at least not their own. As the only hunter-gatherers who still resist ‘contact’, these fine-looking people – strong, slim, fit and stark naked except for a small plant-fibre belt round the waist – usually greet visitors with showers of arrows. Good luck to them.

To reach the Andaman islands (then closer to the Burmese coast, but still out of sight) and Sahul, however, the migrants of 65,000 years ago must have been proficient canoeists. It was in the early 1990s that the African-born zoologist Jonathan Kingdon first suggested that the black skin of many Africans, Australians, Melanesians and ‘negrito’ Asians hinted at a maritime past. For a hunter-gatherer on the African savannah, a very black skin is not needed, as the relatively pale Khoisan and pygmies prove. But out on an exposed reef or beach, or in a fishing canoe, maximum sunscreen is called for. Kingdon believed that the ‘Banda strandlopers’, as he called them, had returned to conquer Africa from Asia, rather than the other way round, but he was ahead of the genetic evidence in coining the idea of an essentially maritime Palaeolithic race.

This remarkable expansion of the human race along the shore of Asia, now known as the ‘beachcomber express’, has left few archaeological traces, but that is because the then coastline is now 200 feet under water. It was a cool, dry time with vast ice sheets in high latitudes and big glaciers on mountain ranges. The interior of many of the continents was inhospitably dry, windy and cold. But the low-lying coasts were dotted with oases of freshwater springs. The low sea level not only exposed more springs, but increased the relative pressure on underground aquifers to discharge near the coast. All along the coast of Asia, the beachcombers would have found fresh water bubbling up and flowing into streams that meandered down to the ocean. The coast is also rich in food if you have the ingenuity to find it, even on desert shores. It made sense to stick to the beach.

The evidence of DNA attests that some of these beachcombers, on reaching India and apparently not before, must have eventually moved inland, because by 40,000 years ago ‘modern’ people were pressing west into Europe and east into what is now China. Abandoning the crowded coast, they resumed their old African ways of hunting game and gathering fruits and roots, becoming gradually more dependent on hunting once more as they inched north into the steppes grazed by herds of mammoths, horses and rhinoceroses. Soon they came across their distant cousins, the descendants of Homo erectus, with whom they last shared an ancestor half a million years before. They got close enough to acquire the latter’s lice to add to their own, so louse genes suggest, and conceivably even close enough to acquire a smattering of their cousins’ genes by interbreeding. But inexorably they rolled back the territory of these Eurasian erectus hominids till the last survivor, of the European cold-adapted sort known as Neanderthal, died with his back to the Strait of Gibraltar about 28,000 years ago. Another 15,000 years saw some of them spilling into the Americas from north-east Asia.

They were very good at wiping out not only their distant cousins, but also much of their prey, something previous hominid species had not managed. The earliest of the great cave painters, working at Chauvet in southern France 32,000 years ago, was almost obsessed with rhinoceroses. A more recent artist, working at Lascaux 15,000 years later, depicted mostly bisons, bulls and horses – rhinoceroses were rare or extinct in Europe by then. At first, modern human beings around the Mediterranean relied mostly on large mammals for meat. They ate small game only if it was slow-moving – tortoises and limpets were popular. Then, gradually and inexorably, starting in the Middle East, they switched their attention to smaller animals, and especially to fast-breeding species, such as rabbits, hares, partridges and smaller gazelles. They gradually stopped eating tortoises. The archaeological record tells this same story at sites in Israel, Turkey and Italy.

The reason for this shift, say Mary Stiner and Steven Kuhn, was that human population densities were growing too high for the slower-reproducing prey such as tortoises, horses and elephants. Only the fast-breeding rabbits, hares and partridges, and for a while gazelles and deer, could cope with such hunting pressure. This trend accelerated about 15,000 years ago as large game and tortoises disappeared from the Mediterranean diet altogether – driven to the brink of extinction by human predation. (A modern parallel: in the Mojave Desert of California, ravens occasionally kill tortoises for food. But only when landfills provided the ravens with ample alternative food and boosted – subsidised – their numbers did the tortoise numbers start to collapse from raven predation. So modern people, subsidised by hare meat, could extinguish mammoths.)

It is rare for a predator to wipe out its prey altogether. In times of prey scarcity, erectus hominids, like other predators, had simply suffered local depopulation; that in turn would have saved the prey from extinction and the hominid numbers could recover in time. But these new people could innovate their way out of trouble; they could shift their niche, so they continued to thrive even as they extinguished their old prey. The last mammoth to be eaten on the Asian plain was probably thought a rare delicacy, a nice change from hare and gazelle stew. As they adjusted their tactics to catch smaller and faster prey, so the moderns developed better weapons, which in turn enabled them to survive at high densities, though at the expense of extinguishing more of the larger and slower-breeding prey. This pattern of shifting from big prey to small as the former were wiped out was characteristic of the new ex-Africans wherever they went. In Australia, almost all larger animal species, from diprotodons to giant kangaroos, became extinct soon after human beings arrived. In the Americas, human arrival coincided with a sudden extinction of the largest, slowest-breeding beasts. Much later in Madagascar and New Zealand mass extinctions of large animals also followed with human colonisation. (Incidentally, given the obsession of ‘show-off’ male hunters with catching the largest beasts with which to buy prestige in the tribe, it is worth reflecting that these mass extinctions owe something to sexual selection.)