The so-called Natufian culture inhabited what is now the Middle East between approximately 14,000 and 11,500 years ago. This period is commonly split into two subperiods, Early Natufian (14,000 to 13,000 years ago) and Late Natufian (13,000 to 11,500). The Natufians were hunter-gatherers who relied primarily on gazelle, although they also cultivated some cereal grains. During the early period at least, they lived year-round in villages in built stone houses. Like all human beings, their way of life depended on the climate. Around 13,000 years ago, their climate began to change, becoming colder and drier, a period known as the Younger Dryas. We know that times were hard in the increasingly arid landscapes of the Younger Dryas, but quite how hard remains unclear. The droughts certainly caused many ponds and rivers to disappear completely and the larger lakes to shrink in size. The people who lived in the south, in today's deserts of the Negev and the Sinai, were most likely hit the hardest. They returned to a completely transient hunter-gatherer way of life, moving from place to place. Survival required improved hunting weapons: game (animals hunted for food) had become scarce, and consequently, success had become essential when a kill was possible. And so we see the invention of the Harif point, a new kind of arrowhead. Further north, the impact of the Younger Dryas may have been less severe. Yet survival still required more than just a return to the ancient mobile hunter-gatherer lifestyle, especially as there were now many more people needing food than had been the case during earlier periods, when the Natufians lived in permanent dwellings. One response was to hunt a much wider range of animals than before, and hence we find in Late Natufian settlements the bones of many small-game species as well as larger, ever-present gazelle. Another response to the changing climate was to continue, and perhaps expand the cultivation of plants. Wild cereals were particularly hard hit by the Younger Dryas owing to a decrease in the concentration of carbon dioxide (CO2) in the atmosphere. This diminution, carefully documented from air bubbles trapped in Antarctic ice, inhibited their photosynthesis and markedly reduced their yields. Consequently, whatever cultivation practices had begun during the Early Natufian period – weeding, transplanting, watering, pest control – may now have become essential to secure sufficient food. And these may have created the first domesticated strains. This appears to be what happened at the village of Abu Hureyra just before its abandonment. When the archaeologist Gordon Hillman studied the cereal grains from the site, he found a few grains of rye from plants that had undergone the transition into domestic forms. When dated, they were shown to lie between 11,000 and 10,500 B.C. – the oldest domesticated cereal grain from anywhere in the world. Along with these grains, Hillman found seeds from the weeds that typically grow in cultivated soil. And so it appears that, as the availability of wild plant foods declined due to the onset of the Younger Dryas, the Abu Hureyra people invested an ever greater amount of time and effort in caring for the wild rye and by doing so unintentionally transformed it into a domestic crop. But even this could not support the village – it was abandoned as people were forced to return to a mobile lifestyle, perhaps carrying pouches of cereal grain. The domesticated rye of Abu Hureyra reverted to its wild state. The geographical range of the Late Natufians also changed. With their increased interest in plant cultivation, the Late Natufians drifted away from the depleted woodlands where their forebears once flourished. They were drawn to the alluvial soils (soils deposited by rivers) of the valleys, not only those of the River Jordan, but also those found by the great rivers of the Mesopotamian plain and in the vicinity of lakes and rivers throughout the Middle East. Large expanses of these rich, fertile soils became available as the rivers and lakes struck during the Younger Dryas Wild, but cultivated, cereals grew well in such soil, especially when close to the meager springs, ponds, and streams that survived the arid conditions.