Agricultural origins in Southwest Asia: the pace of transition

农业被认为大约在1万年前开始于“新月沃土”，这是西南亚的一个地区，包括美索不达米亚平原、叙利亚和巴勒斯坦的沙漠，以及安纳托利亚东部的一些山区。农业的开始是人类历史上最重要的事件之一，是人类第一次从环境的限制中解脱出来，学会了如何根据自己的目的塑造环境。农业也对人类社会产生了深远的影响，粮食生产方法的改进最终导致了人口的快速增长和复杂文明的发展，如古典希腊和罗马。许多研究致力于了解农业的起源，但许多问题仍未得到解答。其中最重要的一个问题是，从狩猎采集到农业的过渡是一个快速的过程还是一个渐进的过程。当我们考虑这个问题时，重要的是我们要区分耕种和驯化，前者是指有意种植和收获作物，后者是指作物发生基因变化，使这些作物更受农民和消费者的欢迎。这些令人满意的特点，包括在植物的穗中保留成熟的种子，这样收割起来更容易，只能作为遗传选择的结果，这就要求农民仔细管理作物，防止它们与野生植物杂交。实验表明，如果遵循严格的耕作方法，驯化可以在最初的种植后很快进行，也许在几年内。相反，如果不遵循严格的实践，那么在驯化发生之前，种植可以持续数百年。因此，从狩猎采集到农业的快速转变意味着我们的祖先在农业的建立中扮演了有意识的角色，而这种转变可能是由一小群开明的人推动的。一个更渐进的转变，虽然不排除我们的祖先的积极作用，但留下了一种可能性，即农业是一万年前新月沃地盛行的社会和环境条件的自然结果。我们一直在通过对小麦的遗传分析来研究这个问题，小麦是一种今天没有广泛种植的小麦，但它是西南亚最早也是最重要的驯化作物之一。我们发现，虽然栽培植物与土耳其东南部卡拉卡达格地区的野生植物最密切相关（这已经为人所知），但栽培植物也具有在伊拉克北部和伊朗西北部野生种群中更常见的遗传特征。我们相信这些结果可以解释，如果einkorn在被驯化之前在新月沃土被种植了很长一段时间，因为在这段时间里，作物可能与不同的野生种群杂交，导致我们今天看到的混合遗传特征。我们现在希望执行一个更复杂的项目来检验这一初步发现。为了做到这一点，我们将研究小麦基因组中第1号染色体的一部分，称为5S-DNA-A1位点，它由一段DNA序列组成，长度为360个核苷酸，从头到尾重复大约500次。单个重复序列非常相似但不完全相同，不同野生种群中存在的特定序列也不同。通过找出哪些序列存在于栽培的玉米中，然后将这些序列与野生植物中的序列进行比较，我们将能够准确地计算出哪些野生种群与栽培植物相关，以及每种情况下这种关系的本质。

Agriculture is thought to have begun about 10 000 years ago in the 'Fertile Crescent', a region of Southwest Asia comprising the plains of Mesopotamia, the deserts of Syria and Palestine, and some of the mountainous areas to the east of Anatolia. The beginning of agriculture was one of the most important events in the human past, being the first occasion on which humans broke free from the limits imposed by the environment and learnt how to shape the environment to their own ends. Agriculture also had far reaching effects on human society, the improved methods of food production eventually resulting in rapid population growth and the development of complex civilisations such as those of Classical Greece and Rome. Much research has been devoted to understanding the origin of agriculture but many questions remain unanswered. One of the most important of these is whether the transition from hunting-gathering to agriculture was a rapid or a gradual process. When we consider this question it is important that we make a distinction between cultivation, which is the deliberate planting and harvesting of crops, and domestication, which results in the crop undergoing genetic changes that make the plants more desirable to the farmer and consumer. These desirable features, which include such things as retention of the mature seeds in the ears of the plant so that harvesting is easier, can only arise as a result of genetic selection, which requires that the farmers manage the crops carefully to prevent them from cross-hybridizing with wild plants. Experiments have shown that if rigorous farming practices are followed, domestication can occur soon after the initial cultivation, perhaps within a few years. In contrast, if rigorous practices are not followed then cultivation can continue for hundreds of years before domestication occurs. A rapid transition from hunting-gathering to agriculture therefore implies that our ancestors played a conscious role in the establishment of agriculture, with the transition possibly being driven by a small group of enlightened people. A more gradual transition, while not precluding an active role for our ancestors, leaves open the possibility that agriculture arose as a natural consequence of the social and environmental conditions that prevailed in the Fertile Crescent ten thousand years ago. We have been studying this question by genetic analysis of einkorn, a type of wheat that is not extensively grown today but which was one of the first and most important crops to be domesticated in Southwest Asia. We discovered that although cultivated plants are most closely related to wild plants from the Karacadag region of southeast Turkey (as was already known), the cultivated plants also have genetic features more commonly found in wild populations from north Iraq and northwest Iran. We believe that these results can be explained if einkorn was cultivated in the Fertile Crescent for a lengthy period before it became domesticated, because during this period the crop might have cross-hybridised with different wild populations, resulting in the mixed genetic signature that we see today. We now wish to carry out a more sophisticated project to test this initial finding. To do this we will study a part of chromosome number 1 of the wheat genome called the 5S-DNA-A1 locus, which is made up of a sequence of DNA, 360 nucleotides in length, that is repeated approximately 500 times in a head to tail fashion. The sequences of the individual repeats are very similar but not identical, and the particular sequences that are present are different in different wild populations. By finding out which sequences are present in cultivated einkorns, and then making comparisons between these sequences and the ones in wild plants, we will be able to work out exactly which wild populations are related to the cultivated plants, and what the nature of the relationship is in each case.