Agricultural origins in Southwest Asia: the pace of transition

农业被认为是在大约10000年前在肥沃的新月地区开始的，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，以及安纳托利亚以东的一些山区。农业的开始是人类历史上最重要的事件之一，是人类第一次摆脱环境强加的限制，学会了如何为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，以及古典希腊和罗马等复杂文明的发展。许多研究致力于了解农业的起源，但许多问题仍然没有答案。其中最重要的一个问题是，从狩猎采集向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栽培和驯化，前者是有意种植和收获作物，后者是导致作物经历基因变化，使作物更受农民和消费者的欢迎。这些令人向往的特征，包括将成熟的种子留在植物的耳朵里，以便更容易收获，只能是遗传选择的结果，这要求农民仔细管理作物，以防止它们与野生植物杂交。实验表明，如果遵循严格的耕作做法，驯化可以在最初种植后不久进行，可能在几年内。相比之下，如果不遵循严格的做法，那么在驯化发生之前，耕作可能会持续数百年。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发挥了有意识的作用，这种转变可能是由一小群开明的人推动的。一种更渐进的过渡，虽然不排除我们的祖先发挥积极的作用，但也有可能农业的产生是一万年前肥沃的新月盛行的社会和环境条件的自然结果。我们一直在通过对一粒粒小麦的遗传分析来研究这个问题，一粒粒小麦是一种今天没有被广泛种植的小麦，但它是西南亚最早也是最重要的作物之一。我们发现，尽管栽培植物与土耳其东南部卡拉卡达格地区的野生植物关系最近(已知)，但栽培植物也具有更常见于伊拉克北部和伊朗西北部野生种群的遗传特征。我们认为，这些结果可以解释，如果一粒粒在肥沃的新月种植了很长一段时间才被驯化，因为在这段时间里，这种作物可能与不同的野生种群杂交，导致了我们今天看到的混合遗传特征。我们现在希望进行一个更复杂的项目来检验这一初步发现。为此，我们将研究小麦基因组1号染色体的一部分，称为5S-DNA-A1基因座，它由一段长度为360个核苷酸的DNA序列组成，以从头到尾的方式重复大约500次。单个重复序列的序列非常相似，但不完全相同，不同野生种群中存在的特定序列是不同的。通过找出栽培一粒粒植物中存在哪些序列，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有关，以及每种情况下这种关系的性质是什么。

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.