Agricultural origins in Southwest Asia: the pace of transition

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.