綜合報(bào)道，英德兩國(guó)科學(xué)家進(jìn)行的一項(xiàng)研究顯示,，7000年前的新石器時(shí)代人類(lèi)消化不了牛奶,，如果強(qiáng)飲會(huì)使人體生病。研究稱(chēng),，現(xiàn)在的非洲人和亞洲人中有幾十億仍缺少能夠生成消化牛奶的酶的基因,，但是幾乎所有歐洲人和美洲人已可以飲用牛奶。
研究結(jié)果稱(chēng),，歐洲人也只是在最近的7000年才學(xué)會(huì)消化牛奶,，公元前5000年的人類(lèi)尸骨中找不到消化牛奶的基因,。隨著乳品業(yè)的出現(xiàn)，早期歐洲人很快開(kāi)始可以忍受牛奶,，他們因此有了相對(duì)于其它地方人類(lèi)的生存優(yōu)勢(shì),。如今，超過(guò)90%的北歐人可以在毫無(wú)不適反應(yīng)的情況下飲用牛奶,，一些非洲人和中東人也可以如此,。然而，其它地方的多數(shù)成年人仍無(wú)法消化這種天然的乳糖,。乳糖的消化靠的是乳糖分解酵素,，如果沒(méi)有這種酶，服用牛奶和乳制品會(huì)造成腹脹和腹泄,。
英國(guó)倫敦學(xué)院大學(xué)的馬克-托瑪斯博士稱(chēng),，他們的研究小組對(duì)公元前5000年到5840年的一些新石器時(shí)代歐洲人的骨骼進(jìn)行了DNA檢測(cè)，結(jié)果沒(méi)有找到乳糖分解酵素基因,。托瑪期說(shuō),，歐洲人從不能忍受牛奶到能夠天天飲用牛奶，這種變化除了強(qiáng)大的自然選擇的原因外,，再?zèng)]有別的理由可以解釋,。
此前，人們一直認(rèn)為某些歐洲人生來(lái)就可以忍受乳糖,，而且這種與生俱來(lái)的能力促使他們開(kāi)始從事乳品業(yè),。但是，從托瑪斯等人的研究成果來(lái)看,，人們是因?yàn)榻?jīng)常接觸牛奶而具備了對(duì)乳糖的忍受力,。托瑪斯稱(chēng)，這是一個(gè)先有雞還是先有蛋的問(wèn)題,。除此之外,，他們還在研究為什么不同地區(qū)人類(lèi)的乳糖忍受力不盡相同。他說(shuō)：“盡管最早的牧農(nóng)出現(xiàn)在南歐,，但是現(xiàn)在仍有大約80%的南歐人仍無(wú)法忍受乳糖,。”
托瑪斯等人的研究成果已經(jīng)在美國(guó)國(guó)家科學(xué)院院刊發(fā)表。
部分英文原文：
Published online before print February 28, 2007, 10.1073/pnas.0607187104
PNAS | March 6, 2007 | vol. 104 | no. 10 | 3736-3741
Absence of the lactase-persistence-associated allele in early Neolithic Europeans
J. Burger,, M. Kirchner, B. Bramanti, W. Haak, and M. G. Thomas
Johannes Gutenberg University, Institute of Anthropology, Saarstrasse 21, D-55099 Mainz, Germany; and Department of Biology, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, United Kingdom
Edited by Walter Bodmer, Cancer Research UK, Oxford, United Kingdom, and approved December 27, 2006 (received for review September 4, 2006)
Lactase persistence (LP), the dominant Mendelian trait conferring the ability to digest the milk sugar lactose in adults, has risen to high frequency in central and northern Europeans in the last 20,000 years. This trait is likely to have conferred a selective advantage in individuals who consume appreciable amounts of unfermented milk. Some have argued for the "culture-historical hypothesis," whereby LP alleles were rare until the advent of dairying early in the Neolithic but then rose rapidly in frequency under natural selection. Others favor the "reverse cause hypothesis," whereby dairying was adopted in populations with preadaptive high LP allele frequencies. Analysis based on the conservation of lactase gene haplotypes indicates a recent origin and high selection coefficients for LP, although it has not been possible to say whether early Neolithic European populations were lactase persistent at appreciable frequencies. We developed a stepwise strategy for obtaining reliable nuclear ancient DNA from ancient skeletons, based on (i) the selection of skeletons from archaeological sites that showed excellent biomolecular preservation, (ii) obtaining highly reproducible human mitochondrial DNA sequences, and (iii) reliable short tandem repeat (STR) genotypes from the same specimens. By applying this experimental strategy, we have obtained high-confidence LP-associated genotypes from eight Neolithic and one Mesolithic human remains, using a range of strict criteria for ancient DNA work. We did not observe the allele most commonly associated with LP in Europeans, thus providing evidence for the culture-historical hypothesis, and indicating that LP was rare in early European farmers.
ancient DNA | dairying | selection
Most mammals lose the ability to digest the milk sugar lactose after weaning because of an irreversible reduction in expression of the intestinal enzyme lactase (i.e. lactase phlorizin hydrolase). This pattern is also seen in most humans, but some continue expressing lactase throughout adult life [lactase persistence (LP)]. This dominant Mendelian trait is common in populations of northern and central European descent and shows intermediate frequencies in southern and eastern Europe (1). Africa and the Middle East show a more complex distribution, with pastoralists often having high frequencies of LP, whereas in their nonpastoralist neighbors, it is usually much less common (2). The T allele of C/T polymorphism located 13,910 bp upstream of the lactase (LCT) gene (–13.910*T) has been shown to associate strongly with LP in Europeans (3), and recent in vitro studies have indicated that it can directly effect LCT gene promoter activity (4). However, different but closely linked polymorphisms associate with LP in most African groups, indicating either that –13.910*T is not causative of LP and/or that the trait has evolved more than once in humans (2, 5, 6).
It has been suggested that the modern frequency of LP in Europe is the result of a relatively recent and strong selection process (7, 8). Although not fully understood, the biological advantages of LP probably include the continuous availability of an energy- and calcium-rich drink that enables a farming community to overcome poor harvests. Because it is unlikely that LP would have provided a selective advantage in the absence of a supply of fresh milk, and because of observed correlations between the frequency of LP and the extent of traditional reliance on animal milk, the culture-historical hypothesis has been proposed (8–12). Under this model, LP was driven from very rare cases of preadaptation to appreciable frequencies only after the cultural practice of dairying arose. However, an opposing view, the reverse cause hypothesis, has also been proposed (8, 13, 14). According to this model, human populations were already differentiated with regard to LP frequency before the development of dairying, and the presence of LP determined the adoption of milk production and consumption practices (15). Based on the decay of long-range haplotypes and variation in closely linked microsatellites, the inferred age of the –13.910*T allele in Europe has been estimated to be between 2,188 and 20,650 years (16) and between 7,450 and 12,300 years (17), respectively. These dates bracket archaeological estimates for the introduction of domestic cattle breeds into Europe (18), and when considered in conjunction with the modern frequency distribution of the –13.910*T allele in Europe, they indicate a strong selective advantage to LP. However, these date estimates do not exclude the possibility of LP being present in Europe at appreciable frequency before the Neolithic.
Analyzing DNA from archaeological human remains is the only direct method to identify the presence of the –13.910*T allele in specific prehistoric populations. Unfortunately, the authenticity of ancient DNA data, particularly when recovered from human remains, cannot be guaranteed because of the problems of modern DNA contamination (19–23) and postmortem damage (24–29). Although generic validation criteria have been proposed for ancient DNA work (30), they are often not tailored to the specific questions being addressed. Gilbert et al. (31) have recently proposed a more flexible approach, whereby the validation criteria used are customized to the particulars of the archaeological source material and the aims of the investigation. In the context of understanding the origins and evolution of LP in Europe, we have engaged this approach to analyze 51 bone samples from early Holocene sites in central and eastern Europe for the preservation of mitochondrial and nuclear DNA and subsequently for the presence of the –13.910*T allele. Using a range of authentication criteria, we obtained high confidence genotypes from eight early Neolithic and one Mesolithic skeletons of central, northeast, and southeast Europe (Fig. 1).
Fig. 1. Locations of archaeological sites.
