综合报道,英德两国科学家进行的一项研究显示,7000年前的新石器时代人类消化不了牛奶,如果强饮会使人体生病。研究称,现在的非洲人和亚洲人中有几十亿仍缺少能够生成消化牛奶的酶的基因,但是几乎所有欧洲人和美洲人已可以饮用牛奶。
研究结果称,欧洲人也只是在最近的7000年才学会消化牛奶,公元前5000年的人类尸骨中找不到消化牛奶的基因。随着乳品业的出现,早期欧洲人很快开始可以忍受牛奶,他们因此有了相对于其它地方人类的生存优势。如今,超过90%的北欧人可以在毫无不适反应的情况下饮用牛奶,一些非洲人和中东人也可以如此。然而,其它地方的多数成年人仍无法消化这种天然的乳糖。乳糖的消化靠的是乳糖分解酵素,如果没有这种酶,服用牛奶和乳制品会造成腹胀和腹泄。
英国伦敦学院大学的马克-托玛斯博士称,他们的研究小组对公元前5000年到5840年的一些新石器时代欧洲人的骨骼进行了DNA检测,结果没有找到乳糖分解酵素基因。托玛期说,欧洲人从不能忍受牛奶到能够天天饮用牛奶,这种变化除了强大的自然选择的原因外,再没有别的理由可以解释。
此前,人们一直认为某些欧洲人生来就可以忍受乳糖,而且这种与生俱来的能力促使他们开始从事乳品业。但是,从托玛斯等人的研究成果来看,人们是因为经常接触牛奶而具备了对乳糖的忍受力。托玛斯称,这是一个先有鸡还是先有蛋的问题。除此之外,他们还在研究为什么不同地区人类的乳糖忍受力不尽相同。他说:“尽管最早的牧农出现在南欧,但是现在仍有大约80%的南欧人仍无法忍受乳糖。”
托玛斯等人的研究成果已经在美国国家科学院院刊发表。
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.