Determination of the parameters of the Guildhall amphitheatre ellipse in London

The paper presents an algorithm for computing the parameters of an ellipse that fits, in a least-squares sense, the surveyed remains of Roman amphitheatre arena walls. The algorithm is validated using ideal data and the near complete remains of the amphitheatre at Sarmizegetusa in Romania. The technique is then applied to the partial remains of the amphitheatre at the Guildhall in the City of London, in the UK, which cover only the Eastern end of the monument. The underlying mathematical model and method of solution is presented. The method was developed with a view to determining firstly whether or not an ellipse is a feasible model for the actual constructed curve geometry, and secondly, where the Western end of the Guildhall amphitheatre might be located. One single ellipse was found to fit the Guildhall remains, with measurement residuals at the level of 30 mm in the eastings and 26 mm in the northings. The ellipse had a semi-major axis of 28.345 m and a semi-minor axis of 22.775 m. The issues of alternative geometries and the practicalities of their use in construction are considered.     

现代人起源六十年之争

现代人起源的争议由来已久,而遗传学的冲击尤大。1987年,美国加州大学伯克利分校的人类学家根据对147名各大洲不同人种妇女胎盘细胞中的线粒体DNA分析,将所有现代人起源追溯到20万年前的一位非洲妇女。这一被称为"夏娃理论"或"走出非洲"的假设对学术界和一般公众产生了巨大的影响①。这意味着人类起源和进化的主干在非洲,其他各大洲的古人类代表都不过是进化中的旁支。后来,有学者对"夏娃理论"的遗传学推理做了检验,认为现代人有一个共同祖母的说法十分可疑②。虽然目前国际学界承认,古代的非洲种群不管在规模还是在古老性上,对全球现代人进化有着最重要的影响,但是这种影响究竟是整体取代还是存在一定的基因交流仍存在争议。本期介绍的是两位美国人类学家对现代人起源之争的历史回顾,对我们了解好似雾里看花的现代人起源之争有一定的帮助。现代人起源争论的关键,在于如何解释遗传学证据与化石材料之间的矛盾。遗传学研究表明,现代人的线粒体DNA和Y染色体变异非常小,表明其更新世祖先种群的规模很小,而且分布非常有限。但古人类化石却表明人类祖先的种群不少,分布也很广。如果古人类以很小的种群在漫长的时间里孤立进化却没有形成新的物种显然不可思议。于是,一种"瓶颈说"被提出来解释这种现象,即人类进化在更新世末遇到一个瓶颈,只有少量基因被保留下来,"夏娃理论"就认为现代人都是一小群非洲人的后代。但是与"夏娃理论"不同,本文作者认为这种瓶颈在更新世一直存在,并提出了一种间断进化模式来说明人科化石记录的地理差异和遗传学解释,即人类进化是一种适应辐射(种群扩张)和物种选择(种群减少)的交替过程。他们认为,现代人起源也是这种过程的产物,大部分更新世人科化石是没有留下后裔的一批种群。在过去两百万年的进化过程中,人类并非源自单一物种,而是来自由间断性物种形成事件所隔开的几个相继的物种,反复的遗传瓶颈大大减少了最后智人种群的遗传变异。作者还指出,现代人起源应该从生物人类学的方法来进行研究,并重视自然选择的作用。     

New radiometric dating of water management features at the prehistoric Purrón Dam Complex,Tehuacán Valley,Puebla, México

Recent investigations at the prehistoric Purrón Dam Complex in the Tehuacán Valley of southern Puebla, México applied radiometric dating to more securely date the complex. Ceramic-based dating in the 1960′s placed the Dam's origin to the Formative Period. While Formative Period origins are widely accepted, the chronology lacks resolution and direct dates. Samples from impounded sediments behind the dam and from prehistoric canal fill were dated through Optically Stimulated Luminescence (OSL) as well as both standard ¹⁴C and accelerated mass spectrometry (AMS) assays. These samples directly date the functioning of the water management features within the complex. The results confirm the Formative Period origins of the Purrón Dam, and offer new insights into the construction and functioning of the dam and irrigation system.     

The Landes de Gascogne (southwest France): periglacial desert and cultural frontier during the Palaeolithic

During the last glacial period, a large part of the Aquitaine basin (southwest France) was a periglacial desert comprising coversands with low-relief dune fields surrounded by loess accumulations. OSL and radiocarbon dates show that the phase of maximum sand deposition coincides with Marine Isotope Stage (MIS) 2. Peats and gleyic palaeosoils intercalated within the sands at some sites indicate that vegetation cover was able to develop locally during short events, possibly D–O interstadials, due to raised groundwater levels in interdunal depressions. Few Palaeolithic sites have been discovered in the coversand area in contrast to the peripheral loess region. Systematic survey along a future highway corridor demonstrates that this paucity of sites is not the result of insufficient survey nor deep site burial, but rather reflects an archaeological reality. This strongly suggests that the sand area was not attractive for hunter–gatherer populations due to its reduced levels of water resources, and available vegetation and animal biomass. The distribution of cultural markers such as art items and projectile points also shows that the coversand area probably acted as a barrier separating two different cultural sub-areas, one in the Pyrenees and Cantabria, the other in the Périgord. As a consequence, the commonly accepted view that southwest France, as a whole, served as a refugia during the cold and arid phases of the Pleistocene should be replaced by a more complex one that reflects the fact that a large part of the territory was almost unoccupied and that human populations were concentrated along alluvial valleys.