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遷徙新大陸模型

维基百科,自由的百科全书

这是本页的一个历史版本,由Sinanshi留言 | 贡献2013年4月18日 (四) 11:32 考古,地理和基因学证据选录编辑。这可能和当前版本存在着巨大的差异。

"Three maps of prehistoric America. (A) then gradual population expansion of the Amerind ancestors from their East Central Asian gene pool (blue arrow). (B) Proto-Amerind occupation of Beringia with little to no population growth for ≈20,000 years. (C) Rapid colonization of the New World by a founder group migrating southward through the ice-free, inland corridor between the eastern Laurentide and western Cordilleran Ice Sheets (green arrow) and/or along the Pacific coast (red arrow). In (B), the exposed seafloor is shown at its greatest extent during the last glacial maximum at ≈20–18 kya [25]. In (A) and (C), the exposed seafloor is depicted at ≈40 kya and ≈16 kya, when prehistoric sea levels were comparable. A scaled-down version of Beringia today (60% reduction of A–C) is presented in the lower left corner. This smaller map highlights the Bering Strait that has geographically separated the New World from Asia since ≈11–10 kya."
早期人类迁徙和殖民美洲的三个阶段图

人类在何时以何种方式迁入美洲的问题成引发了人类学家考古学家巨大的兴趣,并且成为几个世纪以来一直争论不休的话题。学术界给出了许多古印地安人在美洲定居的模型。现代生物化学技术和考古学的结合也极大地促进了人对于这个问题的认识。

目前,对于这个问题的解答工作主要涉及到以下四个互相关联的学科,考古学人类体格学DNA分析学以及语言学。目前,学界大体认可,美洲大陆第一批移民是自白令海峡迁徙而来的亚洲的族群。然而,移民的模式,时间,以及进入美洲的人为何种亚洲族群至今尚不明确。[1]

近年来,学术界不断用已经掌握的方法对一些已经建立起来的理论进行进一步证实或者是证伪,这些理论种较为出名的是认为克洛维斯人是第一个到达美洲的人类种群。[2] 随着发现的深入,过去的假设被重新审查并且新的理论随即被提出。考古证据表明,古印地安人大规模的扩张在上个冰河时期末期, 或者更精确地说,是末次冰盛期(Late Glacial Maixmum),也就是距今16,500–13,000年前。[3]

各派意见综述

Flèches préhistoriques amérindiennes, conservées à Washington

迁徙模型在时间上大致分为两派。[4][5] 第一种叫做"短时理论"(short chronology theory),短时理论认为人类第一次从阿拉斯加迁徙至新大陆的发生晚于距今15,000 – 17,000年前,之后展开了波浪式的移民推进(waves of immigrants)。[6][7] 第二种叫做"长时理论"(long chronology theory)其认为,第一批人类到达美洲大陆所在的半球时间要远早于15,000 – 17,000年前,他们认为可能的时间为 21,000–40,000年前。[8][9] ,然后才是第二波巨大的移民潮。[10][11][12]

一个引发巨大争论的原因是,南部美洲和北部美洲的古印地安人定居点在考古证据上的不连续。一个大体上统一的考古学文化在北部和中部美洲被发现,距今至少有13,500年的历史,考古学家称之为克洛維斯文化[13] 然而,同一时代的南美洲的据点就缺乏这样的一致性,拥有更大的文化多样性。考古学家认为,"克洛维斯先至论"和古印地安人时间框架都不足以解释复杂的美洲石器时代(lithic stage)工具是如何在南美洲出现的。一些理论学家正在寻求一种可以整合南北美洲考古记录的新的殖民模型。

对美洲土著的基因研究发现,“殖民缔造人群”(colonizing founder)可能是单一祖先人群。基于Y染色体微卫星中的美洲单倍型类群Q1a3a(Y-DNA)分叉的时间推算,这个单一祖先人群出现在距今10,000到15,000年前。而这个单一祖先人群的很有可能来自白令海峡 [14][15][16][17][18] [5][19] 这一点仍然不足以说明,在此之前,抑或其他基因也曾试图在这片土地上繁衍并且以失败告终。因为基因测试只能基于现存人口的遗传信息进行。[5]

当白令大陆桥露出海平面的时候,从东北亚步行至阿拉斯加是相对容易的。然而从阿拉斯加到其他北美洲地区路途却十分艰难。我们猜测主要有两个可能的路径,沿大西洋海岸往南,或者是逻辑山脉东侧的内陆通道—麦肯锡走廊(Mackenzie Corridor)[17]。在Laurentide和Corilleran冰盖最大的时候,这两条道路都是极其容易的。Corilleran冰盖西起太平洋,东至Laurentide冰盖,也就是今天加拿大的不列颠-哥伦比亚省和阿尔伯塔省的交界。地理学证据证明,太平洋沿岸路保持畅通是在公元前21,000年之前和公元前13,000年之后。在上一个冰河时期中最冷的一千年,大致在距今23,000到19,000年之前,冰川是的道路充满危险。即使使用船只也十分困难,因为水体中到处都是冰山。况且尚无古代海岸线上有船只的考古证据。在这段时间以前,这些通路是没有结冰的。另外,当气候温暖的时候,土地上被植被覆盖,早期古印第安人可以在这里补充给养,缝补衣服帐篷,以及重置工具等等。[15]海岸线和船只理论有一个模糊的假设,那就是一个在北美大陆上的古印第安人可能已经不是纯粹的陆地狩猎者,而已经习惯于航海或者半航海的生活。[12]另外,”白令人”(北阿拉斯加人)甚至很有可能是由于被上一次冰河期的逼迫下,在20,000年前,向北美内陆和海岸线迁徙, [20] 并且留下占据某些特定局部区域的考古证据。然而,除非他们最终在最后一次冰期结束后仍然生存繁衍,不然他们就不能被当作是“缔造人群”(founding population)。[21]


陆桥论

也被称为白令海峡沿岸论(Bering Strait Theory)或者白令陆桥论(Beringia)。自从1930年起,陆桥论被广泛接受。而早在1590年,耶稣会学者José de Acosta就已经提出这样的初步假设。[22] 陆桥论主张,第一批美洲移民是从西伯利亚来到阿拉斯加。原因则很可能是追赶迁徙的牛群。我们可以通过收集氧的同位素深海泥土的取样发现,在最后一个更新世,也就是距今50,000-10,000年前,海平面比现在低60米左右。那样就有一段至少1000英里宽的大陆桥连接西伯利亚和阿拉斯加。也就是这段时间内,那些追逐大型猎物的猎人在距今大约12,000年前的时候通过大陆桥到达美洲,并且在距今11,000年前最终到达南美洲的最南端。

Shrinking of the Bering land bridge
Shrinking of the Bering land bridge
人类自白令海峡往南移民的无障碍通路表[23]
时间 公元前 B.C. 白令路桥 "大陆桥" 海岸线 Mackenzie 走廊
38,000–34,000 可通行 (开启) 开启 关闭
34,000–30,000 淹没 (关闭) 开启 开启
30,000–22,000 可通行 (开启) 关闭 开启
22,000–15,000 可通行 (开启) 开启 关闭
15,000–today 淹没 (关闭) 开启 开启

Clovis culture

Map showing the approximate location of the ice-free corridor and specific Paleoindian sites (Clovis theory).

This big game-hunting culture has been labeled the Clovis culture, and is primarily identified by its artifacts of fluted projectile points. The culture received its name from artifacts found near Clovis, New Mexico, the first evidence of this tool complex, excavated in 1932. The Clovis culture ranged over much of North America and appeared in South America. The culture is identified by distinctive "Clovis point", a flaked flint spear-point with a notched flute by which it was inserted into a shaft; it could be removed from the shaft for traveling. This flute is one characteristic that defines the Clovis point complex.

Dating of Clovis materials has been by association with animal bones and by the use of carbon dating methods. Recent reexaminations of Clovis materials using improved carbon-dating methods produced results of 11,050 and 10,800 radiocarbon years B.P. (before present). This evidence suggests that the culture flowered somewhat later and for a shorter period of time than previously believed. Michael R. Waters of Texas A&M University in College Station and Thomas W. Stafford Jr., proprietor of a private-sector laboratory in Lafayette, Colorado and an expert in radiocarbon dating, attempted to determine the dates of the Clovis period. The heyday of Clovis technology has typically been set between 11,500 and 10,900 radiocarbon years B.P. (The radiocarbon calibration is disputed for this period, but the widely used IntCal04 calibration puts the dates at 13,300 to 12,800 calendar years B.P.). In a controversial move, Waters and Stafford conclude that no fewer than 11 of the 22 Clovis sites with radiocarbon dates are "problematic" and should be disregarded—including the type site in Clovis, New Mexico. They argue that the datable samples could have been contaminated by earlier material. This contention was considered highly controversial by many in the archaeological community.

Clovis-type artifacts seem to disappear from the archaeological record after the hypothesized Younger Dryas impact event, roughly 12,900 years before the present. The effects of the event possibly caused a decline in post-Clovis human populations and shifts in culture and behavior patterns.[24]

水路移民论

arlier finds have led to a pre-Clovis culture theory encompassing different migration models with an expanded chronology to supersede the "Clovis-first" theory.

Pacific coastal models

Pacific models propose that people first reached the Americas via water travel, following coastlines from northeast Asia into the Americas. Coastlines are unusually productive environments because they provide humans with access to a diverse array of plants and animals from both terrestrial and marine ecosystems. While not exclusive of land-based migrations, the Pacific 'coastal migration theory' helps explain how early colonists reached areas extremely distant from the Bering Strait region, including sites such as Monte Verde in southern Chile and Taima-Taima in western Venezuela. Two cultural components were discovered at Monte Verde near the Pacific Coast of Chile. The youngest layer is radiocarbon dated at 12,500 radiocarbon years (~14,000 cal BP) [來源請求] and has produced the remains of several types of seaweeds collected from coastal habitats. The older and more controversial component may date back as far as 33,000 years, but few scholars currently accept this very early component.[來源請求]

Other coastal models, dealing specifically with the peopling of the Pacific Northwest and California coasts, have been advocated by archaeologists Knut Fladmark, Roy Carlson, James Dixon, Jon Erlandson, Ruth Gruhn, and Daryl Fedje. In a 2007 article in the Journal of Island and Coastal Archaeology, Erlandson and his colleagues proposed a corollary to the coastal migration theory—the "kelp highway hypothesis"—arguing that productive kelp forests supporting similar suites of plants and animals would have existed near the end of the Pleistocene around much of the Pacific Rim from Japan to Beringia, the Pacific Northwest, and California, as well as the Andean Coast of South America. Once the coastlines of Alaska and British Columbia had deglaciated about 16,000 years ago, these kelp forest (along with estuarine, mangrove, and coral reef) habitats would have provided an ecologically similar migration corridor, entirely at sea level, and essentially unobstructed.

Southeast Asians: Paleoindians of the Coast

The boat-builders from Southeast Asia may have been one of the earliest groups to reach the shores of North America.[來源請求] One theory suggests people in boats followed the coastline from the Kurile Islands to Alaska down the coasts of North and South America as far as Chile [2 62; 7 54, 57]. The Haida nation on the Queen Charlotte Islands off the coast of British Columbia may have originated from these early Asian mariners between 25,000 and 12,000.[來源請求] Early watercraft migration would also explain the habitation of coastal sites in South America such as Pikimachay Cave in Peru by 20,000 years ago and Monte Verde in Chile by 13,000 years ago [6 30; 8 383].

"'There was boat use in Japan 20,000 years ago,' says Jon Erlandson, a University of Oregon anthropologist. 'The Kurile Islands (north of Japan) are like stepping stones to Beringia,' the then continuous land bridging the Bering Strait. Migrants, he said, could have then skirted the tidewater glaciers in Canada right on down the coast." [7 64]'

Atlantic coastal model

Archaeologists Dennis Stanford and Bruce Bradley champion the coastal Atlantic route. Their Solutrean Hypothesis is also based on evidence from the Clovis complex, but instead traces the origins of the Clovis toolmaking style to the Solutrean culture of Ice Age Western Europe.[25] The theory suggests that early European people (or peoples) may have been among the earliest settlers of the Americas.[26][27] Citing evidence that the Solutrean culture of prehistoric Europe may have provided the basis for the tool-making of the Clovis culture in the Americas, the theory suggests that Ice Age Europeans migrated to North America by using skills similar to those possessed by the modern Inuit peoples and followed the edge of the ice sheet that spanned the Atlantic. The hypothesis rests upon particular similarities in Solutrean and Clovis technology that have no known counterparts in Eastern Asia, Siberia or Beringia, areas from which, or through which, early Americans are known to have migrated. The theory is largely discounted by most professionals for a variety of reasons, including the fact that the differences between the two tool-making traditions far outweigh the similarities, the several thousand miles of the Atlantic Ocean to be crossed, and the 5,000-year-span that separate the two different cultures.[28][29] Genetic studies of Native American populations have also shown the Solutrean theory to be unlikely, showing instead that the five main mtDNA haplogroups found in the Americas were all part of one gene pool migration from Asia.[30]

Problems with evaluating coastal migration models

The coastal migration models provide a different perspective on migration to the New World, but they are not without their own problems. One of the biggest problems is that global sea levels have risen over 100 metres since the end of the last glacial period, and this has submerged the ancient coastlines which maritime people would have followed into the Americas. Finding sites associated with early coastal migrations is extremely difficult—and systematic excavation of any sites found in deeper waters is challenging and expensive. If there was an early pre-Clovis coastal migration, there is always the possibility of a "failed colonization." Another problem that arises is the lack of hard evidence found for a "long chronology" theory. No sites have yet produced a consistent chronology older than about 12,500 radiocarbon years (~14,500 calendar years) [來源請求], but research has been limited in South America related to the possibility of early coastal migrations.

基因与血型

Schematic illustration of maternal geneflow in and out of Beringia.Colours of the arrows correspond to approximate timing of the events and are decoded in the coloured time-bar. The initial peopling of Berinigia (depicted in light yellow) was followed by a standstill after which the ancestors of indigenous Americans spread swiftly all over the New World while some of the Beringian maternal lineages–C1a-spread westwards. More recent (shown in green) genetic exchange is manifested by back-migration of A2a into Siberia and the spread of D2a into north-eastern America that post-dated the initial peopling of the New World.
Schematic illustration of maternal (mtDNA) gene-flow in and out of Beringia.

学界早在1920年代就指出,在哥伦布到达新大陆前,美洲的绝大部分人口为O型血以及很小部分在北方的A型人口。之后,由Cavalli-Sforza为先驱,开始通过对更早的历史人口迁徙记录更深入的统计学和基因学基因学研究。Jacob BronowskiThe Ascent of Men(1973)中这样说道,

"我们没有理由不相信,第一批来到美洲的一些是来自一个较小且具有亲属关系的O型血人群,然后他们在美洲大陆上繁衍生息,并且向南扩张。之后到来的,同样是一个较小的人群,但是混合了A型和O型血,来到北美洲。"[31]

现代美洲基因学则主要研究人类Y染色体DNA单倍型类群人类粒线体单倍群。基因图谱显示两种迥然不同的基因(genetic episodes),也就是美洲土著和欧洲殖民者的基因。决定前者的基因谱数的是合子的突变和基本单倍型。[5][32][33] [32]由此说明,新大陆上的居民由小部分基础人群(found population)一开始从白令路桥一步步繁衍而来。[5][16][21]微卫星在南美洲的多样性和Y种系特异性基因的分布表明,一个特定印地安人在迁徙在美洲的一开始就被孤立起来。[34] ,其中就包括 Na-Dené, 因纽特人 以及 阿拉斯加土著。在这些人群中发生了和其他美洲土著人群完全不同的基因突变。前者出现了haplogroup Q (Y-DNA) mutations,而后者则多为mtDNA 和atDNA突变。[35][36][37] 这也就暗示了那些最早到达美洲大陆最北端和格林兰的人群恰恰后来的移民而非一开始到达美洲的人群。[38][39]


考古,地理和基因学证据选录

40,000 B.C. – 25,000 B.C.
30,000–20,000 years ago:
  • 70-80年代,考古学家Jacques Cinq-Mars在育空鯥洞(Bluefish Caves)发现有人工雕琢痕迹的猛犸象骨。[41] [42]
  • 2004年,南卡罗来那考古和人类学学院的Albert Goodyear在topper,(Allendale, county, South Carolina),通过炭的同位素法发现了一块早于科利维斯文化的燃烧过的木炭。 [43]然而也有人认为生成这些木炭的原因是森林火灾。[43]也有人对此表示异议。[12][44]
  • 50年代,由于在冰川沙石(Grimshaw, Bow River 以及Lethbridge Alberta附近)下发现了石质器具,地理学家由此认为在Alberta和Laurentide ice sheet之间有一个没有为冰雪所覆盖的走廊。 据此,也表明在冰河世纪以前可能已经有游牧民族在这里生活。[45]1961年一块儿童的头骨在Taber,Alberta附近被发现,是迄今在Alberta发现的最古老的居民。[46]也有人对此表示异议。[47]
  • 剑桥DNA项目组估计人类到达美洲大约是25,000年以前。[48]其他的基因学家对于年代的的估计意见分歧很大,认为美洲的人群可能在范围在21,000年到42,000年间。[11]
  • 西伯利亚猛犸象猎人被认为可能通过无冰走廊到达过北极的深处。这个理论于1970年被地理学家提出。他们的证据是对泥土样本的分析。分析的结论是所有的冰层的年龄不超过17,000。由此说明,无冰走廊的存在。[15]
23,000–16,500 years ago:
  • The Ice Age entombs the northern hemisphere in glaciers, cutting off routes from Siberia to the south.[49]
  • 2002 the presence of the X haplogroup was found in a small percentage of modern indigenous Americans that is known to exist in a few locations in Europe and the Middle East. Subsequent research indicated that this DNA was not the result of genetic mixing after Columbus. However, the time estimates on haplogroup X entering Americas is around 15,000 to 20,000 years ago.[50]
  • Genetic evidence (2007–2009) suggests the Beringia population's first genetic diversification from Asian populations occurred.[51] An article in the American Journal of Human Genetics states "Here we show, by using 86 complete mitochondrial genomes, that all Native American haplogroups, including haplogroup X, were part of a single founding population.[11][51][52]
16,500–13,000 years ago:
  • Receding glaciers reopened an ice-free corridor through Canada between Alaska and the rest of the Americas. Massive flooding would have created large lakes covering vast areas of north America with glacial waters.[53]
  • Age estimates based on Y-chromosome micro-satellite place diversity of the so called "American Haplo" Q1a3a1 at around 10,000 to 15,000 years ago.[5]
  • Mass extinction of large fauna begins due to hunting and perhaps climate change. The dire wolf, Smilodon, American lion, giant beaver, ground sloths, mammoths, American mastodon, American camel and American equine all become extinct by 11,000 years ago.[54][55]
  • Pre-Clovis sites uncovered from 1973 to 1978 Meadowcroft Rockshelter in Pennsylvania site indicated occupancy as early as 16,000 years ago and possibly as long as 19,000 years ago. Dates in excess of 19,000 years have been claimed for the deepest occupation layer uncovered.[56]
  • pre-Clovis sites found in Monte Verde, located along Chinchihuapi Creek, in Chile. A crew of eighty people, led by Tom Dillehay of the University of Kentucky, excavated the site from 1977 to 1985.[57] A coastal migration could explain how people arrived in Monte Verde.[57]
  • 2000, archaeologists say people were living at Cactus Hill, Virginia where stone tools and charcoal from a fire pit are found.[58]
15,000–13,000 years ago:
  • The Taima Taima mastodon kill/butchering site in Falcon, Venezuela was first excavated by J.M. Cruxent in the 1960s and 1970s. It is one of the earliest archaeological sites that is pre-Clovis. In 1976 a broken El Jobo point (red arrow) was found inside the pubic cavity of a partially disarticulated and butchered young mastodon whose bones had been cut, with a jasper flake found near the left ulna of the animal.[59]
  • Peñon woman found by an ancient lake bed near Mexico City in 1959.[60]
  • El Abra sites located in the valley east of the city of Zipaquirá, Colombia. First excavated by Gonzalo Correal and associates in the late 1970s and early 1980s. 3,072 pieces found indicate it was inhabited continuously for over 7,000 years.[61]
  • At Paisley Caves in the Cascade Range of Oregon, archaeologists find a scattering of human coprolites, or fossil feces in 2003.[62] The mitochondrial DNA extracted from coprolites linked the cave dwellers to two genetic groups of early Americans that arose 14,000 to 18,000 years ago.[62] These two genetic groups were the founding Paleo-Indians and later Na-Dené migration.[16][35]
13,500–12,000 years ago:
  • The Ice Age is ending, melting glaciers have raised sea levels 120 meters and submerged the land bridge between Alaska and Siberia. Geologic evidence indicates that by 11,500 years ago, the Cordilleran and Laurentide ice sheets had retreated far enough to open a habitable ice-free corridor between them. The exposed land was dry and probably restored enough to support plants and animals, which the migrating hunter-gatherer followed.[63]
  • Clovis theory – People were living near Clovis, New Mexico where tools from this era were found in the 1930s. This find gave rise to the widely held "Clovis First" theory that people spread through the Americas only after the Ice Age.[64] The Clovis culture was believed replaced by several more localized regional cultures, such as the Folsom tradition, from the time of the Younger Dryas cold climate period.[11]
  • Peru coastal region inhabitants fished with nets and bone hooks, collecting seafood such as crabs and sea urchins.[65]
12,000–10,000 years ago:
  • Ice age over, climate similar to present temperatures. Old migration theories believe first widespread migration in South America and subsequently a dramatic rise in population all over the Americas, introduced in the 1930s.[66]
  • The Maritimes of Canada are settled by Paleo-Indians. Sites in and around Belmont, Nova Scotia have evidence indicating small seasonal hunting camps, perhaps re-visited over many generations.[67]
  • Luzia Woman's skull and other bones excavated in the Lagoa Santa, Brazil area by French archaeologist Annette Laming-Emperaire in the 1970s.[68] By 2006, Lagoa Santa sites had produced no fewer than 75 well-preserved ancient skulls.[68]
  • 1994, University of California, Riverside anthropologist R. Erv Taylor examined seventeen of the Spirit Cave artifacts near Fallon, Nevada from the 1940s using mass spectrometry. The results indicated that a mummy was approximately 9,400–10,200 years old — older than any previously known North American mummy.[69]
  • Unique markers found in DNA recovered from an Alaskan tooth were found in specific coastal tribes, and were rare in any of the other indigenous peoples in the Americas. This finding lends substantial credence to a migration theory that at least one set of early peoples moved south along the west coast of the Americas in boats.[70]
9,000–8,000 years ago:
  • Remains, known as Kennewick Man, are found in 1996 on the Columbia River near Kennewick, Washington. A skull and more than 300 bones and bone fragments were found at the site, making up among the oldest, best preserved, and most complete human remains ever found in North America. Initial radiocarbon dating indicated the remains were between 7,000 and 9,500 years old.[71] A leaf-shaped projectile found on the body was long, broad and had serrated edges, all fitting the definition of a Cascade point. This type of point is a feature of the Cascade phase, occurring in the archaeological record from roughly 6,000 to over 8,500 years ago.
  • 1930s-1990s no major Central American archaeological sites that go back more than 9,000 years have been found. Isolated finds of stone tools in Belize, Nicaragua and Costa Rica indicate that such sites almost certainly exist. Lack of funding for exploration in the areas has postponed likely finds.[68]
  • Tehuacan Valley of Mexico – people are living in rock shelters and using stone cooking pots, which were left in the center of the hearth. Maize was cultivated to be used in the same valley between 7,000–6,000 years ago.[72]

參見

参考文献

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