遷徙新大陸模型

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和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。

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。 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。

各派意见综述

迁徙模型在时间上大致分为两派。 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）。 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。 ，然后才是第二波巨大的移民潮。

Flèches préhistoriques amérindiennes, conservées à Washington

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。 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 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。

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）。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。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。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， 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）。

人类自白令海峡往南移民的无障碍通路表

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

陆桥论

Shrinking of the Bering land bridge

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。 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。

基因与血型

更多信息：Genetic history of indigenous peoples of the Americas

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 Bronowski在The 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。"

现代美洲基因学则主要研究人类Y染色体DNA单倍型类群和人类粒线体单倍群。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。 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。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。 ，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。 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。

考古，地理和基因证据选录

更多信息：Archaeology of the Americas和Late Glacial Maximum

40,000 B.C. – 25,000 B.C. Paleolithic people move into Beringia across the Bering Land Bridge into western Alaska.
30,000–20,000 years ago: Mammoth bones, believed to have been chipped by humans, are found at the Yukon's Bluefish Caves sites investigated in the 1970s and 1980s by archeologist Jacques Cinq-Mars and his team. In 2004, Albert Goodyear of the South Carolina Institute of Archaeology and Anthropology announced radiocarbon dating of a bit of charcoal found in the Topper Site that preceded Clovis culture, near Allendale County, South Carolina. However, these deposits may have been made by forest fires. (Note: The dates given for the Old Crow and Topper digs have not been completely accepted by the archaeology community.) Ice-free corridor running north and south through Alberta and the continental glacier called Laurentide ice sheet. Introduced by geologists in the 1950s when stone tools were found in the Grimshaw, Bow River and in Lethbridge Alberta, under glacial sand and gravel; they are believed to be pre-glacial and may indicate nomadic humans occupied the area. A child's skull found in 1961 near Taber, Alberta is believed to be of one of the oldest inhabitants discovered in Alberta. (Note: The conclusions reached in Alberta on dates have not been accepted by the entire archaeology community.) Cambridge DNA Services estimates humans entered the Americas around 25,000 years ago. Other geneticists have variously estimated that peoples of Asia and the Americas were part of the same population from about 21,000 to 42,000 years ago. Siberian mammoth hunters were believed to have penetrated far into the Arctic where ice-free corridors north during the time are believed found. Theory first introduced by geologists in the late 1970s when core samples indicate the ice is no older than 17,000 years old.
23,000–16,500 years ago: The Ice Age entombs the northern hemisphere in glaciers, cutting off routes from Siberia to the south. 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. Genetic evidence (2007–2009) suggests the Beringia population's first genetic diversification from Asian populations occurred. 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.
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. 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. 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. 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. 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. A coastal migration could explain how people arrived in Monte Verde. 2000, archaeologists say people were living at Cactus Hill, Virginia where stone tools and charcoal from a fire pit are found.
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. Peñon woman found by an ancient lake bed near Mexico City in 1959. 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. At Paisley Caves in the Cascade Range of Oregon, archaeologists find a scattering of human coprolites, or fossil feces in 2003. 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. These two genetic groups were the founding Paleo-Indians and later Na-Dené migration.
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. 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. 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. Peru coastal region inhabitants fished with nets and bone hooks, collecting seafood such as crabs and sea urchins.
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. 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. Luzia Woman's skull and other bones excavated in the Lagoa Santa, Brazil area by French archaeologist Annette Laming-Emperaire in the 1970s. By 2006, Lagoa Santa sites had produced no fewer than 75 well-preserved ancient skulls. 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. 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.
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. 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. 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.

參見

• 人类单地起源说
• 德內－葉尼塞語門
• Paleo-Indians period (Canada)
• 史前哥伦比亚跨海联系
• 各国（岛屿）人类居时间住列表