Genetic Divergence Disclosing a Rapid Prehistorical Dispersion of Native Americans in Central and South America

An accurate estimate of the divergence time between Native Americans is important for understanding the initial entry and early dispersion of human beings in the New World. Current methods for estimating the genetic divergence time of populations could seriously depart from a linear relationship with the true divergence for multiple populations of a different population size and significant population expansion. Here, to address this problem, we propose a novel measure to estimate the genetic divergence time of populations. Computer simulation revealed that the new measure maintained an excellent linear correlation with the population divergence time in complicated multi-population scenarios with population expansion. Utilizing the new measure and microsatellite data of 21 Native American populations, we investigated the genetic divergences of the Native American populations. The results indicated that genetic divergences between North American populations are greater than that between Central and South American populations. None of the divergences, however, were large enough to constitute convincing evidence supporting the two-wave or multi-wave migration model for the initial entry of human beings into America. The genetic affinity of the Native American populations was further explored using Neighbor-Net and the genetic divergences suggested that these populations could be categorized into four genetic groups living in four different ecologic zones. The divergence of the population groups suggests that the early dispersion of human beings in America was a multi-step procedure. Further, the divergences suggest the rapid dispersion of Native Americans in Central and South Americas after a long standstill period in North America.

精准估算美洲原住民群体间的遗传分化时间，对于理解人类首次迁入新大陆的历程以及早期在美洲的扩散模式具有重要意义。当前用于估算群体遗传分化时间的方法，在处理群体规模各异且存在显著群体扩张的多群体场景时，极易严重偏离与真实分化时间的线性关联。为此，我们提出一种全新的群体遗传分化时间估算指标，以解决这一问题。计算机模拟实验证实，在存在群体扩张的复杂多群体场景中，该新指标与群体分化时间始终保持极佳的线性相关性。我们依托该新指标，并结合21个美洲原住民群体的微卫星（microsatellite）数据，对美洲原住民群体的遗传分化情况开展了研究。结果显示，北美群体间的遗传分化程度高于中美洲与南美洲群体间的分化水平。但所有分化程度均未达到足以支撑人类首次迁入美洲的两波或多波迁徙模型的显著性水平，无法为该模型提供令人信服的证据。我们进一步借助邻域网（Neighbor-Net）方法探究了美洲原住民群体的遗传亲缘关系，结合遗传分化分析结果，可将这些群体划分为四个分别栖息于不同生态区域的遗传类群。各群体类群的分化情况表明，人类在美洲的早期扩散是一个多阶段的过程。此外，该分化结果还提示，美洲原住民在北美经历长期停滞期后，快速扩散至中美洲与南美洲地区。