一国际研究小组最近宣称，他们最新完成了迄今为止年代最为久远的史前生物的基因测序工作，成功获得距今70万年的古代野马的基因组序列。这是科学家首次完成对中更新世时期物种的基因测序工作。
更新世，又称洪积世，是地质时代第四纪的早期，距今180万年到11000年左右，其显著特征是气候变冷，有冰期与间冰期的明显交替，而这一时期的大多数动植物属种则与现代相似。在该项研究中，科学家所研究的对象即是来自加拿大西北部育空地区永久冻土中发现的70万年前的野马骨骼残骸。
在有机体死亡之后，其化石中会残留该物种的DNA分子，虽然不可能是整个的基因组，但残留的DNA片段仍有机会被重新组合起来。如果有足够的DNA片段残留下来，科学家就可以重建消失物种的完整基因组序列。在过去几年间，科学家已经获得了一些古人类的完整基因组序列，但从时间上看，则远远没有70万年这样久远。70万年这一时间比之前DNA测序物种最早年份记录提高了大约10倍。
利用基因测序，科学家能够进一步了解过去70万年马血统进化过程中的主要基因变化情况。他们可以追踪史前野马向近代马种转变过程中基因的变化，以获得现代马匹基因构成的详细信息。通过对70万年前的史前野马、43000年前的古代马、六个现代马种以及驴的基因组进行对比研究，科学家发现，所有现代马科动物都有一个共同的祖先，生活在400万至450万年前。这一发现表明，马、驴以及斑马的最初进化起源时间,或者说地球上第一匹野马的出现时间，远比之前科学家所认知的要早。
该项研究由丹麦自然历史博物馆科学家领导完成，研究小组成员则包括来自丹麦、中国、加拿大、美国、瑞士、英国、挪威、法国、瑞典和沙特阿拉伯十个国家的科学家，其成果刊发在最新出版的《自然》杂志上。（生物谷 Bioon.com）
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生物谷推荐的英文摘要
Nature doi:10.1038/nature12323
Recalibrating Equus evolution using the genome sequence of an early Middle Pleistocene horse
Ludovic Orlando, Aurélien Ginolhac, Guojie Zhang, Duane Froese, Anders Albrechtsen, Mathias Stiller, Mikkel Schubert, Enrico Cappellini, Bent Petersen, Ida Moltke, Philip L. F. Johnson, Matteo Fumagalli, Julia T. Vilstrup, Maanasa Raghavan, Thorfinn Korneliussen, Anna-Sapfo Malaspinas, Josef Vogt, Damian Szklarczyk, Christian D. Kelstrup, Jakob Vinther, Andrei Dolocan, Jesper Stenderup, Amhed M. V. Velazquez, James Cahill, Morten Rasmussen, Xiaoli Wang, Jiumeng Min, Grant D. Zazula, Andaine Seguin-Orlando, Cecilie Mortensen, Kim Magnussen, John F. Thompson, Jacobo Weinstock, Kristian Gregersen, Knut H. Røed, Véra Eisenmann, Carl J. Rubin, Donald C. Miller, Douglas F. Antczak, Mads F. Bertelsen, Søren Brunak, Khaled A. S. Al-Rasheid, Oliver Ryder, Leif Andersson, John Mundy, Anders Krogh, M. Thomas P. Gilbert, Kurt Kjær, Thomas Sicheritz-Ponten, Lars Juhl Jensen, Jesper V. Olsen, Michael Hofreiter, Rasmus Nielsen, Beth Shapiro, Jun Wang & Eske Willerslev
The rich fossil record of equids has made them a model for evolutionary processes1. Here we present a 1.12-times coverage draft genome from a horse bone recovered from permafrost dated to approximately 560–780 thousand years before present (kyr BP)2, 3. Our data represent the oldest full genome sequence determined so far by almost an order of magnitude. For comparison, we sequenced the genome of a Late Pleistocene horse (43 kyr BP), and modern genomes of five domestic horse breeds (Equus ferus caballus), a Przewalski’s horse (E. f. przewalskii) and a donkey (E. asinus). Our analyses suggest that the Equus lineage giving rise to all contemporary horses, zebras and donkeys originated 4.0–4.5 million years before present (Myr BP), twice the conventionally accepted time to the most recent common ancestor of the genus Equus4, 5. We also find that horse population size fluctuated multiple times over the past 2 Myr, particularly during periods of severe climatic changes. We estimate that the Przewalski’s and domestic horse populations diverged 38–72 kyr BP, and find no evidence of recent admixture between the domestic horse breeds and the Przewalski’s horse investigated. This supports the contention that Przewalski’s horses represent the last surviving wild horse population6. We find similar levels of genetic variation among Przewalski’s and domestic populations, indicating that the former are genetically viable and worthy of conservation efforts. We also find evidence for continuous selection on the immune system and olfaction throughout horse evolution. Finally, we identify 29 genomic regions among horse breeds that deviate from neutrality and show low levels of genetic variation compared to the Przewalski’s horse. Such regions could correspond to loci selected early during domestication.
