当地媒体日前援引一项最新研究报告说，多数现代哺乳动物的起源可以追溯到恐龙消失很久之后出现的生物大爆发时期。这一研究报告对关于现代地球生命的一个基本理论提出了挑战。
报道说，当今的主流观点是，恐龙在大约６５００万年前白垩纪结束时灭绝，其原因可能是一颗小行星撞上了地球，引起尘埃遮天蔽日，从而导致地球变冷，恐龙赖以生存的植物遭到毁灭。
根据这一理论，当“恐怖的蜥蜴”渐渐消失时，一直在附近静等的哺乳动物欣然接管了它们的地盘，并迅速变得多样化，形成了我们今天所看到的哺乳动物物种的各个家族。
但是，英国《自然》周刊最新刊登的一篇论文说，哺乳动物多样性通过两个重要阶段逐渐形成。第一个阶段发生在白垩纪结束几百万年前，另一个阶段发生在白垩纪结束几百万年后。
德国慕尼黑工业大学的奥拉夫·比宁达－埃蒙茨及其同事绘制了当今几乎所有４５００个哺乳动物物种的基因“族谱”。
他们利用以物种进化速度为基础的分子钟，让时光倒流，估算出这些哺乳动物何时发生基因变化。
在１．６亿年的跨度中，哺乳动物在大约９３００万年前经历了首次多样化大爆发。当今灵长类动物、啮齿动物和有蹄动物的祖先最早出现在大约７５００万年前或稍晚一些。
但是，哺乳动物非但没有普遍受益于白垩纪的结束，反而因为大灾难而受到沉重打击。许多哺乳动物物种像恐龙一样遭受灭顶之灾。
第二次多样化发生在大约３５００万年前，此时距恐龙的衰落已经过了漫长的岁月。第二次物种大爆发尤其重要，因为它产生了现今哺乳动物的家族。
论文作者说，第二次物种大爆发的原因尚不明了，但这可能与全球气温上升有关。
论文的作者之一、美国自然历史博物馆脊椎动物馆馆长罗斯·麦克菲说：“现在的重要问题是，什么原因让现代哺乳动物的祖先过了这么久才变得多样化。这就好比它们在恐龙灭绝之后前来参加聚会，但只是在附近徘徊，而它们的远亲却过得很快乐。显然，我们对大规模灭绝之后结束的宏观生态学机制知之甚少。”
部分英文原文：
Nature 446, 507-512 (29 March 2007) | doi:10.1038/nature05634; Received 26 October 2006; Accepted 31 January 2007
The delayed rise of present-day mammals
Olaf R. P. Bininda-Emonds1,11, Marcel Cardillo2,11, Kate E. Jones4, Ross D. E. MacPhee5, Robin M. D. Beck6, Richard Grenyer7, Samantha A. Price8, Rutger A. Vos9, John L. Gittleman10 & Andy Purvis2,3
Lehrstuhl für Tierzucht, Technical University of Munich, 85354 Freising-Weihenstephan, Germany
Division of Biology, and,
NERC Centre for Population Biology, Imperial College, Silwood Park campus, Ascot SL5 7PY, UK
Institute of Zoology, Zoological Society of London, Regents Park, London NW1 4RY, UK
Division of Vertebrate Zoology, American Museum of Natural History, New York, New York 10024, USA
School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales 2052, Australia
Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, UK
National Evolutionary Synthesis Center (NESCent), Durham, North Carolina 27705, USA
Department of Zoology, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
Institute of Ecology, University of Georgia, Athens, Georgia 30602, USA
Present addresses: Institut für Spezielle Zoologie und Evolutionsbiologie mit Phyletischem Museum, Friedrich-Schiller-Universität Jena, 07743 Jena, Germany (O.R.P.B.-E.); Centre for Macroevolution and Macroecology, School of Botany and Zoology, Australian National University, Canberra 0200, Australia (M.C.).
Correspondence to: Olaf R. P. Bininda-Emonds1,11 Correspondence and requests for materials should be addressed to O.R.P.B.-E. (Email: olaf.bininda@uni-jena.de).
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Abstract
Did the end-Cretaceous mass extinction event, by eliminating non-avian dinosaurs and most of the existing fauna, trigger the evolutionary radiation of present-day mammals? Here we construct, date and analyse a species-level phylogeny of nearly all extant Mammalia to bring a new perspective to this question. Our analyses of how extant lineages accumulated through time show that net per-lineage diversification rates barely changed across the Cretaceous/Tertiary boundary. Instead, these rates spiked significantly with the origins of the currently recognized placental superorders and orders approximately 93 million years ago, before falling and remaining low until accelerating again throughout the Eocene and Oligocene epochs. Our results show that the phylogenetic 'fuses' leading to the explosion of extant placental orders are not only very much longer than suspected previously, but also challenge the hypothesis that the end-Cretaceous mass extinction event had a major, direct influence on the diversification of today's mammals.
