當(dāng)?shù)孛襟w日前援引一項(xiàng)最新研究報(bào)告說,，多數(shù)現(xiàn)代哺乳動(dòng)物的起源可以追溯到恐龍消失很久之后出現(xiàn)的生物大爆發(fā)時(shí)期。這一研究報(bào)告對關(guān)于現(xiàn)代地球生命的一個(gè)基本理論提出了挑戰(zhàn),。
報(bào)道說,，當(dāng)今的主流觀點(diǎn)是，恐龍?jiān)诖蠹s６５００萬年前白堊紀(jì)結(jié)束時(shí)滅絕，其原因可能是一顆小行星撞上了地球,，引起塵埃遮天蔽日,，從而導(dǎo)致地球變冷，恐龍賴以生存的植物遭到毀滅,。
根據(jù)這一理論,，當(dāng)“恐怖的蜥蜴”漸漸消失時(shí)，一直在附近靜等的哺乳動(dòng)物欣然接管了它們的地盤,，并迅速變得多樣化,，形成了我們今天所看到的哺乳動(dòng)物物種的各個(gè)家族。
但是,，英國《自然》周刊最新刊登的一篇論文說,，哺乳動(dòng)物多樣性通過兩個(gè)重要階段逐漸形成。第一個(gè)階段發(fā)生在白堊紀(jì)結(jié)束幾百萬年前,，另一個(gè)階段發(fā)生在白堊紀(jì)結(jié)束幾百萬年后,。
德國慕尼黑工業(yè)大學(xué)的奧拉夫·比寧達(dá)－埃蒙茨及其同事繪制了當(dāng)今幾乎所有４５００個(gè)哺乳動(dòng)物物種的基因“族譜”。
他們利用以物種進(jìn)化速度為基礎(chǔ)的分子鐘,，讓時(shí)光倒流,，估算出這些哺乳動(dòng)物何時(shí)發(fā)生基因變化。
在１．６億年的跨度中,，哺乳動(dòng)物在大約９３００萬年前經(jīng)歷了首次多樣化大爆發(fā),。當(dāng)今靈長類動(dòng)物、嚙齒動(dòng)物和有蹄動(dòng)物的祖先最早出現(xiàn)在大約７５００萬年前或稍晚一些,。
但是,，哺乳動(dòng)物非但沒有普遍受益于白堊紀(jì)的結(jié)束，反而因?yàn)榇鬄?zāi)難而受到沉重打擊,。許多哺乳動(dòng)物物種像恐龍一樣遭受滅頂之災(zāi),。
第二次多樣化發(fā)生在大約３５００萬年前，此時(shí)距恐龍的衰落已經(jīng)過了漫長的歲月,。第二次物種大爆發(fā)尤其重要,，因?yàn)樗a(chǎn)生了現(xiàn)今哺乳動(dòng)物的家族。
論文作者說,，第二次物種大爆發(fā)的原因尚不明了,，但這可能與全球氣溫上升有關(guān)。
論文的作者之一,、美國自然歷史博物館脊椎動(dòng)物館館長羅斯·麥克菲說：“現(xiàn)在的重要問題是,，什么原因讓現(xiàn)代哺乳動(dòng)物的祖先過了這么久才變得多樣化。這就好比它們在恐龍滅絕之后前來參加聚會,，但只是在附近徘徊,，而它們的遠(yuǎn)親卻過得很快樂,。顯然，我們對大規(guī)模滅絕之后結(jié)束的宏觀生態(tài)學(xué)機(jī)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: [email protected]).
Top of page
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.
