CNRS, Université Montpellier 2, UMR 5554, ISEM, Montpellier, France.
Mol Biol Evol. 2013 Jan;30(1):5-13. doi: 10.1093/molbev/mss211. Epub 2012 Sep 4.
It is widely assumed that our mammalian ancestors, which lived in the Cretaceous era, were tiny animals that survived massive asteroid impacts in shelters and evolved into modern forms after dinosaurs went extinct, 65 Ma. The small size of most Mesozoic mammalian fossils essentially supports this view. Paleontology, however, is not conclusive regarding the ancestry of extant mammals, because Cretaceous and Paleocene fossils are not easily linked to modern lineages. Here, we use full-genome data to estimate the longevity and body mass of early placental mammals. Analyzing 36 fully sequenced mammalian genomes, we reconstruct two aspects of the ancestral genome dynamics, namely GC-content evolution and nonsynonymous over synonymous rate ratio. Linking these molecular evolutionary processes to life-history traits in modern species, we estimate that early placental mammals had a life span above 25 years and a body mass above 1 kg. This is similar to current primates, cetartiodactyls, or carnivores, but markedly different from mice or shrews, challenging the dominant view about mammalian origin and evolution. Our results imply that long-lived mammals existed in the Cretaceous era and were the most successful in evolution, opening new perspectives about the conditions for survival to the Cretaceous-Tertiary crisis.
人们普遍认为，生活在白垩纪的哺乳动物祖先体型很小，它们躲在避难所里躲过了大规模的小行星撞击，并在恐龙灭绝 6500 万年后进化成了现代形态。大多数中生代哺乳动物化石的小体型基本支持了这一观点。然而，古生物学并不能确定现存哺乳动物的祖先，因为白垩纪和古近纪的化石与现代谱系不太容易联系起来。在这里，我们使用全基因组数据来估计早期胎盘哺乳动物的寿命和体重。通过分析 36 个完全测序的哺乳动物基因组，我们重建了祖先基因组动态的两个方面，即 GC 含量进化和非同义替换与同义替换的比率。将这些分子进化过程与现代物种的生活史特征联系起来，我们估计早期胎盘哺乳动物的寿命超过 25 年，体重超过 1 公斤。这与现代灵长类动物、有蹄类动物或食肉动物相似，但与老鼠或鼩鼱明显不同，这对哺乳动物起源和进化的主流观点提出了挑战。我们的结果表明，长寿的哺乳动物在白垩纪时期就已经存在，并且在进化中是最成功的，为了解白垩纪-第三纪大灭绝时期的生存条件提供了新的视角。
