Origin of the metazoan phyla: Molecular clocks confirm paleontological estimates

The time of origin of the animal phyla is controversial. Abundant fossils from the major animal phyla are found in the Cambrian, starting 544 million years ago. Many paleontologists hold that these phyla originated in the late Neoproterozoic, during the 160 million years preceding the Cambrian fossil explosion. We have analyzed 18 protein-coding gene loci and estimated that protostomes (arthropods, annelids, and mollusks) diverged from deuterostomes (echinoderms and chordates) about 670 million years ago, and chordates from echinoderms about 600 million years ago. Both estimates are consistent with paleontological estimates. A published analysis of seven gene loci that concludes that the corresponding divergence times are 1,200 and 1,000 million years ago is shown to be flawed because it extrapolates from slow-evolving vertebrate rates to faster-evolving invertebrate rates, as well as in other ways.

动物门类（animal phyla）的起源时间颇具争议。在距今5.44亿年前起始的寒武纪（Cambrian）地层中，已发现大量来自各主要动物门类的化石。不少古生物学家主张，这些动物门类起源于新元古代（Neoproterozoic）晚期，即寒武纪生命大爆发（Cambrian fossil explosion）前的1.6亿年时段内。我们对18个蛋白质编码基因位点（protein-coding gene loci）开展了分析，估算得到原口动物（protostomes，涵盖节肢动物（arthropods）、环节动物（annelids）与软体动物（mollusks））与后口动物（deuterostomes，涵盖棘皮动物（echinoderms）和脊索动物（chordates））的分化时间约为6.7亿年前，而脊索动物与棘皮动物的分化时间约为6.0亿年前。两项估算结果均与古生物学（paleontological）估算结果一致。此前一项针对7个基因位点的已发表研究认为，对应的分化时间分别为12亿年和10亿年前，但该分析被证实存在缺陷：其不仅从演化速率较慢的脊椎动物（vertebrate）类群外推至演化速率更快的无脊椎动物（invertebrate）类群，还存在其他方法论层面的问题。