Howard Hughes Medical Institute and Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
Department of Biological Sciences, Vanderbilt University, Nashville, TN 37235, USA.
Curr Biol. 2017 Oct 9;27(19):R1081-R1088. doi: 10.1016/j.cub.2017.08.054.
The origin of animals, one of the major transitions in evolution, remains mysterious. Many key aspects of animal evolution can be reconstructed by comparing living species within a robust phylogenetic framework. However, uncertainty remains regarding the evolutionary relationships between two ancient animal lineages - sponges and ctenophores - and the remaining animal phyla. Comparative morphology and some phylogenomic analyses support the view that sponges represent the sister lineage to the rest of the animals, while other phylogenomic analyses support ctenophores, a phylum of carnivorous, gelatinous marine organisms, as the sister lineage. Here, we explore why different studies yield different answers and discuss the implications of the two alternative hypotheses for understanding the origin of animals. Reconstruction of ancient evolutionary radiations is devilishly difficult and will likely require broader sampling of sponge and ctenophore genomes, improved analytical strategies and critical analyses of the phylogenetic distribution and molecular mechanisms underlying apparently conserved traits. Rather than staking out positions in favor of the ctenophores-sister or the sponges-sister hypothesis, we submit that research programs aimed at understanding the biology of the first animals should instead embrace the uncertainty surrounding early animal evolution in their experimental designs.
动物的起源是进化中的主要转变之一，其仍然神秘莫测。通过在稳健的系统发育框架内比较现存物种，可以重建动物进化的许多关键方面。然而，关于两个古老动物谱系——海绵动物和栉水母动物——以及其余动物门之间的进化关系仍然存在不确定性。比较形态学和一些系统基因组学分析支持海绵动物是其余动物的姐妹群的观点，而其他系统基因组学分析则支持栉水母动物，即肉食性、凝胶状海洋生物的一个门，是姐妹群。在这里，我们探讨了为什么不同的研究得出不同的答案，并讨论了这两种替代假说对理解动物起源的意义。古代进化辐射的重建极具挑战性，可能需要更广泛地对海绵动物和栉水母动物的基因组进行采样，改进分析策略，并对明显保守特征的系统发育分布和分子机制进行批判性分析。我们不主张在栉水母动物-姐妹群假说或海绵动物-姐妹群假说中站队，而是认为旨在理解最初动物生物学的研究计划应在其实验设计中接受早期动物进化的不确定性。
