A rooted bacterial tree is necessary to understand early evolution, but the position of the root is contested. Here, we model the evolution of 11,272 gene families to identify the root, extent of horizontal gene transfer (HGT), and the nature of the last bacterial common ancestor (LBCA). Our analyses root the tree between the major clades Terrabacteria and Gracilicutes and suggest that LBCA was a free-living flagellated, rod-shaped double-membraned organism. Contrary to recent proposals, our analyses reject a basal placement of the Candidate Phyla Radiation, which instead branches sister to Chloroflexota within Terrabacteria. While most gene families (92%) have evidence of HGT, overall, two-thirds of gene transmissions have been vertical, suggesting that a rooted tree provides a meaningful frame of reference for interpreting bacterial evolution.

扎根的细菌树是了解早期进化所必需的，但是根的位置存在争议。在这里，我们对11,272个基因家族的进化进行建模，以鉴定其根源，水平基因转移的范围（HGT）和最后一个细菌祖先（LBCA）的性质。我们的分析扎根在主要进化枝Terrabacteria和Gracilicutes之间的树上，表明LBCA是一种自由生活的鞭毛状，杆状双膜生物。与最近的提议相反，我们的分析拒绝了候选Phyla辐射的基础位置，而该位置将姊妹分支到了Terrabacteria中的Chloroflexota。尽管大多数基因家族（92％）具有HGT的证据，但总体而言，三分之二的基因传递是垂直的，这表明有根的树为解释细菌的进化提供了有意义的参考框架。