The ability to sequence genes and, more recently, whole genomes has transformed our understanding of the tree of life by elucidating the tremendous diversity of microorganisms and by placing plants, animals, and fungi as branches nested among microbial lineages (1–3). The resulting evolutionary tree divides life into three domains: the exclusively microbial Bacteria and Archaea, and Eukarya, organisms whose cells contain nuclei (including ciliates, amoebae, and animals). Yet, the ordering of the earliest branching events on the tree and the nature of now-extinct ancestors remains unclear. On page 588 of this issue, Coleman et al. (4) provide a new estimate of the root of the bacterial tree of life, that is, the ancestor from which all bacterial species are derived. Knowledge of the root of the bacterial tree is important because it defines the evolutionary starting point for the tremendous diversity of Bacteria and offers glimpses into the nature of the first bacterial cells.

对基因序列的能力，最近，全基因组已经阐明微生物的巨大的多样性，并通过将植物，动物和真菌嵌套微生物谱系（其中分支机构改变了我们理解生命之树的1 - 3）。由此产生的进化树将生命分为三个领域：唯一的微生物细菌和古细菌，以及真核生物，它们的细胞含有细胞核（包括纤毛虫、变形虫和动物）。然而，树上最早分支事件的顺序以及现已灭绝的祖先的性质仍不清楚。在本期第 588 页上，Coleman等人。( 4) 提供了对细菌生命树的根的新估计，即所有细菌物种的祖先。细菌树根的知识很重要，因为它定义了细菌巨大多样性的进化起点，并提供了对第一个细菌细胞本质的一瞥。