A rooted bacterial tree: the common ancestor of all living bacteria was at a different location from where previously supposed

The root of the bacterial phylogeny, that is, the common ancestor of all living bacteria, was explored by Gergely Szöllösi, the leader of the Lendület (Momentum) Evolutionary Genomics Research Group at ELTE, and his British and Australian colleagues, with a novel approach. As their article in Science shows, the root of the tree was at a different location from where previously supposed.

2021. május 17.

Tracing the vertical and horizontal lines of descent back to the root of the bacterial family tree.

Gergely Szöllősi

Just as in plants and animals, the genomes of Bacteria are home to many different genes. However, bacterial genes are not only inherited vertically – from mother to daughter – but are also frequently exchanged horizontally between potentially distant family members.

Such horizontal gene sharing drives the rapid spread of antibiotic resistance among pathogenic Bacteria. This combination of vertical and horizontal ancestry complicates how we think about the evolutionary relationships of bacteria and the construction of evolutionary trees. Coleman et al. used phylogenetic methods that simultaneously consider the vertical and horizontal transmission of genes and found that, on average, genes travel vertically two-thirds of the time.

This result implies that a single tree can provide a meaningful backbone of vertical inheritance for interpreting bacterial evolution.

Furthermore, accounting for both vertical and horizontal transmission allowed the authors to trace the history of modern genes, and associated traits, back along the tree. Their inferences revealed that the common ancestor of all modern Bacteria was already a complex free-living organism, which was able to swim, sense its environment, and defend itself against viruses. It gave rise to two major descendant lineages, the Gracilicutes (which include the famous gut bacterium Escherichia coli) and the Terrabacteria (which include the photosynthetic Cyanobacteria, among other lineages); host-associated Bacteria such as the recently-discovered CPR, despite their high diversity, appear to have evolved later from free-living ancestors.

These findings provide valuable insights into the earliest stages of evolution on our planet.

You can read the article here.