A massive analysis of almost every bacterial genome sequenced to date suggests a new shape for the tree of life. One of its core branches appears to be a union of two other branches. Descendants of that line became the energy centers of plant and animal cells. In other words, the results of this distant union between microorganisms ‘allowed us to walk on the Earth,’ said James Lake, a University of California at Los Angeles cell biologist and author of the analysis. ‘It’s responsible for the oxygen in the atmosphere, and brought in the organelles that made plants grow and let us breathe air.’ The details of early cellular evolution are murky, with few fossils having remained intact for the billions of years since self-replicating chemicals assumed cellular form. But at a general level, scientists know that the earliest organisms were single-celled, nucleus-lacking creatures called prokaryotes. These are broken down into five groups: bacteria, archaea, clostridia, actinobacteria and so-called gram-negative bacteria. To most people, prokaryotes are just a bunch of microscopic bugs, but to microbiologists they’re as richly varied as the animal kingdom. Scientists think the first eukaryotes – single-celled creatures with nuclei and complex […]