Early Life Didn’t Just Divide, It United

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.

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 internal structures, from which the entire plant and animal kingdom descended — evolved from prokaryotes, with some prokaryotes absorbed wholesale into these new and complicated organisms.

The best-known examples of this absorption are mitochondria and chloroplasts, the structures that generate energy in animal and plant cells. Both belong to the gram-negative class, as do cyanobacteria, which several billion years ago probably transmuted Earth’s early atmosphere from a toxic soup to oxygen-rich air through photosynthesis.

According to scientists, the origin of this gram-negative group is not singular. Instead it appears to have been produced through a fusion of actinobacteria and clostridium. Were mammals derived from a union of insect and amphibian, the story-of-life rearrangement would be comparably profound.

The scientist's hypothesis is based on pattern analyses run on the genome sequences of more than 3,000 types of bacteria. For three years, he refined algorithms that teased out relationships between the different genes, then proposed different taxonomies to explain them.

The union likely took the form of endosymbiosis, in which one of the prokaryotes literally swallowed the other, and the two grew together.

Read more about the hypothesis in the original article here.