Evolution of bacteria

Background

The evolution of bacteria began with their divergence from the archaeal/eukaryotic lineage, though the exact timing is debated. Molecular clock studies using various genetic data suggest the last universal common ancestor (LUCA) lived roughly 3.5–4.0 billion years ago, but the divergence date is not precisely fixed. Evidence of permineralized microfossils of early prokaryotes was discovered in the Australian Apex Chert rocks, dating back roughly 3.5 billion years ago during the Precambrian time. The identity of the last bacterial common ancestor (LBCA) remains unresolved; Thermotogota is only one of several deep-branching phyla, not canonically considered the LBCA.

By the Siderian period, roughly 2.45 billion years ago, oxygen had appeared in ancient rocks, indicating that oceanic, photosynthetic cyanobacteria evolved during this period. These were the first microbes to produce oxygen as a byproduct of their metabolism. Some scientists argue cyanobacteria could have lived as early as 2.7 billion years ago, before the Great Oxygenation Event. The rise in atmospheric oxygen eventually led to the evolution of Pseudomonadota, though molecular clock estimates for this event vary widely and no single date is canonically established.

Conflicting theories remain regarding bacterial origins. Some scientists argue that the lack of identifiable morphology in microfossils prevents drawing accurate evolutionary timelines. Bacteria evolve through natural selection, and their rapid reproduction (as little as 20 minutes) allows fast adaptation, which has become an issue regarding antibiotic resistant bacteria.

The Vast Temporal Canvas

The evolutionary narrative that leads to modern humans stretches back approximately four billion years, encompassing essentially the entire history of life on Earth. Rather than focusing narrowly on the last few million years of primate development, this framework situates Homo sapiens within the full arc of biological history, from the earliest single-celled organisms to the most recent genetic changes occurring during and after the Last Glacial Period. This sweeping perspective means that the story of human origins is inseparable from the broader story of how complex multicellular life emerged, diversified, and eventually produced the lineages that would give rise to our species. The timeline thus functions not merely as a human genealogy but as a compressed history of the biosphere, where every major transition—from the first cells to the first animals, from aquatic to terrestrial existence, from simple vertebrates to the sophisticated primates of today—serves as a necessary chapter in the unfolding narrative that culminates in the modern human form.