GENETICS × EVOLUTION

A Third Branch of Life

1977 CE

Phanerozoic · Cenozoic · Quaternary · Holocene · Meghalayan

Comparing a single molecule present in everything alive, the RNA of the ribosome, split the microbial world in two. What had been filed together as bacteria turned out to contain a second group as distant from bacteria as we are. Life had three great branches instead of two, and the new one, the archaea, had been sitting in plain sight in every sample anyone had ever taken.

Why it matters

Everything before animals is placed on this map. It has since moved once more, and in our favour: eukaryotes now appear to arise from inside the archaea rather than beside them, which is why the ancestor of our own cells could later be found among them.

Dating & uncertainty

The result came from comparing ribosomal RNA, a molecule present in every living thing. It was resisted for a decade, and the boundary between archaea and eukaryotes has since moved again: eukaryotes now appear to arise from within the archaea rather than beside them.

Sources

1. Woese & Fox (1977), PNAS · Phylogenetic structure of the prokaryotic domain: the primary kingdoms
2. Woese (1981), Nature · Are archaebacteria merely derived prokaryotes?
3. Zaremba-Niedzwiedzka et al. (2017), Nature · Asgard archaea illuminate the origin of eukaryotic cellular complexity

Earth history as one day

23:59:59.9991933 µs before midnight

if Earth’s 4.54 billion years were compressed into a single day