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 hold 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 in the collections all along under a name that hid it.

Why it matters

Everything before animals is placed on this map. It also told the search for our own origins where to look: once the archaea stood as a group of their own, the ancestor of complex cells had an address, and phylogenomics has since put it there.

Dating & uncertainty

1977 is the date of the publication, not of the agreement that followed it: the claim was contested for a decade before it was accepted. The boundary it drew has since moved again, this time between the archaea and ourselves, since eukaryotes now look like a branch inside that group rather than a neighbouring one.

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