A GENOMIC TIMESCALE FOR THE RISE IN OXYGEN AND ORIGIN OF EUKARYOTES

The earliest biomarker evidence of eukaryotes is at 2.7 Ga and the earliest fossils appear 2.1 Ga. The fossil record of cyanobacteria has been argued to extend to 3.5 Ga but the biomarker evidence at 2.7-2.8 Ga usually is considered the earliest record of cyanobacteria. The origin of oxygenic photosynthesis may have occurred at some time later than the origin of cyanobacteria. Geologic evidence bearing on the origin and rise in oxygen likewise has been debated. Although the existence of banded iron formations prior to ~3 Ga sometimes has been used as evidence for the early evolution of oxygenic photosynthesis, oxygen-independent mechanisms of iron deposition are known.

Genomic analyses have shown that horizontal gene transfer occurred during the origin of eukaryotes following symbiosis. However, the number of symbiotic events and their relationship with changes in Earth's environment are unclear. We used genomic sequence data from prokaryotes and eukaryotes to obtain time estimates for the origin of cyanobacteria, and of eukaryotes. Horizontal gene transfer provided a means to time the symbiotic events in the origin of eukaryotes.

Our protein sequence analyses support more than one symbiotic event in the origin of eukaryotes. Cyanobacteria appear slightly before the major (undisputed) evidence of the rise in oxygen and mitochondria appear just after the rise in oxygen. The estimates for the origin of cyanobacteria and eukaryotes are consistent (within one SE) with the earliest biomarker evidence for those two groups (~2.7 Ga.). It has been proposed that a major rise in oxygen ~2.4-2.2 Ga lowered global temperatures and may have triggered the Paleoproterozoic glaciations. If this is true, and given our time estimates, the evolutionary innovation of oxygenic photosynthesis may have had a relatively rapid impact on the environment as it set the stage for further evolution of the eukaryotic cell.