Molecular Clock Dates for Bacterial Origins are consistent with Impact Bottleneck Scenarios

The Late Heavy Bombardment (LHB) and other late accretion impactor scenarios are often invoked as habitability constraints on the Hadean/Eoarchean Earth. These hypotheses either describe an “impact frustration” where life would not arise until high impact fluxes abated, or “impact bottlenecks” with Bacteria and Archaea representing surviving lineages that subsequently diversified. Phylogenomics studies using relaxed molecular clocks have frequently used these early impact fluxes, especially the LHB, as older-bound constraints on extant life’s early diversification. However, the intensity, timing, and sterilization potential of these scenarios is poorly constrained, and lacks consensus. We propose inverting this hypothesis testing, evaluating late accretion impact hypotheses using molecular clocks that do not presuppose impact frustration or bottlenecks as constraints. However, in the absence of these constraints, previous studies lack the precision to discriminate between these hypotheses. Our recently developed molecular clock approach, using horizontal gene transfers as “cross cutting events” between lineages, overcomes this limitation, and provides sufficient precision to test the proposed biological impact of specific planetary hypotheses such as the LHB.   Using this methodology, we show that major bacterial groups likely diversified between 3.75 and 3.55 Ga, with the last common ancestor of extant Bacteria likely existing shortly after 3.8 Ga.  These ages are consistent with the LHB impact bottleneck hypothesis, wherein bacteria and archaea represent survivors of early Archean cataclysms that extinguished most primordial biodiversity ~3.9 Ga. Future work extending this methodology to Archaea can potentially provide an independent test of the impact bottleneck hypothesis.