A genomic timescale for the origin of eukaryotes

Area of Science:

• Evolutionary Biology
• Genomics
• Astrobiology

Background:

• Genomic analyses suggest horizontal gene transfer (HGT) via symbiosis was crucial for eukaryote origins.
• The precise timing and number of symbiotic events remain unclear.
• Establishing an early eukaryote timeline is vital for understanding their link to environmental changes like oxygenation.

Purpose of the Study:

• To determine the timing and number of symbiotic events in early eukaryote evolution.
• To correlate early eukaryotic diversification with environmental shifts, particularly the rise of oxygen.
• To refine the evolutionary timescale of eukaryotes using advanced genomic methods.

Main Methods:

• Utilized refined sequence alignment, site selection, and time estimation techniques.
• Analyzed protein sequences from complete prokaryotic and eukaryotic genomes.
• Applied phylogenetic methods to reconstruct evolutionary history and divergence times.

Main Results:

• Eukaryotes exhibit faster evolution than prokaryotes; those from eubacteria evolve faster than those from archaebacteria.
• Identified an early divergence of archaebacteria around 4 billion years ago (Ga).
• Supported at least two HGT events at 2.7 Ga and 1.8 Ga, with cyanobacteria originating at 2.6 Ga and Giardia diverging at 2.2 Ga.

Conclusions:

• Evidence supports two symbiotic events in eukaryote origins: one pre-mitochondrial and a subsequent mitochondrial event.
• The emergence of cyanobacteria preceded evidence of oxygen (2.4–2.2 Ga), indicating rapid environmental impact.
• Oxygenic photosynthesis likely facilitated further eukaryotic cell evolution.