While the nature or function of these filaments remain unknown, these observations suggest that they may help mediate physical attachment and symbiotic interactions with partner organisms. Notably, prokaryotic filament structures—flagella, pili, and archaella—have been repeatedly

Phylogenetic analyses suggest FECA may have also had low aerotolerance but did not respire oxygen, like HC1 (Fig. 5g). This strongly indicates that the prokaryote-eukaryote transition was steep in both cell structure and physiology—from anaerobic to aerobic and from simple to complex internal cell structure.