Figure 1. Timeline of estimated and known evolutionary developments of Life on Earth, and parallel changes in development of the immune system components [Ga = Giga/ billion / 10 ⁹ years; Ma = Mega/ million /10 ⁶ years; PRR = pattern-recognition receptors; PAMPs = pathogen associated molecular patterns; DAMPs = damage (or danger)-associated molecular patterns; NK = natural killer cells].

Note: representative non-linear timeline scale.