Gravity has been a pervasive factor throughout evolutionary history, and biological systems are assumed to have used this external clue for orientation rather early to find and stay in optimal living conditions, which offer ecological advantages. Consequently, already early during evolution, unicellular organisms developed organelles for active movement and sensors for diverse environmental stimuli. This chapter summarizes some aspects and ideas concerning evolution and refinement in gravisensing, and focuses on the fundamental question concerning common origins in the underlying signal transduction chains.

Development of gravisensing during evolution

As discussed in several chapters: To introduce a detectable change in the cell, gravity has to interact with a mass. Consequently, size matters! To be more precise: size and specific density difference matters. Published values for the specific density of prokaryotes are scarce. Guerrero and coworkers (1984) published a value of 1.16 g ml—1 for Allochromatium vinosum, a purple sulfur bacterium. This value might vary drastically due to living conditions, feeding and metabolic status, etc. The sedimentation rate is only 0.12 µms—1. In contrast, sedimentation rates in eukaryotes are generally one to two orders of magnitude higher. Size makes the difference. Prokaryotes are typically at least one order of magnitude smaller than eukaryotes. Brownian motion, as well as low Reynolds numbers, prohibit any active steering mechanism in free-swimming species. But, there is a way out: limiting diffusion dimensions.