Although the sequence of a protein largely determines its function, proteins can 
adopt different folding states in response to changes in the environment, some 
of which may be deleterious to the organism. All organisms--Bacteria, Archaea 
and Eukarya--have evolved a protein homeostasis, or proteostasis, network 
comprising chaperones and folding factors, degradation components, signalling 
pathways and specialized compartmentalized modules that manage protein folding 
in response to environmental stimuli and variation. Surveying the origins of 
proteostasis networks reveals that they have co-evolved with the proteome to 
regulate the physiological state of the cell, reflecting the unique stresses 
that different cells or organisms experience, and that they have a key role in 
driving evolution by closely managing the link between the phenotype and the 
genotype.