Present in bacteria and in eukaryotes. In the latter it is made of up fibrils and tubules. It shapes the cell, can change form of the cell, transport products, divide the cell, etc. The article shows how many, many different proteins are involved. the authors are of the opinion from their review that this complexity was present before the last common ancestor of the eukaryotes was formed. the implication is that early cellular forms of life were highly complex:-http://jcb.rupress.org/content/194/4/513.full-Abstract:-"The cytoskeleton is a system of intracellular filaments crucial for cell shape, division, and function in all three domains of life. The simple cytoskeletons of prokaryotes show surprising plasticity in composition, with none of the core filament-forming proteins conserved in all lineages. In contrast, eukaryotic cytoskeletal function has been hugely elaborated by the addition of accessory proteins and extensive gene duplication and specialization. Much of this complexity evolved before the last common ancestor of eukaryotes. The distribution of cytoskeletal filaments puts constraints on the likely prokaryotic line that made this leap of eukaryogenesis."-It is very unlikely that early life was quite complex. There is no such thing as 'simple' life.