In macropinocytosis, cells take up micrometre-sized droplets of medium into 
internal vesicles. These vesicles are acidified and fused to lysosomes, their 
contents digested and useful compounds extracted. Indigestible contents can be 
exocytosed. Macropinocytosis has been known for approaching 100 years and is 
described in both metazoa and amoebae, but not in plants or fungi. Its 
evolutionary origin goes back to at least the common ancestor of the amoebozoa 
and opisthokonts, with apparent secondary loss from fungi. The primary function 
of macropinocytosis in amoebae and some cancer cells is feeding, but the 
conserved processing pathway for macropinosomes, which involves shrinkage and 
the retrieval of membrane to the cell surface, has been adapted in immune cells 
for antigen presentation. Macropinocytic cups are large actin-driven processes, 
closely related to phagocytic cups and pseudopods and appear to be organized 
around a conserved signalling patch of PIP3, active Ras and active Rac that 
directs actin polymerization to its periphery. Patches can form spontaneously 
and must be sustained by excitable kinetics with strong cooperation from the 
actin cytoskeleton. Growth-factor signalling shares core components with 
macropinocytosis, based around phosphatidylinositol 3-kinase (PI3-kinase), and 
we suggest that it evolved to take control of ancient feeding structures through 
a coupled growth factor receptor. This article is part of the Theo Murphy 
meeting issue 'Macropinocytosis'.