Secreted signal that acts globally to specify positional identity along the anterior/posterior axis during development. May play critical roles in patterning both mesodermal and neural tissues and in establishing the skeletal pattern. Signals through activin receptors type-2, ACVR2A and ACVR2B, and activin receptors type-1, ACVR1B, ACVR1C and TGFBR1 leading to the phosphorylation of SMAD2 and SMAD3.