What is in this picture ?

clipped from www.abc.net.au Have a guess Hints •I spread myself around the globe by utilising insects, birds, mammals, and the weather •While my existence is beneficial, in spring many curse me •My presence can help scientists work out the deposition dates of ancient rock assemblages and help forensic detectives solve crimes. »What am I? clipped from www.abc.net.au Have a guess Hints •I am alive •I am an integral part of you •In classical Greek medicine, I was associated with air, springtime, and a merry personality »What am I? clipped from www.abc.net.au Have a guess Hints •Rarely seen, I am a common coastal inhabitant •I live in sediment and often build tube-like burrows •I reproduce by dropping my tail, which contains my sexual-organs »What am I? clipped from www.abc.net.au Have a guess Hints •I was discovered in 1976 •I was the subject of a 2000 movie starring Gary Sinise •I was once believed to be evidence of alien life »What am I clipped from www.abc.net.au Have a guess Hints •I am microscopic in size •I am part of an amphibious animal •I am undergoing an essential life process »What am I?