What Evidence Have Some Scientists Claimed for Ancient Life on Mars

What evidence have some scientists claimed for ancient life on Mars (although the hypothesis has been strongly disputed by many other scientists) ?

A) an apparent thumbprint on a rock at the Mars Pathfinder landing site
B) peroxides in the "soil" (regolith) found by experiments on the Viking landers
C) microscopic features in a Martian meteorite similar to biologically generated features on Earth
D) calcium carbonate deposits similar to seashells in a sample of Martian limestone