Then, Mars was much warmer and wetter than at present. It had a much thicker atmosphere. Streams, rivers, maybe even an ocean existed. These microorganisms quite possibly played a critical role in the formation of the carbonate globules that are found with the fractures. Carbonate is a mineral that on earth is commonly produced by the action of microorganisms.
Limestone is an example.
Furthermore, the carbonate globules in ALH84001 are similar in size and texture to carbonate precipitates that are often formed by terrestrial bacteria. The McKay group demonstrated that these carbonate globules contained fine-grained secondary phases of single domain magnetites and iron sulfides. These minerals probably formed in water solutions at temperatures amenable to microbial life. This result is extremely significant. On earth, magnetites and iron sulfides can be produced by sulfur-eating bacteria.