An interesting study paper and article recently showed up:

Paper: Constraints on the Metabolic Activity of Microorganisms in Atacama Surface Soils Inferred from Refractory Biomarkers: Implications for Martian Habitability and Biomarker Detection

Article: Is Mars' soil too dry to sustain life?

The article comes to some very interesting conclusions (my underlines):
... All life on Earth is built with "left-handed" amino acid molecules. However, when a cell dies, some of its amino acids change at a known rate into the reflecting "right-handed" structure, eventually balancing into a 50-50 ratio over many years. By looking at this ratio in the driest Atacama soils, the scientists found microbes there that have been dead for at least 10,000 years. Finding even the remnants of life this old is extremely rare, and surprising for a sample sitting in the surface of Earth.

Getting Ready for Mars:

Mars is 1,000 times drier than even the driest parts of the Atacama, which makes it less likely that microbial life as we know it exists on the planet's surface, even with some access to water. However, even in the driest areas of Chile's desert, remnants of past microbial life from wetter times in the Atacama's history were clearly present and well preserved over thousands of years. This means that because scientists know that Mars was a wetter, more vibrant planet in its past, traces of that ancient life might still be intact.
Perhaps those blueberries might really turn out to be 10,000 year old dessicated/fossilised martian fruit after all, eh?