Views of a Martian lake bed (left) compared to Earth features that were formed by microbial mats (right).
The Martian sandstone, about 3.7 billion years old, appears to have layers of stacked microbial mats, pockets eroded into those layers, and places where sections of mat rolled up or cracked after drying out.
Nora Noffke, who mustered the evidence, says:
This hypothesis is testable and warrants further scrutiny in current and near-term missions to Mars.
Source: Ancient Sedimentary Structures in the <3.7 Ga Gillespie Lake Member, Mars, that Compare in Macroscopic Morphology, Spatial Associations, and Temporal Succession with Terrestrial Microbialites, by Nora Noffke, Astrobiology, Dec 11, 2014