  This paper outlines our recent attempts to model the growth and form of
microbialites from the perspective of the statistical physics of evolving
surfaces. Microbialites arise from the environmental interactions of microbial
communities (microbial mats). The mats evolve over time to form internally
laminated organosedimentary structures (stromatolites). Modern day
stromatolites exist in only a few locations, whereas ancient stromatolitic
microbialites were the only form of life for much of the Earth's history. They
existed in a wide variety of growth forms, ranging from almost perfect cones to
branched columnar structures. The coniform structures are central to the heated
debate on the oldest evidence of life. We proposed a biotic model which
considers the relationship between upward growth of a phototropic or
phototactic biofilm and mineral accretion normal to the surface. These
processes are sufficient to account for the growth and form of many ancient
stromatolities. These include domical stromatolites and coniform structures
with thickened apical zones typical of Conophyton. More angular coniform
structures, similar to the stromatolites claimed as the oldest macroscopic
evidence of life, form when the photic effects dominate over mineral accretion.
