  Mathematical models have recently been used to cast doubt on the biotic
origin of stromatolites. Here by contrast we propose a biotic model for
stromatolite morphogenesis which considers the relationship between upward
growth of a phototropic or phototactic biofilm ($v$) and mineral accretion
normal to the surface ($\lambda$). These processes are sufficient to account
for the growth and form of many ancient stromatolities. Domical stromatolites
form when $v$ is less than or comparable to $\lambda$. Coniform structures with
thickened apical zones, typical of Conophyton, form when $v >> \lambda$. More
angular coniform structures, similar to the stromatolites claimed as the oldest
macroscopic evidence of life, form when $v >>> \lambda$.
