Microbial communities dominated life on Earth for at least 3 billion years. Evolution of life prior to 2 billion years ago consisted of advances in biochemical processes that are not reflected in the morphology of organisms, in contrast to the morphological record of evolution in younger, multi-cellular fossils. The structure of enzymes and information recorded in the genomes of extant microbes provide a record of many evolutionary advances in biochemistry. Additional evolutionary constraints from the rock record allow us to test and calibrate genomic “clocks”, as well as characterize early evolutionary processes. Organic biomarkers record the existence of specific biochemical pathways associated with certain groups of organisms, which makes their analysis an extremely useful tool. An additional, as yet untapped, source of evolutionary information resides in the morphology of partially preserved microbial structures.
Microbial communities can create complex structures as a result of motility in response to chemical cues in their environment. Some of these cues are explicitly manipulated by the organisms, such as quorum sensing responses where individuals release a specific chemical to trigger a behavior in other individuals; other cues are predominantly environmental, such as nutrient or oxygen gradients. Mathematical models of the behavior of specific organisms under well defined environmental conditions can reproduce very complex patterns from easily defined behaviors. However, extending these models to morphological features expected in natural environments with unknown microbial communities is beyond our current understanding of microbial community behavior in general. Laboratory experiments with biofilms and behavior modeling are starting to bridge this gap.
The problem of interpreting microbial behavior from structures preserved in the rock record can also be approached from characterizing abiotic processes. Diffusion and surface reaction dynamics can produce structures that are morphologically similar to biological ones in the absence of specifically biological behaviors. Biological structures that have not yet been reproduced with abiotic models include peaked and ridged morphologies. These may require microbial motility to form, making them potential morphological biomarkers. A significant number of peaked stromatolites and microbialites are preserved in Earth’s early rock record. The challenge now is to use the details of their morphology to extract more specific constraints on ancient microbial behavior.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616