How to unravel the history of life when it had only just begun? One breed of paleontologist just keeps an eye peeled while tromping across some of the world’s oldest rocks. In the December issue of Astrobiology, a group of such “geobiologists” reports the discovery of the oldest known “microbially induced sedimentary structures,”  which are just distinctive shapes in sedimentary rock formed by layers of now-gone slime. All manner of bacteria, fungi, protozoa, and algae can live in their own intimate ecosystem layered on the bottom of anything from lakes and rivers to lagoons and shallow seas. Such microbial mats bind sand grains so that when, say, a strong current rips off a bit of mat and rolls it up, the rolled mat of sand has a chance to be found embedded in rock eons later. The Astrobiology group reports such rolled up fragments several centimeters thick today on a North Carolina barrier island (right), turned into 2.9-billion-year-old rock in southern Africa (middle, sliced-open crosswise) and turned into 3.5-billion-year-old rock in Western Australia (left). No vestige of life, micro- or megascopic, older than the Australian example has been found. Such microbially induced sedimentary structures can thus identify once-inhabited environments on early Earth—providing a history of “slime through time”—and possibly on early Mars.