Tiny but deadly
Almost 2 million years ago, the only animals around were bacteria. But it was no paradise, as those guys were hungry. Scientists looked at 1.9 billion-year-old rocks with advance imaging techniques, and saw evidence of little bacteria snacking on bigger ones. Their technique involved taking elements from seawater to help break down the larger bacteria's shells, and the byproduct of that process was gaseous CO2. So ancient Earth smelled like fa---, um, rotten eggs.