Life originated on Earth at least 3.4 billion years ago (Australia), and perhaps as early as 3.6 billion (Greenland). The Earth formed 4.5 billion years ago and was cool enough to start trying for life at 4 billion. Life appeared early and quick. -Stromatolites are mats of old microbes, but also new ones. They have always been around.-"Studying the diet of ancient microbes isn't as simple as with large, multi-cellular organisms. For instance, paleontologists can look at the teeth of dinosaurs and, based on the scratches and scrapes left by years of chewing, make a good guess at the types of plants and animals they ate. Microbes have no teeth, but they do leave a lasting mark on what they eat. Namely, their metabolism prefers specific isotopes of the same chemical element. Certain ratios (or fractionations) of isotopes can be linked to biological reactions, showing that the elements of interest were processed by microbes and not non-biological reactions or weathering. For Strelley Pool, the team focused on isotopes of sulfur found in the kerogen laminae. They discovered fractionations consistent with an Archean sulfur cycle, indicating that sulfate-respiring microbes were at least living there when the mats were formed. This is also true for modern stromatolites ... meaning that ancient and modern stromatolites may have more in common than their looks alone. "Our results do not prove that sulfate-respiring microbes produced the stromatolitic laminations—they just indicate that sulfate-respiring microbes were present within the microbial mat," explains Bontognali. "This finding highlights important similarities between these early ecosystems and their modern counterparts. Sulfur-respiring microbes are commonly found at depth within modern microbial mats, and—in some lithifying mats—they play an important role in forming laminations. Thus, it is legitimate to hypothesize that similar biological processes might have contributed to the formation of the Strelley Pool stromatolites."- Read more at: http://phys.org/news/2012-11-million-year-old-diet.html#jCp