Genes as fossils
When exactly did oxygen first appear in Earth’s atmosphere? Although many physical and chemical processes are thought to be responsible for that profound transformation, scientists have tried to answer at least part of that question by looking for the origin of oxygenic photosynthesis — the process that organisms use to split water to make oxygen — in rocks that are billions of years old. One way they try to pinpoint the start of that process is by searching for biological links between the distant past and the present. Specifically, they study molecules known as biomarkers that are produced by modern organisms and can be traced to the origins of certain biological processes because they are found in rocks that are 2.5 billion years old.One biomarker that had been proposed for such research is a type of lipid, or fat molecule, known as 2-methylhopanoid. This substance was thought to be a good...