The Earth formed 4.5 billion years ago, and life less than a billion years after that. Although life as we know it is dependent on four major macromolecules – DNA, RNA, proteins and lipids – only one is thought to have been present at the beginning of life: RNA.
It is no surprise that RNA likely came first. It is the only one of those major macromolecules that can both replicate itself and catalyze chemical reactions, both of which are essential for life. Like DNA, RNA is made from individual nucleotides linked into chains. Scientists initially understood that genetic information flows in one direction: DNA is transcribed into RNA, and RNA is translated into proteins. That principle is called the central dogma of molecular biology. But there are many deviations.
