Geologists recently have learned more about the conditions on our planet that could have made life possible as long as 4 billion years ago. And thanks to the DNA revolution, biologists now can trace the development of modern organisms — ranging from simpler forms such as mushrooms and flowers to more complicated ones such as humans — back to primitive microbes that lived in the ocean more than 2.7 billion years ago. At some point in between, lifeless molecules — combinations of atoms — learned to eat, breathe, move and reproduce. No one's certain when and how they did it, however. As early as 150 million years after the Earth formed, 4.5 billion years ago, the planet had cooled enough to have an atmosphere, an ocean and some dry land. Chemical reactions between hot water and rocks produced more and more complex molecules. Later, probably sometime between 4 billion and 3.8 billion years ago, these inorganic molecules grew and clotted together in "protocells." They formed organic compounds necessary for life. At some point in this process came the crucial, still-unexplained step: the advance from clumps of molecules to living cells containing an early version of DNA known as RNA. DNA contains the instructions to make proteins, the building blocks of every living thing. "I believe life began very early, before 4 billion years ago". However, the evidence for such a quick start was limited to "squiggles in the rocks" in Greenland and Australia that might represent ancient fossils. The oldest positive evidence for living microbes is dated at 2.7 billion years ago.