Earth’s history is written in stone. The rocks you see on the ground and the rocks beneath your feet contain layer upon layer of geological narrative. These layers were there 500,000 times further than recorded human history. Like scrapbook, these layers filled with mementos in the past.
The earliest pages tell a story of a billion years ago, when our planet was a red-hot ball of liquid metal, no breathable air, and inhospitable to life. Then, 3.5 billion years ago, the arrival of oxygen, a form of gaseous waste product puffed out by photosynthetic bacteria, brought the turning point. The oxygen reacted with silicon, phosphorus, and calcium to make new chemicals, which could dissolve in water. Occasionally, chemicals that contain chemicals so concentrated that they produced a crystalline film over rocks and even living things. Acritarchs, those early organisms, became the first fossils.
Acritarch fossils show early steps of evolution, but they are difficult to be interpreted. Oxygen burns molecules, as a fire burns tinder. As a result, early creatures were forced to adapt or die. Survivors teamed up, one living inside another, to get used to the rise in oxygen level. Those survivors, 2-billion-year-old creatures, finally became plants, mushrooms, and even human.