What were the earliest life forms? How do we know?<\/p>\n\n\n\n
Scientists believe they have pieced together a history of the successive types of life forms that have prevailed on Earth from the earliest bacteria to modern man. The earliest life forms existed in the Precambrian period.<\/p>\n\n\n\n
Keep reading for a brief overview of Precambrian life.<\/p>\n\n\n\n\n\n\n\n
The Precambrian period is the earliest geologic age of the Earth. Bryson explains that scientists can infer the history of Precambrian life (the earliest life forms) from geologic information about the early Earth: Anaerobic bacteria must have been the first prevalent lifeform on Earth because Earth\u2019s early atmosphere didn\u2019t contain much oxygen. Then, at some point, cyanobacteria developed photosynthesis and started producing oxygen as a byproduct. Oxygen is actually toxic to most anaerobic organisms, but strains of anaerobic bacteria have survived to this day in swamps or other places where they are shielded from oxygen.<\/p>\n\n\n\n
Bryson recounts how, once oxygen became abundant in the atmosphere, mitochondria started producing energy for cells by oxidizing nutrients. This facilitated the growth of more complex cells called eukaryotes, which eventually formed multicellular organisms. Scientists speculate that mitochondria may have begun as a separate organism that invaded bacterial cells but then formed a symbiotic relationship with them. This is because mitochondria have their own DNA that they keep separate from the rest of the cell\u2019s DNA. <\/p>\n\n\n\n