The source of all life is water. Once the Earth’s crust had cooled off sufficiently, the first oceans began to form. All kinds of substances mixed with the water, forming a primordial soup. Just over three billion years ago, this soup began to generate life: at first in the form of cells without a nucleus. Cells with a nucleus followed and so began the evolution of life into the millions of different organisms we know today.
Very primitive blue-green algae living in the oceans produced so much oxygen that the atmosphere became toxic – at least, toxic to the life forms that then existed. Slowly but surely, however, other organisms evolved that could survive the presence of oxygen. Like us, for example.