The period of Earth's history that began 2.5 billion years ago and ended 544 million years ago is known as the Proterozoic; it is divided up, rather arbitrarily, into the Paleoproterozoic (2.5 to 1.6 billion years ago), Mesoproterozoic (1.6 billion to 900 million years ago) and Neoproterozoic (900 to 543 million years ago). Near the beginning of the Proterozoic, stable continents first appeared and began to accrete, a long process taking about a billion years.
Where was this oxygen coming from? Cyanobacteria, photosynthetic organisms that produce oxygen as a byproduct, had first appeared 3.5 billion years ago, but became common and widespread in the Proterozoic. Their photosynthetic activity was primarily responsible for the rise in atmospheric oxygen.
Read about the Llano uplift in central Texas, which is of Proterozoic age.
Find out more about the Precambrian paleontology and geology of North America at the Paleontology Portal.
Holland, H.D. 1994. Early Proterozoic atmospheric change. Pp. 237-244. In: Bengtson, S. (ed.) Early Life on Earth. Columbia University Press, New York.