Ancient rocks hold evidence for life before oxygen

Somewhere between Earth’s creation and where we are today, scientists have demonstrated that some early life forms existed just fine without any oxygen. The 2.52 billion-year-old sulfur-oxidizing bacteria are exceptionally large, spherical-shaped, smooth-walled microscopic structures much larger than most modern bacteria, but similar to some modern single-celled organisms that live in deepwater sulfur-rich ocean settings today, where even now there are almost no traces of oxygen.
Paleontology News — ScienceDaily