Stromatolite

This is easily the oldest fossil we’ve ever featured. This small, dark rock is a stromatolite, a sedimentary rock consisting of layer upon layer of oxygen-producing microorganisms. These single-celled microbes were so simple they lacked a DNA packaging nucleus, but were responsible for possibly the most drastic changes the earth has undergone. By producing oxygen as a waste product, they were the only major source of atmospheric oxygen at the time, and therefore critical for the development of more complex life.
Some stromatolites date back an astounding 3.4 billion years, making them the oldest record of life on planet Earth.When these microbes lived, Earth’s atmosphere had no oxygen, the oceans were euxinic—void of oxygen and rich in hydrogen sulfide (H2S).