Figure 18.6: Evidence of Early Photosynthesis (A) The alternating red and dark layers in these “banded iron formations” resulted from a reaction between oxygen and dissolved iron, which produced iron oxide. The oxygen was produced by Earth’s first photosynthetic organisms, beginning about 2.5 billion years ago. (B) A vertical section through a fossil stromatolite. Fossil stromatolites provide an early record of photosynthetic organisms. (C) These rocklike structures are living stromatolites that thrive in the very salty waters of Shark Bay in Western Australia.