The oldest known fossil of a living organism is a stromatolite, dated to around 3.5 billion years ago. These layered structures are formed by the activity of cyanobacteria, which are ancient photosynthetic bacteria. Stromatolites provide valuable insight into early life on Earth and the evolution of ecosystems. Their presence indicates the earliest known biological activity and the crucial role of microorganisms in shaping Earth's atmosphere by contributing to oxygen production during the process of photosynthesis.