A microorganism, or microbe, is a microscopic organism that can exist as a single cell or in colonies. The concept of unseen microbial life has roots in ancient times, such as in Jain scriptures from the sixth century BC. Scientific study began in the 1670s when Anton van Leeuwenhoek first observed microbes under a microscope. Louis Pasteur later discovered that they caused food spoilage, debunking spontaneous generation, while Robert Koch identified microorganisms as the cause of diseases like tuberculosis and cholera.
Microorganisms are incredibly diverse, including unicellular organisms from all three domains of life: Archaea, Bacteria, and Eukaryota. The latter includes unicellular protists as well as multicellular organisms. Microbes thrive in varied environments, from extreme deserts to deep sea and polar regions. Some can survive harsh conditions, such as extreme temperatures and radiation. Evidence shows that 3.45 billion-year-old Australian rocks contained microorganisms, marking the earliest direct evidence of life on Earth.
