A Roman farmer warned against building near swamps because of invisible creatures
In the 1st century BC Marcus Terentius Varro advised against siting a farmhouse near a swamp, where tiny unseen creatures drifted through the air and entered through the mouth and nose. He was guessing, but he was close. Proof that microbes cause disease took nearly two thousand more years.
Ideas of invisible life are old. By the 6th century BC Jain teachers in India described nigodas, minute beings living in clusters everywhere, even inside plants and people, and surviving only a fraction of a second. Avicenna suggested in 1020 that tuberculosis might spread between people. The 15th-century Turkish scholar Akshamsaddin wrote that diseases pass from person to person through living seeds too small to see, and in 1546 Girolamo Fracastoro argued that epidemics travel by seedlike particles, sometimes even across distances.
Seeing came in the 1670s, when Antonie van Leeuwenhoek experimented with microbes under single-lens microscopes of his own making. Proving their role took two famous experiments. Louis Pasteur boiled broths and left them open to air only through filters or curved necks that trapped dust, and nothing grew, which showed that life in spoiled broth arrives from outside rather than arising spontaneously. In 1876 Robert Koch found anthrax bacteria in the blood of sick cattle, passed the disease to healthy animals with small blood samples and with bacteria grown in broth, and turned the results into his postulates for linking a germ to an illness.
Microbes span all three domains of life, and two of them, bacteria and archaea, contain nothing else. They thrive from geysers to the deep sea, and Deinococcus radiodurans shrugs off intense radiation. Single cells were the first life, around 3.5 billion years ago, and for about 3 billion years they were the only life; bacteria sealed in 220-million-year-old amber look much like modern ones.
They also evolve fast. Bacteria reproduce quickly and swap genes directly, even between very different species, which has helped produce superbugs that resist many antibiotics. In 2012 Japanese researchers described Parakaryon myojinensis, a microbe that keeps its genetic material in a membrane like complex cells do while otherwise resembling bacteria, a possible intermediate between the two.
Source: Microorganism
