The discovery of microbes was anticipated by Charles Darwin and some of his contemporaries, who speculated that life originated from miniscule organisms that left little impression in the fossil record. It was not until 1673 when Antonie Van Leeuwenhoek first observed such life that microbiology was born. Buttressed by Louis Pasteur’s experiments, microbiology grew into a large field, but microbes themselves acquired a poor reputation after Pasteur’s Germ Theory. This hampered studies of microbes, leaving much unknown–it was not until 1977 that Carl Woese defined archaea as a distinct kingdom of life separate from bacteria.

Prior to Woese’s discovery, life was arranged into two domains: prokaryotes (like bacteria) whose cells have no nucleus, and eukaryotes (like animals) that do. Woese suggested that archaea (or archaebacteria, as it was first called) was as distinct from bacteria as bacteria is from eukaryotes, with a different composition of its cell wall and membrane and the ability to live in extreme environments. One theory to the origin of the first eukaryote is a union between a bacterial cell and an archaea, an evolutionary leap that has never been repeated in nature.  Microbial life does not simply maintain more complex life, it created it.

Besides laying the evolutionary groundwork for complex life, microbes created the environment for it to flourish. These organisms were the first photosynthetic beings on Earth, making it habitable for furture life. Microbes are not something acquired after an animal is born, they are there right from the beginning, intermingling with an organism's development. These beings  preceded complex life by billions of years–all evolutionary adaptations are in response to this biome of life. Microbes are not just hitching a ride in their hosts, but operating a complex symbiotic relationship that allows both to survive. They may aid in digestion, circulation, reproduction, or any other basic life process.

The natural world is littered with innumerable examples of these symbiotic relationships. The Hawaiian bobtail squid nurtures Aliivibrio fischeri, which in turn provides bioluminescent camouflage. The pufferfish uses bacteria to produce tetrodotoxin, a deadly poison that protects the fish from predation. But these relationships are not just used against predators, they can also be a means of communication. Hyenas used a scent gland to mark territory, but the bacterial makeup of these glands varies widely between species and social group, allowing the hyena to leave a distinctive trace and respond to others.

In humans, these symbiotic relationships begin from our earliest days. Human milk is not actually digestible by the human body alone, it instead nourishes Bifidobacterium longum bacteria in the gut that in turn creates fatty acids for the baby. The biomes in our mouths and lungs weed out aggressive pathogens that would otherwise make us sick, while out bodies provide them nourishment and a stable environment.

When we die, our immune system which controls this elaborate biome collapses, allowing unregulated microbes to begin to decompose our bodies, a burst of life accompanying our individual demise. The necrobiome as it is called is a fascinating arena of life in its own right–just as the microbiome allows insight into a person’s life, this thanatomicrobiome allows study into a person’s death.