Life on Earth began over 3.7 billion years ago with the appearance of prokaryotes, the most primitive life known to exist. Prokaryotes, better known as bacteria, possess no nucleus and no advanced cellular machinery. They are unicellular and are only a small fraction of the size of a plant or animal cell. Despite their primitive construction, prokaryotes are the most populous life on the planet, outnumbering every other form of life, combined, by many orders of magnitude. Without prokaryotes, no other life would exist.

Bacteria created the atmosphere’s oxygen levels, beginning around 2.5 billion years ago. These initial photosynthesizers, called cyanobacteria, still exist today. Their ancestors lived in a world with no atmospheric oxygen and used energy from the sun and chemicals in primordial oceans to create their own food, similar to modern plants. Cyanobacteria generated oxygen gas, a poison to all early life, as waste. Over the next 300 million years, the levels of oxygen in the atmosphere and the ocean built up due entirely to these microscopic creatures. Primordial species died in mass extinctions as the oxygen levels rose, but oxygen-tolerant life evolved to fill the empty niches. Modern life would not exist without these early oxygen-creating bacteria.

The smallest life on Earth has the largest role: breaking down and recycling all waste. Husks and carcasses of dead plants and animals and excreted matter of all types contain vital nutrients and stored energy. Without a way of returning those nutrients to the ground, life would quickly deplete every available nutrient on the planet. Many species of bacteria feed on these energy sources, breaking down waste to its smallest molecules and returning them to the ground, where they re-enter the food chain. Some species of bacteria even consume oil, and helped rapidly break down and eliminate large volumes of oil from the Deepwater Horizon spill in the Gulf of Mexico in 2010.

Without prokaryotes, society would never experience a wide range of foods. Anything fermented, such as beer, wine, yogurt, buttermilk, sour cream, pickles, olives and sourdough bread owes its existence to various species of beneficial bacteria which produce food-preserving acids as metabolic by-products. Prokaryotes also help make cheese, insulin for diabetics, vinegars, sauerkraut, vitamins, soy sauce and hundreds of other foods and medicines across the world.

Often overlooked and thought of in unflattering terms, gut bacteria perform many tasks in exchange for food and shelter. The resident bacterial population in a single human colon is an order of magnitude greater than the entire number of human cells in the host. This huge reserve of metabolic activity helps digest food, stimulates peristalsis, acts in concert with the immune system to drive out pathogens, and produces vitamin K to help blood clot. The human body cannot undertake any of these tasks alone and survive: bacteria are essential to human survival.

In addition to colonizing the digestive tract, prokaryotes colonize every exterior surface on a human body from the moment of birth. These bacteria exist in a mutually beneficial relationship with their host. The bacteria have a spot to live on and colonize. In return, these species defend their “home”, the host’s skin, from pathogenic bacteria and fungi that opportunistically invade through the skin. The host’s immune system spends less energy in this arrangement, allowing it to focus on other tasks, like fighting viruses and destroying precancerous cells.