Normally we think of an ecosystem in reference to the environment in which we humans live.

From the perspective of a microscopic organism living inside us, however, the human body is the ecosystem and indeed the whole universe. Each microbe is perfectly adapted to live within the range of conditions that occurs in its particular internal organ or its location on the skin.

The basic functioning of an ecosystem is the same whether it is micro or macro in scale. There is competition for territory, energy and nutrients, which tends toward equilibrium and a balance among species.

The human body consists of about 1,000 trillion cells. These comprise the skin, bones, blood, muscles, organs, connective tissue and bacteria. Nine hundred trillion of these cells, about 90 percent, are not human.

The average human body is a zoo of bacteria and yeast, collectively known as the microbiome. Researchers estimate that upward of 36,000 different species live in the human body, and no two people have the same microbiotic assemblage. About 170 different species of bacteria live inside the typical human digestive tract.

Maintaining this gut microbiota can stop pathogenic microorganisms from colonizing the gastrointestinal tract. Broad-spectrum antibiotics can allow unfriendly pathogens to proliferate, often with acute health implications. Certain probiotic foods can help to maintain or re-establish the equilibrium as the resident bacteria salvage energy that the host individual does not have the ability to digest.

Each person is a unique ecosystem with unique communities of bacteria and yeast. The population, diversity and activities of these communities fluctuate with changes in our inner ecology. For example, 700 species of bacteria inhabit the human mouth, and the mix is different for each of us.

More than 99 percent of the human genome is microbial rather than human, part of our long evolutionary heritage. We are a walking colony of bacteria that are crucial for maintaining health. The effects of our microbiome are profound. Among other things they help to digest food, synthesize vitamins and guide the immune system.

Imbalanced microbiomes are implicated in all kinds of health conditions including obesity, diabetes, heart disease, lung disease, various gastrointestinal disorders and even anxiety and Alzheimer’s.

As in any ecosystem it is the richness and diversity that matters rather than which species are present. There are healthful benefits from the presence of a wide range of microbial guests, many of which co-evolved with our genome.

Even the colonies that live on our hands are unique. Researchers have identified individuals with up to 90 percent accuracy by identifying the bacteria left behind on objects that they touched. The average human hand contains about 150 species of bacteria, with only about 13 percent shared by any two people. On average only 17 percent of the species on the left hand also live on the right hand.

Scientists and law enforcement personnel are working to make it possible in the near future to use bacterial assemblages as a forensic tool to identify people by the unique collections of bacteria that they leave behind at a crime scene.

In the meantime we need to change our thinking about who we are. Instead of an individual human body, we are a multi-organism ecosystem.