13.2: Microbes are the simplest but most successful organisms on earth.

Humans are large organisms, and being large comes with some “baggage.” We need a skeletal system to support our weight, and a respiratory system to take in oxygen and get rid of carbon dioxide. We need a circulatory system to move oxygen, carbon dioxide, and other molecules around our bodies, and a digestive system to take in food and break it down. We even need a nervous system so that our brain knows what distant parts of our body are doing.

Microbes—the most abundant organisms on earth—don’t have skeletal, respiratory, circulatory, digestive, or nervous systems, because they are too small to need them. Take an amoeba, for example—its volume is about a million billion (10¹⁵) times smaller than a human. When you are that tiny, the force of gravity is trivial, so the amoeba needs no skeleton to support it. Plenty of oxygen diffuses inward across its cell membrane, and carbon dioxide diffuses outward, so an amoeba does not need a respiratory system; and because every part of its interior is close to the body surface, it doesn’t need a circulatory system to transport gases. An amoeba doesn’t need a digestive system either: it eats by enclosing food items in a piece of its cell membrane and digests the food with the same enzymes it uses to recycle its own proteins, lipids, and carbohydrates. And no part of an amoeba is far enough from any other part to require a specialized nervous system for communication.

Most microbes are even smaller than an amoeba: a typical bacterium or archaeon is about one thousand million billion (10¹⁸) times smaller than a human, and an influenza virus is about one thousand billion trillion (10²⁴) times smaller than you are (FIGURE 13-2).

They may be invisible, but microbes could never be considered unsuccessful just because of their (lack of) size. They are actually more successful than humans in almost every imaginable way.

Microbes are genetically diverse. More than 500,000 kinds of microbes have been identified by their unique nucleotide sequences, and further studies will almost certainly distinguish millions of additional microbial species.

Microbial species live in almost every habitat on earth; among them, they can eat almost anything. As you read these lines, more than 400 species of microbes are thriving in your intestinal tract (FIGURE 13-3), 500 more species thrive in your mouth, and nearly 200 species call your skin home. The microbes that live in and on you eat mostly what you eat—some of the bacteria in your mouth and intestine compete with you, trying to digest your food before you can, and others use the waste products you release after you have broken down the food. Others feed on the leftovers released by the breakdown of your cells during the normal process of cell renewal.

Living conditions in the human body are relatively moderate. Other microbes inhabit some of the toughest environments on earth—in the almost boiling water of hot springs, at depths a mile below the earth’s surface, and more than a mile deep in the oceans, where hydrothermal vents emit water at 400° C (750° F).

Microbes are abundant. Surface seawater contains more than 100,000 bacterial cells per milliliter, and diatoms (protists in the eukarya domain) are as abundant there as are the bacteria. These densities translate to about 8,000 million billion trillion (8 × 10³⁰) individuals of just these two kinds of microbes in the world’s oceans. Your own body is a testament to the abundance of microbes: it contains about 100 trillion cells, but only one-tenth of those cells are actually human cells—the remaining 90 trillion cells are the microbes that live in and on you (FIGURE 13-4). You’re a minority in your own body.