Biology Now, 2e

270 ■ CHAPTER 15 Bacteria and Archaea

BIODIVERSITY

T

he gathering crowd jostled for position

around the table. The scientists behind the

table smiled as they handed out long, thin

cotton swabs. Holly Menninger thanked them

for the swab, then stared down at her shirt. She

had been told what to do: lift your shirt, insert the

swab into your belly button, swirl it around, place

it into a test tube, seal the top. “Oh, the things we

do for science,” Holly thought. Then she swirled.

After returning her tube, Menninger—an

entomologist by training and, at the time, coordi-

nator of the New York Invasive Species Research

Institute at Cornell University—struck up a

conversation with the researchers manning the

table, including Rob Dunn of North Carolina

State University. Dunn, an applied ecologist and

writer with an infectious passion for science, had

initiated the project to answer a simple question:

What lives on our skin? The belly button, as small

and strange as it is, was an excellent place to look.

If you pick away the lint, the leftover schmutz

in your belly button isn’t dirt; it’s alive. Our bodies

are ecosystems, home to an estimated 39 trillion

resident microbes (microscopic organisms),

primarily bacteria. Considering that the average

human body is made up of 30 trillion human

cells, that’s about a 1:1 ratio of microbial cells to

human cells. (The old 10:1 ratio is a miscalcu-

lation that has unfortunately become enshrined

in popular culture.) The human microbiome—

the complete collection of microbes that live in

and on our cells and bodies—affects human gut

health, brains, and even body odor.

Microbes live in our guts, on our eyelashes,

and yes, in our belly “holes,” as Dunn calls them.

In fact, the “belly hole” is an ideal place for a

scientific study of resident microbes because it

is a protected, moist patch of skin and one of the

few areas that individuals don’t regularly wash.

Dunn knew that his students, if they could deter-

mine which microbes were swimming around

in individual volunteers’ navels, could then dig

into the more burning question: Why do we each

have the microbes we do?

“We know that which microbes you have on

your skin influences your risk of infection, how

attractive you are to other people, and how

attractive you are to mosquitoes,” says Dunn.

“So this question of ‘What determines which

microbes are on your skin?’ is super intriguing.”

Of the three domains of life on Earth,

Bacteria was the first to split off from the

shared ancestor of the Archaea and Eukarya

(Figure 15.1). Some fossil evidence places that

split at about 3.48 billion years ago, yet a 2016

discovery in Greenland suggests that bacteria

existed as early as 3.7 billion years ago, close to a

period of time when Earth was being bombarded

by asteroids (Figure 15.2).

Domain

Bacteria

Domain

Archaea

Domain

Eukarya

Kingdom

Plantae

Kingdom

Protista

Kingdom

Fungi

Kingdom

Animalia

Common

ancestral cell

or universal

ancestor

Figure 15.1

Two o f t he t hr ee domains o f lif e

Bacteria and Archaea are two distinct domains. They share many

characteristics that are not found in the Eukarya.

4 cm

Stromatolite

Figure 15.2

Earth’s first life left its mark in

Greenland rocks

In 2016, Australian scientists found stromatolites,

the fossilized secretions of bacteria, in rocks

exposed by receding glaciers. These rocks formed

3.7 million years ago in a part of the world that is

now Greenland.