Biology Now, 2e

Navel Gazing ■ 273

to generate that tree, they showed that life is

clearly dominated by prokaryotes (Figure 15.5).

Merry Microbes

In 2010, as a gag, an undergraduate student in

Dunn’s lab decided to make a Christmas card

with streaks of microbes taken from people in

the lab. She asked for belly button swabs from

her coworkers, spread each sample on a petri

dish, and grew the bacteria into colonies. As

she grew bacteria from multiple individuals, it

quickly became clear that there was more vari-

ety among the microbes growing on people in

the lab than had been expected.

“Then it went from being a fun lab project to a

serious question,” says Dunn. The Christmas gag

had raised an important question: Why do one

person’s microbes differ from another’s? From

his experience studying ecology, Dunn knew that

microbes are critical in our lives. But neither Dunn

nor others knew which factors determine which

particular skin microbes a person has, or lacks.

So Dunn and his team began to study the

locally collected swabs. An early part of the

experiment engaged participants by visually

depicting each person’s microbial menagerie.

The team plated each sample onto a petri dish

and then took a picture of it for the microbes’

owner. Although prokaryotes are single-celled

organisms, some form colonies or long chains

of identical cells, produced by repeated splitting

begun by one original cell. This bacterial pattern

of reproduction resulted in bright, dramatic

patterns on the plates (Figure 15.6).

Under the microscope, the individual microbes

were even more diverse and elaborate. Bacteria

and Archaea cells are quite variable in shape, rang-

ing from rods to spheres to spirals (Figure 15.7).

Still, they all have a basic structural plan. Most

bacteria and many archaeans have a protective

cell wall that surrounds the plasma membrane.

Some have an additional wrapping around that

cell wall called a capsule. The capsule, made of

slippery biomolecules, works like an invisibility

cloak: it helps disease-causing bacteria evade the

immune system that protects organisms like us

from foreign invaders.

Dunn’s team observed many bacteria whose

surface was covered in short, hairlike projections

called pili (singular “pilus”), a common bacterial

Clostridiales

Bacillus

Micrococcus

Staphylococcus

Figure 15.6

Belly button bacterial biodiversity

Each petri dish contains colonies of the bacteria

collected from one person’s belly button sample.

Q1: Where in the figure would you place the first life found on Earth?

Q2: Identify where in the figure the Bacteria split off from the

ancestor of Archaea and Eukarya.

Q3: The figure (and thus the study) demonstrates that Archaea and

Eukarya are more closely related to each other than to Bacteria. How

is that illustrated?

Figure 15.5

The most recent tree of life

Researchers sequenced DNA from individuals in 3,083 genera across

all three domains. This information was combined by phylum, and the

placement of each phylum in the tree was based on its relatedness to other

phyla, as measured by DNA similarity.

Bac

teria

Arc

hae

a

Eu

ka

ry

a