The Ecology Book

91

See also: Early theories of evolution 20–21 ■ Evolution by natural selection 24–31

■ The role of DNA 34–35 ■ A system for identifying all nature’s organisms 86–87

ORDERING THE NATURAL WORLD

grains that all cells need in order to

make protein—and devised what he

called the “three-domain system.”

This gave him a new perspective

on the branches of Charles Darwin’s

evolutionary “tree of life.” Woese

found big differences in the

chemical makeup of ribosomes

among tiny microbes, with one

group as far from other prokaryotes

as bacteria are from humans.

Revising the tree of life

Woese’s third domain of organisms,

known as archaea, is superficially

similar to bacteria, but has some

strange properties. Many thrive in

extreme habitats. Some—uniquely

among living things—generate

methane in oxygen-deprived places,

such as deep marine sediments, or

inside warm digestive cavities,

such as those of belching, flatulent

plant-eating mammals. Other

archaea inhabit lakes that are ten

times saltier than seawater, or hot

acidic pools fed by geothermal heat

that would kill anything else.

A decade before Woese proposed

his theory, Robert H. Whittaker

had recognized animals, plants,

and fungi as separate eukaryotic

kingdoms, with all other eukaryotes

placed in the protist kingdom,

and bacteria constituting a

fifth kingdom. Whittaker’s protist

kingdom covered eukaryotic

organisms such as amoebas that

did not fit the other categories.

Some protists were closer to

animals, some closer to plants,

and others not close to either.

They did not match the tree of life

model, in which clades—groups

of organisms with a common

ancestry—spring as branches

from the previous fork.

Woese sought a classification

system that reflected the intricacies

of evolution—with main branches

on the tree of life splitting into

smaller ones, and even tinier twigs

that end in the leaves of individual

species. In the future, the complex

tree of life may reveal even more

evolutionary categories. ■

Carl Woese’s three-domain tree

Kingdom of their own

For most of the history of

biology, fungi were considered

to be plants. Even the great

classifier of organisms Carl

Linnaeus included them in his

kingdom Plantae. It was only

with the invention of more

powerful microscopes that

the differences in fungi began

to be better understood. It is

now known that chitin, a

complex carbohydrate and

component of fungus cell

walls, is not found in plants.

Also, fungi make their food

by digesting rotted material,

whereas plants make food by

absorbing light energy in

photosynthesis.

DNA analysis shows that

fungi are far removed from

plants in the evolutionary

tree of life: they are, in fact,

genetically closer to the

branch that gives rise to

animals. These same studies

show that certain aquatic

molds—traditionally classified

as fungi—are not related to

fungi, while some disease-

causing microbes are fungi

that have evolved to become

microscopic parasites.

Fungi, such as this bright

yellow jelly fungus growing on a

fallen tree, are no longer classified

as plants. Fungi are genetically

closer to animals.

Bacteria

Cyanobacteria

Bacteroides

Purple bacteria

Archaea

Thermoproteus

Methanococcales

Extreme

halophiles

Eukaryotes

Animals

Plants

Fungi

Protists

According to Carl Woese, all organisms can be

separated into three main categories or “domains.”

These divisions are based on similarities in the

ribosome structure found in the cells of the groups

of organisms within each domain.

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