Lesson Overview
Protist Classification—The Saga Continues
Lesson Overview
21.1 Protist Classification
—The Saga Continues
Lesson Overview
Protist Classification—The Saga Continues
THINK ABOUT IT
Some of the organisms we call “protists” live quietly on the bottom of
shallow ponds, soaking up the energy of sunlight. Others swim
vigorously in search of tiny prey. Some, such as diatoms, sparkle in
coastal waters. Still others drift in the human bloodstream, destroying
blood cells and killing nearly a million people a year.
What kind of life is this, capable of such beauty and such destruction?
Lesson Overview
Protist Classification—The Saga Continues
The First Eukaryotes
What are protists?
Lesson Overview
Protist Classification—The Saga Continues
The First Eukaryotes
What are protists?
Protists are eukaryotes that are not members of the plant, animal, or fungi
kingdoms.
Lesson Overview
Protist Classification—The Saga Continues
The First Eukaryotes
More than a billion years ago, the first eukaryotes appeared on
Earth.
Single-celled eukaryotes are still with us today and are often called
“protists”—a name that means “first.” Traditionally, protists are
classified as members of the kingdom Protista.
Protists are eukaryotes that are not members of the plant, animal, or
fungi kingdoms.
Lesson Overview
Protist Classification—The Saga Continues
The First Eukaryotes
Although most protists are unicellular, quite a few are not. Brown algae
called kelp are the largest protists. They contain millions of cells arranged
in differentiated tissues.
Kelp are considered protists because they are related more closely to
certain unicellular protists than to members of any other kingdom.
Otters wrap themselves in giant kelp to keep from drifting out to sea while
they sleep.
Lesson Overview
Protist Classification—The Saga Continues
The “Protist” Dilemma
Biologists have discovered that “protists” display a far greater degree of
diversity than any other eukaryotic kingdom.
Euglena, brown algae, diatoms, and slime molds are examples of
protists.
Lesson Overview
Protist Classification—The Saga Continues
The “Protist” Dilemma
In addition to their diversity, biologists also found that many “protists”
are far more closely related to members of other eukaryotic kingdoms
than they are to other “protists.”
By definition, the members of a living kingdom should be more like one
another than like members of other kingdoms. This is not true of
protists, which means that reclassification is necessary.
In the past, scientists sorted protists into three groups: plantlike protists,
animal-like protists, and funguslike protists. However, this solution
began to fail as biologists learned that many protists do not fit into any
of these groups.
Biologists also discovered that many of the animal-like and funguslike
protists are so similar that they belong in a single group, not two.
Lesson Overview
Protist Classification—The Saga Continues
Multiple Kingdoms?
The most recent studies of protists divide them into six major clades,
each of which could be considered a kingdom.
Lesson Overview
Protist Classification—The Saga Continues
Multiple Kingdoms?
This cladogram represents an understanding of protist relationships
supported by current research.
Lesson Overview
Protist Classification—The Saga Continues
Multiple Kingdoms?
Surprisingly, the plant, animal, and fungi kingdoms fit right into these six
clades. Animals and fungi actually emerge from the same protist
ancestors.
Protists were the first eukaryotes, and evolution has had far more time
to develop differences among protists than among more recently
evolved eukaryotes like plants and animals.
By finding the fundamental divisions among protists, we also identify the
most basic differences among all eukaryotes.
Lesson Overview
Protist Classification—The Saga Continues
What “Protist” Means Today
Biologists assembling the Tree of Life favor the classification shown in
the cladogram.
Lesson Overview
Protist Classification—The Saga Continues
What “Protist” Means Today
Even though the biologist building the Tree of Life prefer a different
classification, the word “protist” remains in common usage, even among
scientists.
Bear in mind that “protists” are not a single kingdom but a collection of
organisms that includes several distinct clades.
Lesson Overview
Protist Classification—The Saga Continues
Protists—Ancestors and Descendants
How are protists related to other eukaryotes?
Lesson Overview
Protist Classification—The Saga Continues
Protists—Ancestors and Descendants
How are protists related to other eukaryotes?
Today’s protists include groups whose ancestors were among the very last
to split from the organisms that gave rise to plants, animals, and fungi.
Lesson Overview
Protist Classification—The Saga Continues
Protists—Ancestors and Descendants
Microscopic fossils of eukaryotic cells, like Tappania plana shown,
have been found in rocks as old as 1.5 billion years.
Genetic and fossil evidence indicates that eukaryotes evolved from
prokaryotes and are more closely related to present-day Archaea
than to Bacteria.
The split between Archaea and Eukarya may have come as early
as 2.5 billion years ago. Since that time, protists have diversified
into as many as 300,000 species.
Lesson Overview
Protist Classification—The Saga Continues
Protists—Ancestors and Descendants
Most of the major protist groups have remained unicellular, but two have
produced multicellular organisms. Plants, animals, and fungi arose from
the ancestors of these multicellular groups.
Lesson Overview
Protist Classification—The Saga Continues
Protists—Ancestors and Descendants
The roots of all eukaryotic diversity, from plants to animals, are found
among the ancestors of protists.