Protistan life began 2.5 billion years ago, when the accumulation of oxygen in the atmosphere
created competition for resources. Living cells that could best use the oxygen for energy
production gained the upper hand. As a result, the prokaryotic cells that survived developed
more complex parasitic and predation relationships, eventually leading to the evolution of
eukaryotic cells. Until the introduction of eukaryotes, the protista were simply a part of the
kingdom of the bacteria.
The evolution of simple eukaryotic cells lead to the formation of the kingdom of protista. The
first step in the evolutionary process was membrane infolding, which occurred in a select
number of prokaryotes, and which was responsible for the creation of a nuclear envelope, as
well as several organelles inherent to the endomembrane system. The second was the
occurrence of endosymbiosis, which introduced the mitochondria and chloroplast as
organelles of the endomembrane system. These organelles began as parasitic prokaryotes
within the eukaryotic host, but eventually developed into organelles especially key in energy
production. Membrane infolding, as well as the mutual relationship of endosymbiosis, created
a successful kingdom of protista. Later, plants, animals, and fungi would evolve into more
complex forms of the protista.
Protista are structurally the simplest eukaryotes in existence. As a result, the protista that
affect us humans are parasitic. Most protistans are unicellular; some are colonial, and others
are multicelluar organisms that closely relate to the single protist cells. Protista are either
photosynthetic, as in the algae, or they feed off of bacteria, smaller protista, or organic
material suspended in water. Because they are the simplest eukaryotes, protista have very
little locomotion capabilities. As a result, the predator protista have evolved to exist mainly in
aquatic habitats where they can use the water to move. Water is useful to the algae, as well.
Algae have evolved to reside in wet habitats that still are exposed to sunlight.
Protists contain all of the ancestor cells of today’s eukaryotic kingdoms. Because of their
great diversity, earlier biologists placed no real classification for protests under the original
two-kingdom classification system. Biologists eventually included protista as a separate entity
into the five-kingdom system. Even today, there is debate of splitting the protista into several
new kingdoms, as well.
Here are some of the more important protista groups:
Rhodophytes: These are the red algae, which contain pigments which allow them to capture
blue light very efficiently. As a result, the rhodophytes have evolved deeper in the ocean,
where blue light can penetrate deeper.
Red algae
Chromobionts: these are the kelp, but are also considered the simple, or ‘lower fungi’. They
are important for protection and ecological habitats for fish, but they also use oils in foods
they produce to stay afloat, so they have evolved to create a mutually beneficial situation for
the creatures that inhabit their environment. A similar group, the chlorobionts, are the green
algae and are believed to be the ancestors of modern plants. Similar to plants, the
chromobionts produce starch and their cell walls contain cellulose.
Ciliates: Considered the most advanced and evolved protista because they can reproduce
both sexually and asexually, many ciliates are parasitic. In sexual reproduction, they have
evolved to swap their micronuclei with a partner to increase complexity. A common ciliate
protista is the paramecium; ciliates are also responsible for the disease dysentery in humans.
Animal-like Flagellates: These are responsible for many diseases which we host. Like most
protista, they thrive in wet habitats, such as the moist tissues of plants and animals, making
humans an attractive place in which to reproduce. The flagella in these protista evolved to
expedite travel throughout the wet environments. The flagella parasites can be transferred
through their habitats. These habitats include water and blood. Protists that cause diarrhea,
African sleeping sickness, and trichomonads like the STD trichomonas vaginalis are
examples of animal-like flagellates that exist in these environments.
Protistans have many evolutionary links to the later kingdoms:
Link to Animals: The protozoans (AKA first animals)
(ex. Amoeboid, ciliated, and animal-like flagellates)
Many of these use aerobic respiration, mitotic and meiotic division
Link to Plants: photosynthetic types: algae, multicelled organisms
(ex. Rhodophytes, Chromobionts )
Link to Fungi: the slime molds form spore-bearing structures and the fact that fungi are
heterotrophs
(ex. Arasiomycota)
Evolution of Parasites:
There are a few implications to the survival and evolution of the final two groups of protista. A
pathogens prospect for survival depends on not only its ability to extract resources from its
host, but also its ability to regulate the quantity that it extracts from the host. In other words,
its not only important that these parasites such as ciliates and animal-like flagella produce
descendants for the continuation of their species, but that also means they need to maintain
the survival of their host. As a result, the protistan parasites that extract more mild and
moderately from their hosts have survived.