The Life That Spawned A Quarter-Million Descendant Species
Other parasitic protists are responsible for other diseases, such as African sleeping sickness, toxoplasmosis, and amoebic dysentery (which infects billions of people every year).
"In fact, every human alive has probably been sick or annoyed at one time or another because of some protist in their system," says Jere Lipps, professor of Integrative Biology at the University of California in Berkeley.
Yet not all protists are parasites, and many do more good than harm. Without protists, our planet would have much less oxygen today, and there would've been less oxygen in the past. Although cyanobacteria were responsible for the initial rise of oxygen on Earth, the appearance and evolution of photosynthetic protists in the form of algae dramatically increased the amount of oxygen in the atmosphere.
Protists are important in other ways, too. The balance of carbon dioxide in the atmosphere is controlled, in part, by the activities of protists. Decomposing protist species like the ciliates help recycle nutrients. Photosynthetic protists make up the base of the food chain in the ocean. And without protists, animals, plants, and many other life forms may never have developed.
So just what are protists? Most are single-celled eukaryotes. Because they are composed of only one cell they tend to be microscopic, although some can grow up to 10 centimeters, others live in large cooperative colonies, and a few are multi-celled (kelp, for instance, is a multi-cellular protist algae). Protists are found in nearly every wet environment, including marine and fresh water bodies, damp terrestrial habitats such as leaf litter on a forest floor, and in the moist insides of larger organisms.
But the macroscopic life forms make up only a small part of the eukaryotic tree of life. While animals, plants, and fungi constitute just three small twigs, the rest of the tree positively bristles with protist branches.
The reason so many life forms are lumped into one category called "protist" is a hangover from earlier forms of classification. For a long time, we were only aware of life forms that were visible to the naked eye. But then in 1673, Antoni van Leeuwenhoek used a microscope of his own design to peer into the microscopic world. Leeuwenhoek made the first descriptions of protists and bacteria, which he called "animalicules."
Today there are about 200,000 named species of protists, although the are likely millions that remain undescribed, and over the course of history there have been many millions of protist species that have gone extinct.
Arranging the vast numbers of different protists into descriptive categories has proved to be a headache. At first, protists were broadly divided into the protozoa (animal-like creatures that ingest food), the algae (plant-like organisms that create food through photosynthesis), and fungus-like protists that absorb their food. But there was a lot of crossover between these groups, so the categories were further broken down into the flagellates, ciliates, amoebae, algae, and parasitic protists. Later discoveries led to even more categories. Yet genetic studies now show that these categories don't reflect the unique evolutionary origins of the different protist types. According to Malcolm Walter, director for the Australian Centre for Astrobiology, genome analysis has shown that many of the groups are not at all closely related to one another.
"We consider protists as a group more for our convenience than as a reflection of close kinship," says Walter. "A better name for this group of organisms would be 'eukaryotes that are neither animals, fungi, or plants.'"
Why do protists have so much diversity - more than animals, plants and fungi combined? One answer is time. There may have been as much as two or more billion years between the time of protist origin and the origin of multi-cellular eukaryotes, so protists had a long time to evolve different body forms.
Some scientists have suggested that environmental changes also could have triggered massive evolutionary radiations. For instance, the differentiation of protists may have coincided with the increasing rates of oxygen in the atmosphere. As oxygen levels rose, the eukaryotes that could adapt to atmospheric oxygen evolved to fill all the newly available ecologic opportunities. Such entries into new environments would have led to many evolutionary adaptations over time.
The Mystery of their History
Before eukaryotes evolved to form large, multi-cellular life forms, there were only the protists. The name "protist" means "the very first." It is thought that animals, plants, and fungi derived from different groups of protists: sponges and animals are believed to have originated from the choanoflagellates, plants from green algae, and fungi from water and slime molds.
Before the origin of eukaryotes, prokaryotes were the only kind of life around. The earliest eukaryotes probably evolved from prokaryotes, and thus were similar to prokaryotes in many ways. There are several hypotheses for the origin of the eukaryotic nucleus: one says that prokaryotes developed more and more folded membranes until an organism with a nuclear envelope arose. Another theory suggests that one prokaryotic cell engulfed another, but rather than be digested as food it became the cell's nucleus. (This theory, called endosymbiosis, is also thought to be the means by which mitochondria in animal cells and chloroplasts in plant cells first formed).
Archezoans are believed to have been the first protist species, and they can still be found today. The archezoan Giardia lamblia, for instance, causes the intestinal infection giardiasis in humans. Ribosomal RNA sequencing suggests that the archezoans are closely related to the prokaryotes. And like prokaryotes, archezoans lack mitochondria. Some believe this proves that archezoans diverged prior to mitochondria development. Recent evidence has shown, however, that some archezoans have genes for mitochondrial proteins, suggesting they may have lost their mitochondria.
"We don't really know what acritarchs represented, [but] they seem to be the fossilized cysts of some kind of protistan alga," says Lipps. "Some might even be prokaryotes."
Many of the protists living today have no fossil record, so scientists can't rely on fossils to provide an accurate reflection of all protist history. But according to Lipps, when there is a protist fossil record - as is the case with skeletonized protists - it is even more complete than the fossil records for plants and animals.
"When we get to the fossilized protists, those that have shells or other kinds of skeletons, their record is excellent," says Lipps. "However, each group, being evolutionarily independent of the others, has a different time of origin in the fossil record and a different history."
Because of the independent evolution of the different protist groups, the huge numbers of species that developed over time, and the lack of early fossils, there are many gaps in the protist fossil record. Scientists have not found many protist fossils that would help explain important divergence events.
"At about 1 billion years ago, the fossil record shows the appearance of the first known macroscopic algae," says Walter. "However, not much material is preserved between the origin of the protists and the appearance of macroscopic algae to suggest any divergence or sophistication. Likewise, there is little preserved between the appearance of macroscopic algae and the first appearance of animals at approximately 570 million years ago, and as yet no transitional organism between protists and animals has been found in the fossil record."
One way to overcome such gaps in the fossil record is to use a controversial technique known as "molecular clock dating." General rates of genetic change are used to estimate how long it took for differences to occur between two organisms that share a common ancestor. For instance, molecular clock dating of protists has placed the date of their emergence at 2.7 billion years ago. But Lipps is skeptical about dates obtained through this technique.
"Molecular data can't really be used with confidence to date protists," says Lipps. For one thing, he says, "the geologic evidence comes from geochemicals, not real fossils. Since no fossils are considered in the molecular studies, a huge part of their evolution would not show up."
The ultimate answer to the origin of protists is still out of our purview. While the current evidence suggests the earliest eukaryotes emerged sometime between 2.7 and 2.1 billion years ago, Lipps thinks it is possible that protists could have emerged even earlier. He admits, however, that the data does not yet support that theory.
"We don't have any answers for this yet," says Lipps, "so speculation is rampant, and driving some of us on."
The resilience and diversity of protists may make them ideal candidates for life on other planets. With as many types of protists that have evolved to cope with the environmental demands of Earth, how many kinds of protists could have developed on other kinds of habitable worlds? Lipps, for one, believes that protist-type organisms could very well inhabit other planetary systems.
Lipps has many protist-related projects underway, including investigations of the role of protists in marine food webs over the past 3.8 billion years, of the radiations and extinctions of particular protist groups at various times in the past, and of possible protistan habitats on Jupiter's moon, Europa.
Walter, meanwhile, is conducting studies of protist biomarkers. He hopes that the biomarker composition of the 2.1 billion-year old acritarch fossils may shed light on their biology.