Protists are a diverse group of eukaryotic organisms that are not classified as animals, plants, or fungi. They include microscopic forms such as algae, amoebae, and ciliates, as well as macroscopic forms such as kelp, slime molds, and water molds. Protists are found in almost every habitat on Earth, and play important roles in the ecology and evolution of life.
But which group of protists is most closely related to animals? This question has puzzled scientists for a long time, as protists exhibit a wide range of characteristics and lifestyles that do not fit neatly into the traditional categories of plants and animals. However, with the advent of molecular phylogenetics, which uses DNA sequences to reconstruct the evolutionary history of organisms, the answer has become clearer.
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The Opisthokonts: The Protist Ancestors of Animals and Fungi
According to molecular phylogenetics, the protist group that is most closely related to animals is the opisthokonts. This group includes several clades of protists that share a common feature: they have a single flagellum at the posterior end of their cells. The name opisthokont means “rear pole” in Greek, referring to this distinctive trait.
The opisthokonts are divided into two major branches: the holozoans and the holomycetes. The holozoans include the animals and their protist relatives, such as choanoflagellates, filastereans, ichthyosporeans, and nucleariids. The holomycetes include the fungi and their protist relatives, such as microsporidians, rozellids, and cryptomycetes.
The Choanoflagellates: The Protist Sisters of Animals
Among the holozoans, the protist group that is most closely related to animals is the choanoflagellates. These are unicellular or colonial protists that have a single flagellum surrounded by a collar of microvilli. They use their flagellum to create a water current that brings food particles into their collar, where they are ingested by phagocytosis. Choanoflagellates are mostly marine or freshwater organisms that live in various habitats, such as plankton, sediments, or biofilms.
Choanoflagellates are considered the sister group of animals, meaning that they share a common ancestor with animals that is not shared by any other living group. This common ancestor is estimated to have lived about 800 million years ago, during the Neoproterozoic era. Choanoflagellates and animals have many similarities in their cell structure and gene expression, suggesting that they inherited these features from their common ancestor.
One of the most striking similarities between choanoflagellates and animals is the presence of cadherins and tyrosine kinases in their cells. These are proteins that are involved in cell adhesion and signaling, and are essential for the development and function of multicellular tissues in animals. Choanoflagellates use these proteins to form colonies or rosettes of cells that resemble the simplest forms of animal tissues.
The Origin of Multicellularity: From Protists to Animals
The evolution of multicellularity is one of the major transitions in the history of life. Multicellular organisms have many advantages over unicellular ones, such as increased size, complexity, differentiation, and specialization. However, multicellularity also poses many challenges, such as coordination, communication, cooperation, and conflict among cells.
How did multicellularity arise from unicellular ancestors? This question is still debated by scientists, but one possible scenario is that it involved several steps:
- First, unicellular protists formed transient or permanent aggregations or colonies of cells that were held together by physical or chemical interactions.
- Second, these aggregations or colonies developed mechanisms to regulate cell division, differentiation, and death within the group.
- Third, these mechanisms led to the emergence of distinct cell types with different functions and behaviors within the group.
- Fourth, these cell types became interdependent and coordinated by signaling molecules and pathways.
- Fifth, these signaling molecules and pathways became more complex and sophisticated over time, leading to the formation of tissues and organs.
Choanoflagellates may represent an intermediate stage in this scenario, as they can form colonies or rosettes of cells that have some degree of coordination and differentiation. However, they do not have true tissues or organs like animals do. Therefore, choanoflagellates may provide clues about how multicellularity evolved in animals from their protist ancestors.
Conclusion
Protists are a diverse group of eukaryotic organisms that are not classified as animals, plants, or fungi. They include microscopic forms such as algae, amoebae, and ciliates, as well as macroscopic forms such as kelp, slime molds, and water molds. Protists are found in almost every habitat on Earth, and play important roles in the ecology and evolution of life.
The protist group that is most closely related to animals is the opisthokonts, which have a single flagellum at the posterior end of their cells. The opisthokonts are divided into two major branches: the holozoans and the holomycetes. The holozoans include the animals and their protist relatives, such as choanoflagellates, filastereans, ichthyosporeans, and nucleariids. The holomycetes include the fungi and their protist relatives, such as microsporidians, rozellids, and cryptomycetes.
The protist group that is most closely related to animals within the holozoans is the choanoflagellates. These are unicellular or colonial protists that have a single flagellum surrounded by a collar of microvilli. They use their flagellum to create a water current that brings food particles into their collar, where they are ingested by phagocytosis. Choanoflagellates are considered the sister group of animals, meaning that they share a common ancestor with animals that is not shared by any other living group.
Choanoflagellates and animals have many similarities in their cell structure and gene expression, suggesting that they inherited these features from their common ancestor. Choanoflagellates may also provide clues about how multicellularity evolved in animals from their protist ancestors, as they can form colonies or rosettes of cells that have some degree of coordination and differentiation.
Therefore, by studying protists, we can learn more about the origin and diversity of life on Earth, especially the evolution of animals from their protist ancestors.
