If you have ever wondered which kingdom of life is most closely related to animals, you might be surprised by the answer. It is not plants, as you might expect, nor is it fungi, as some people might think. In fact, the closest relatives of animals are a group of microscopic organisms that belong to the kingdom Protista.
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What are Protists?
Protists are a diverse group of eukaryotic organisms that do not fit into any other kingdom. They are usually unicellular, but some are multicellular or colonial. They can be found in almost any environment, from freshwater to marine, from soil to snow. They can be autotrophic, heterotrophic, or mixotrophic, meaning they can make their own food, consume other organisms, or do both.
Protists are classified into several major groups based on their characteristics and evolutionary relationships. Some of the most well-known protist groups are algae, protozoa, slime molds, and water molds. However, there are many more groups that are less familiar to most people.
How are Protists Related to Animals?
Protists and animals share a common ancestor that lived about 1.5 billion years ago. This ancestor was a single-celled organism that had a nucleus and other membrane-bound organelles. It also had a flexible cell membrane that allowed it to change shape and move around. This ancestor gave rise to two major lineages: the opisthokonts and the bikonts.
The opisthokonts include animals, fungi, and some protists, such as choanoflagellates and nucleariids. The bikonts include plants, algae, and some protists, such as ciliates and dinoflagellates. The opisthokonts and the bikonts are separated by a major evolutionary event: the loss of the second flagellum in the opisthokonts.
A flagellum is a whip-like structure that helps some cells move and sense their environment. Most protists have one or two flagella, but opisthokonts have only one or none. This loss of the second flagellum may have been an adaptation to living in low-oxygen environments or to becoming multicellular.
Which Protists are the Closest Relatives of Animals?
Among the opisthokonts, the closest relatives of animals are the choanoflagellates and the nucleariids. Choanoflagellates are single-celled or colonial protists that have a single flagellum surrounded by a collar of microvilli. They feed on bacteria and other small particles by creating a water current with their flagellum and trapping them with their collar. Choanoflagellates are very similar to the feeding cells of sponges, which are the simplest animals.
Nucleariids are single-celled protists that have no flagella but have pseudopodia, which are extensions of their cytoplasm that help them move and feed. They feed on bacteria and fungi by engulfing them with their pseudopodia. Nucleariids are very similar to the cells of fungi, which are also opisthokonts.
According to molecular evidence, choanoflagellates are more closely related to animals than nucleariids are. This means that animals evolved from a choanoflagellate-like ancestor that became multicellular and developed specialized tissues and organs.
Why is this Relationship Important?
Understanding the relationship between protists and animals can help us learn more about the origin and evolution of animal life. By comparing the features and genes of protists and animals, we can infer what their common ancestor was like and how it diversified into different lineages.
For example, by studying choanoflagellates, we can learn how multicellularity arose in animals and how cell signaling and differentiation occurred. By studying nucleariids, we can learn how fungi diverged from animals and how they adapted to different lifestyles.
Moreover, understanding the relationship between protists and animals can help us appreciate the diversity and complexity of life on Earth. Protists are not just simple or primitive organisms; they are highly diverse and sophisticated in their own ways. They play important roles in various ecosystems and biogeochemical cycles. They also interact with other organisms in beneficial or harmful ways.
For instance, some protists form symbiotic relationships with animals, such as corals and zooxanthellae or termites and trichonymphs. Some protists cause diseases in animals, such as malaria or sleeping sickness. Some protists even parasitize other protists, such as plasmodium or toxoplasma.
Conclusion
In conclusion, protists are a diverse group of eukaryotic organisms that do not fit into any other kingdom. They are the closest relatives of animals, and they share a common ancestor that lived about 1.5 billion years ago. Among the protists, the choanoflagellates are the closest relatives of animals, and they are very similar to the feeding cells of sponges. By studying protists and animals, we can learn more about the origin and evolution of animal life and the diversity and complexity of life on Earth.
