Choanoflagellates are single-celled organisms that have captured the attention of evolutionary biologists due to their unique position in the tree of life. These microscopic eukaryotes are at the center of an intriguing scientific question regarding the origins of animals. Their study provides insights into how complex life forms may have emerged from simpler ancestors.
What Are Choanoflagellates?
Choanoflagellates are small, between 3 to 10 micrometers in diameter, and are found in various aquatic environments, including marine, brackish, and freshwater habitats. They can exist as free-swimming individual cells or form simple colonies. Each cell features an ovoid or spherical body with a single flagellum at one end, surrounded by a funnel-shaped “collar” made of 30 to 40 interconnected microvilli.
The flagellum generates water currents, propelling the organisms through water. This water flow traps bacteria and detritus against the microvilli, which are then engulfed for feeding. This filter-feeding mechanism plays a role in the carbon cycle by connecting different trophic levels within aquatic food webs.
Defining the Animal Kingdom
Animals, belonging to the Kingdom Animalia, are eukaryotic and multicellular organisms. Their cells contain membrane-bound organelles and are organized into specialized tissues, such as nerve and muscle tissue, which perform specific functions.
Animals are heterotrophic, meaning they obtain energy and nutrients by consuming other organisms. Unlike plants and fungi, animal cells lack rigid cell walls, allowing for greater flexibility and movement.
Shared Traits with Animals
Choanoflagellates bear morphological similarities to choanocytes, the feeding cells found in sponges. Both choanoflagellates and choanocytes possess a flagellum surrounded by a collar of microvilli, which they use to generate water currents and filter food particles. This structural resemblance was noted as early as 1841.
Beyond morphology, choanoflagellates share molecular and genetic similarities with animals. Analyses of various genetic sequences, including 18S rDNA and nuclear protein-coding genes, support a close relationship. Choanoflagellates synthesize homologues of genes involved in cell adhesion and communication in multicellular animals, suggesting these genetic components were present in their shared ancestor.
Their Place in the Tree of Life
Despite similarities in cell structure and genetic makeup, choanoflagellates are not classified as animals. The primary reason is that they are predominantly unicellular organisms, though some species can form simple colonies. Animals are multicellular organisms characterized by complex tissue structures, specialized organs, and distinct embryonic developmental stages, which choanoflagellates lack.
Choanoflagellates are recognized as the sister group to the animal kingdom, meaning they are the closest living unicellular relatives of animals. This position on the evolutionary tree suggests that animals evolved from a choanoflagellate-like ancestor. Studying choanoflagellates provides a model for understanding the evolutionary transition from single-celled organisms to the diverse multicellularity observed in the animal kingdom.