T. Cavalier-Smith proposes that eukaryotes and archaebacteria are sisters, sharing a common ancestor that evolved from a Gram-positive eubacterium. This theory challenges previous ideas that eukaryotes originated from a fusion of an archaebacterium and an α-proteobacterium. Instead, Cavalier-Smith argues that eukaryotic cell properties, such as phagotrophy, endomembrane system, cytoskeleton, nucleus, and mitosis, evolved independently and synergistically with the symbiogenetic origin of mitochondria from an α-proteobacterium. The common ancestor of eukaryotes and archaebacteria, a neomuran, underwent significant changes, including the replacement of peptidoglycan with N-linked glycoproteins. This transformation enabled the evolution of phagotrophy, which was crucial for the development of the endomembrane system, cytoskeleton, nuclear organization, and sexual life cycles. Cilia evolved through autogenous specialization of the cytoskeleton, not symbiogenesis. The ancestral eukaryote was uniciliate with a single centriole, and the eukaryote tree is rooted at the divergence between opisthokonts and anterokonts. Anterokonts include the Amoebozoa and a vast biciliate clade called bikonts. The origin of chloroplasts and the symbiogenetic incorporation of a red alga into a corticoflagellate to create chromalveolates may have occurred after the Varangerian snowball Earth melted, stimulating the Cambrian explosion. The neomuran theory is supported by genetic and fossil evidence, and the origin of eukaryotes is estimated to have occurred around 850 million years ago. The paper also discusses the phylogenetic classification of protozoa, the evolution of eukaryotic cells, and the role of co-evolution in the origin of eukaryotes. Key points include the shared neomuran characters, the origin of mitochondria and chloroplasts, and the importance of phagotrophy in eukaryotic evolution. The paper emphasizes the need for a balanced approach to molecular and ultrastructural evidence in phylogenetic studies.T. Cavalier-Smith proposes that eukaryotes and archaebacteria are sisters, sharing a common ancestor that evolved from a Gram-positive eubacterium. This theory challenges previous ideas that eukaryotes originated from a fusion of an archaebacterium and an α-proteobacterium. Instead, Cavalier-Smith argues that eukaryotic cell properties, such as phagotrophy, endomembrane system, cytoskeleton, nucleus, and mitosis, evolved independently and synergistically with the symbiogenetic origin of mitochondria from an α-proteobacterium. The common ancestor of eukaryotes and archaebacteria, a neomuran, underwent significant changes, including the replacement of peptidoglycan with N-linked glycoproteins. This transformation enabled the evolution of phagotrophy, which was crucial for the development of the endomembrane system, cytoskeleton, nuclear organization, and sexual life cycles. Cilia evolved through autogenous specialization of the cytoskeleton, not symbiogenesis. The ancestral eukaryote was uniciliate with a single centriole, and the eukaryote tree is rooted at the divergence between opisthokonts and anterokonts. Anterokonts include the Amoebozoa and a vast biciliate clade called bikonts. The origin of chloroplasts and the symbiogenetic incorporation of a red alga into a corticoflagellate to create chromalveolates may have occurred after the Varangerian snowball Earth melted, stimulating the Cambrian explosion. The neomuran theory is supported by genetic and fossil evidence, and the origin of eukaryotes is estimated to have occurred around 850 million years ago. The paper also discusses the phylogenetic classification of protozoa, the evolution of eukaryotic cells, and the role of co-evolution in the origin of eukaryotes. Key points include the shared neomuran characters, the origin of mitochondria and chloroplasts, and the importance of phagotrophy in eukaryotic evolution. The paper emphasizes the need for a balanced approach to molecular and ultrastructural evidence in phylogenetic studies.