Final answer:
The MRCA of all protists probably had a cytoskeletal system for cilia or flagella, was aerobic with mitochondria from an endosymbiotic relationship, and may have had a cell wall during part of its life cycle, though confirmation is needed.
Step-by-step explanation:
The MRCA (most recent common ancestor) of all protists likely possessed several key characteristics crucial to eukaryotic life. Firstly, this ancestor would have had a cytoskeletal system capable of supporting structures like cilia or flagella, enabling mobility. As protists encompass a broad and diverse group of organisms, including those part of the six eukaryotic supergroups, the early eukaryotes were probably unicellular organisms. Additionally, evidence suggests that the MRCA had mitochondria, pointing toward an aerobic lifestyle. These mitochondria are theorized to be derived from an aerobic alpha-proteobacterium that lived symbiotically within the host cell, a defining eukaryotic feature. Furthermore, the possibility of a cell wall present at least during part of the life cycle is considered, but confirmation requires further evidence. Importantly, the exact nature of this ancestor and whether it had a fully formed nucleus at the time of endosymbiotic events is still a matter of scientific investigation.
Understanding the evolution of protists and their relationships within the domain Eukarya helps us comprehend the progression from unicellularity to multicellularity, with the early eukaryotic ancestors estimated to have emerged around 1.5 billion years ago. Genetic analysis and various endosymbiotic events have shaped the complexity of modern eukaryotes, resulting in the diverse life forms we observe today in the protist kingdom.