Final answer:
The anaerobic parasite crucial in the understanding of eukaryotic evolution is Giardia lamblia, a protist from the Excavata supergroup. It is unique for its mitosome organelles and significance as a human pathogen causing diarrhea.
Step-by-step explanation:
The single-celled anaerobic parasite that is an intermediate in the evolution of eukaryotic cells is called Giardia lamblia. This parasite is a member of the Excavata supergroup and specifically the subgroup Fornicata within this group. Giardia lamblia lacks conventional mitochondria but contains organelles known as mitosomes, which are thought to have evolved from mitochondria.
Giardia lamblia is a protist that inhabits anaerobic environments and uses glycolysis for energy generation. It possesses two nuclei and multiple flagella, and while it does not have functional mitochondria, it has mitosomes for iron and sulfur metabolism. Its unusual cellular structure and metabolism have made it a subject of interest in understanding eukaryotic evolution and the evolution of mitochondrial organelles. This organism is also notable for its role as a human parasite, causing diarrhea when water supplies are contaminated with its cysts. Understanding Giardia lamblia and its characteristics helps in piecing together the evolutionary history of eukaryotes and the transitions from prokaryotic to eukaryotic cellular structures.