Final answer:
Eukaryotic cells evolved from a symbiotic relationship between prokaryotic cells, according to the endosymbiotic theory.
Step-by-step explanation:
The first eukaryotic cells are believed to have evolved from a symbiotic relationship between prokaryotic cells. According to the endosymbiotic theory, smaller prokaryotes were engulfed by larger prokaryotes, leading to a mutually beneficial relationship. The smaller cells, known as endosymbionts, provided the larger cells, known as hosts, with energy or organic molecules, while the hosts provided a safe environment and nutrients for the endosymbionts.