Endosymbiosis is the process that sparked the origin of eukaryotic cells on Earth.
It’s the idea that a prokaryote engulfed a bacteria capable of aerobic respiration.
Then, this turned into mitochondria and became part of a eukaryotic cell.
Because they both symbiotically gain from the encounter, they asexually multiply and evolve this way.
Evolution of eukaryotes from prokaryotes
Long ago, mitochondria were a living organism all by themselves. It lived freely doing aerobic respiration.
The idea of endosymbiosis starts with a mitochondrion latching onto a prokaryote. Next, the prokaryote engulfs it as part of its cytoplasm.
So the mitochondrion leverages the nutrient-rich surrounding of the prokaryote. And in return, the prokaryote receives energy from the mitochondria powerhouse during its permanent residence.
Eukarya have a nucleus
One of the key differences between eukaryotic cells is that they have organs that are sealed off from the rest of the environment.
They have a nucleus that houses chromosomes and DNA. Further to this, eukarya have organelle like mitochondria (through endosymbiosis) and chloroplasts all within its cell membrane.
Even though single-cell eukarya exist, eukaryotes are mostly multicellular. For example, the plant and animal kingdoms are mostly multicellular.