Cells with a nucleus

For roughly the first two billion years, life was small and simple: bacteria and archaea, cells with no nucleus. Sometime around two billion years ago, a much larger kind of cell appears. It keeps its DNA in a nucleus and has internal compartments. That is a eukaryote.

The leading explanation is that one cell took in another and did not digest it. The swallowed cell became the mitochondrion, the compartment that makes usable energy. Later, a similar capture in one lineage became the chloroplast, which is why plants can use sunlight. That partnership is why complex life has enough energy to grow large.

Plants, fungi, and animals are all eukaryotes. Without this cell, there are no bodies with tissues, no brains, and no hands. Bacteria are still here and still most of life's mass. They are not a failed draft. They are the other two domains, still running.

The date is an estimate from fossils and molecular clocks. Different papers put the origin of eukaryotes anywhere from about 1.5 to 2.5 billion years ago. Two billion is the middle of that range.