The ancestor of eukaryotes is believed to be a prokaryotic cell that was engulfed by another prokaryote, likely an archaeon. This event, known as endosymbiosis, led to the formation of the mitochondrion, a key organelle found in all eukaryotic cells.

Here's a breakdown:

* Prokaryotes: Single-celled organisms lacking a nucleus and other membrane-bound organelles.

* Eukaryotes: Organisms with cells containing a nucleus and other membrane-bound organelles.

The prevailing theory, known as the endosymbiotic theory, suggests the following:

1. Ancestral archaeon: This archaeon engulfed a bacterium.

2. Endosymbiosis: Instead of digesting the bacterium, the archaeon kept it alive, allowing it to perform a beneficial function within the cell.

3. Evolution of the mitochondrion: Over time, the engulfed bacterium evolved into the mitochondrion, responsible for cellular respiration and energy production.

While the exact identity of the ancestral prokaryotes is still debated, this theory is widely accepted and explains the origin of eukaryotes. The discovery of other organelles, such as chloroplasts in plant cells, further supports the idea of endosymbiosis playing a significant role in eukaryotic evolution.

In summary, the ancestor of eukaryotes was a prokaryotic cell that underwent endosymbiosis with another prokaryote, leading to the development of key organelles like the mitochondrion. This evolutionary event ultimately paved the way for the diverse array of eukaryotic life forms we see today.