A cell must have an organelle to convert sugars to ATP before it develops an organelle to convert sunlight to sugars.

The mitochondria have been carbon dated to be older than chloroplasts.

Mitochondria are smaller than chloroplasts, so the engulfment of the prokaryotic ancestor of the mitrochondria would have occured more easily.

All eukaryotic cells contain mitochondria, whereas only plants and algae contain chloroplasts.