Many mammals have evolved eyes that can see in the dark. That involves more than just becoming bigger. Their eyes have more light-sensitive rod cells, and these cells have changed at a microscopic level. They have converted the nucleus at the middle of each cell into a light-collecting lens.
In almost all complex cells, DNA is tightly packed around the edge of the nucleus but lightly packed towards its middle. But in the rod cells of nocturnal mammals, it’s the other way round. This inverted arrangement collects light that hits the rod cells and funnels it through to the retina underneath. By moving its DNA around, each cell has become a little optic fibre.
Read more: Nocturnal mammals see in dark by turning displaced DNA into lenses