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This creates a cell with two sets of chromosomes (figure B).

The chromosomes then swap some genes to make new chromosomes which have the genes of both parent cells mixed together (figure C).

In early cells the two pairs of chromosomes then split apart to make two cells (figure D).

Evolution then selected the best of these cells to go on to make the next generation. So sex speeded up evolution.

Diploid
The most advanced eukaryotes kept two pairs of chromosomes through most of their lives, like figure C. A cell like this is called diploid. This let evolution speed up even more.