Insects      Reptiles      Birds          Monotremes         Marsupials       Mammals        Humans

  XY      XX   | XY   XX  | XY     XX  |     XY      XX        |   XY       XX    |      XY         XX   |XY       XX|

                                            Evolution

The Y chromosome has steadily shed its genes throughout the entire course of evolution

beginning with about 900 genes and retaining only 55 to 27 in humans.

This internal process drives the transformation of life over time:

from instinctual behaviour toward emotional attachment, the foundation of intelligence.