Why do mammalian red blood cells lack a nucleus?
Red blood cells are initially produced in the bone marrow with a nucleus. They then undergo a process known as enucleation in which their nucleus is removed. Enucleation occurs roughly when the cell has reached maturity. According to one research (Ji, et al., 2008), the way this occurs in mice is that a ring of actin filaments surrounds the cell, and then contracts. This cuts off a segment of the cell containing the nucleus, which is then swallowed by a macrophage. Enucleation in humans most likely follows a very similar mechanism.

The absence of a nucleus is an adaptation of the red blood cell for its role. It allows the red blood cell to contain more hemoglobin and, therefore, carry more oxygen molecules. It also allows the cell to have its distinctive bi-concave shape which aids diffusion. This shape would not be possible if the cell had a nucleus in the way. Because of the advantages it gives, it is easy to see why evolution would cause this to occur. However, since little is known about the genes the control enucleation, it is still not a fully understood process.