Parahuman

A parahuman or para-human is a human-animal hybrid or chimera. Scientists have done extensive research into the mixing of genes or cells from different species, e.g. adding human (and other animal) genes to bacteria and farm animals to mass-produce insulin and spider silk proteins, and introducing human cells into mouse embryos. Despite most hybrids and chimeras either being sterile or having the genitals of one their parent species, the idea of fertile parahumans being produced and starting a new species is a quite interesting topic.

Reasons...?
There are many reasons why parahumans could be produced. Some of these reasons include for medical purposes (ex: Designing a mouse with a human immune system to study AIDS) and a much more controversial reason for serving classes (ex: soldiers, servants, playmates).

If a line of parahumans could be created using germline engineering, if they also bred true, and if they were different enough from ordinary humans to be unable to breed with us, then they would qualify as a species. Parahumans created using only somatic genetic engineering would have human children.

Another key difference is that a germ-line parahuman would have to be modified before birth, while a somatic parahuman could be an adult human who chooses to be modified. Which one is more ethical is a matter of debate. An argument for the former is that no harm is done to a person born with modified genes because the person would have had no control over their genes in the first place. An argument for the latter being more ethical is that the changes would be made with informed consent.