Michael Egnor v. P.Z. Myers on abortion

The Discovery Institute's Michael Egnor and blogger P.Z. Myers are in the midst(?-not sure if Myers will respond again) of an argument regarding abortion. The back and forth started with a post by Myers (who is also a biology professor) in which he described himself as pro-abortion, called an unborn child at 17 weeks "a mere embryo," claimed the unborn wasn't a human life, and noted that he voted for the couple behind Birthornot.com to abort.

Egnor commented on Myers' statements. Myers responded and essentially argued that treating the unborn like human beings devalues other human beings. Interestingly, although he's a biology professor, Myers didn't provide a single scientific argument for why the unborn aren't human beings. He provided a photo of an egg being fertilized and what I believe is a human embryo alongside a photo of a group of young women. He concluded,

Maybe when Egnor graduates to something beyond the 101 level, he'll learn that human cells are not equivalent to a full human life. An "unborn child" (what a silly euphemism!) is not suddenly a person at conception: development is a gradual process of epigenesis, in which information and complexity expand over time, and the person does not form in an instant. There is no black-and-white boundary between non-personhood and personhood — it's an arbitrary line drawn in a continuum.

Note how Myers can't seem to separate the biological question (is the unborn a human being?) from the philosophical question (is the unborn a person?).

Egnor posted a response in which he attempts to separate the above questions and see what criteria Myers believe instills human beings with the right to life.

Myers asserts that personhood is a continuum, and that some human beings on that continuum are not persons with the right to life, and some are. What is the inflection point? What minimal characteristic(s) of a human being entitle that human being to a right to life? Please note that I am not asking about age (28 weeks of gestation, full term infant, etc), but rather about the characteristics of a human being at a particular age that represent the threshold for the right to life.

To reiterate, here's my view:

The right to life depends only on being human.

Myers disagrees.

So here's my question for Myers:

What characteristic(s) must a human being have (awareness, rationality, independent existence, etc) for that human being to have a right to life?