You need to be more precise in your phrasing. I do have a biology education and as you have stated it you're wrong. Biologist know perfectly well when life begins. Had you said when "a human life begins" your argument would be more correct as what defines a "human life" is more ambigous than the definition of life iteself.
The biological definition of life is: A distinctive characteristic of a living organism from dead organism or non-living thing, as specifically distinguished by the capacity to grow, metabolize, respond (to stimuli), adapt, and reproduce.
A fertilized ovum, zygote, blastocyst, which ever term you prefer, meets all these conditions of the definition of life. Thus biologist do know when life begins.
Now when does "a" human life or sentient life begin? That's more ambiqious question and you would have a point there but more of a philosophical or ethical point than a scientific one.
http://www.biology-online.org/dictionary/Life