Antolin Garcia-Torres has pleaded not guilty.
Three jurors on the case have been excused for medical reasons - that meant eight alternates were needed. The prosecution went first, showing the jury of six women and six men the last known photo of then 15-year-old Sierra, taken the morning of her disappearance.
The prosecution outlined its case by telling the jury about DNA evidence it will present that it says links Garcia-Torres to Sierra's death - including a strand of her hair found on a rope that investigators found in the trunk of his car - and DNA on a pair of Sierra's dirt-stained jeans that very likely belongs to Garcia-Torres.
The defendant sat calmly in court, looking at times at the jury and other times at a TV monitor where evidence was displayed.
Sierra disappeared in 2012 while on her way to school in Morgan Hill. Her body has never been found.
If convicted, 25 year old Garcia-Torres could face the death penalty.