“The Tangled Tree” by David Quammen tells us what DNA sequencing is teaching about evolution.   Perhaps the most startling discovery to come out of this new field—the study of life’s diversity and relatedness at the molecular level—is horizontal gene transfer (HGT), or the movement of genes across species lines.  For instance, we now know that roughly eight percent of the human genome arrived sideways via viral infection.  It’s not just about Darwin anymore.