Complex sentence: Lawrence Keeley writes in "War Before Civilization" that examples exist with low percentage rates of casualties in tribal battle, and that some tribal battles were much more lethal as a percentage of population than, for example, the Battle of Gettysburg.
Split into simpler sentences: Lawrence Keeley writes in "War Before Civilization" that examples exist with low percentage rates of casualties in tribal battle. He also points out that some tribal battles were much more lethal as a "percentage" of population than, for example, the Battle of Gettysburg.