Where did you find evidence for how violence has changed over time?
For prehistoric times, the main evidence is from forensic archaeology: the proportion of skeletons that had bashed-in skulls, or arrowheads embedded in bones, together with archaeological evidence such as fortifications. For homicide over the last millennia or so, there are records in many parts of Europe that go back to the Middle Ages. And we know from documents of the era that crucifixions and all manner of gory executions took place in the ancient world.
For data about wars, there are many databases that estimate war deaths, and in recent eras, governments and social scientists have tracked just about every aspect of life, so we really can get a clear view of things like child abuse, spousal abuse, rape and so on.
How do you explain the decline in violence?
I don't think there is a single answer. One cause is government, that is, third-party dispute resolution: courts and police with a monopoly on the legitimate use of force. Everywhere you look for comparisons of life under anarchy and life under government, life under government is less violent. The evidence includes transitions such as the European homicide decline since the Middle Ages, which coincided with the expansion and consolidation of kingdoms; the transition from tribal anarchy to the first states. Watching the movie in reverse, in today's failed states violence goes through the roof.
http://www.newscientist.com/article/mg21228340.100-steven-pinker-humans-are-less-violent-than-ever.htmlA good read. Definitely check out the 20 worst things people have done interactive graph
http://www.newscientist.com/embedded/20worst