Such events leave lots of evidence -- in the fossil record and in the genetic record. No one saw O.J. whack his wife, but the evidence most certainly points in that direction.
Especially the EDTA (laboratory preservative) in the blood stains.
Evidence is wonderful, but when people take evidence and present it as fact we are all in trouble. There is barely a single "scientifically determined" date pertaining to the age of the earth, and the origin of mankind that I learned as a young man that stands today. For that matter, today's "scientific fact" is almost always tomorrow's outdated theory. The examples abound. When people are dealing with issues like apes, evolution and the origin of man, and they present their unproven theories in a fashion that makes them appear as facts, it's not the bad science that offends, it's the same old damn lies that offend peoples' sensibilities.