In the years since the human genome has been mapped, it's been found that over 90% of our DNA is "junk", and not needed to make a human being. A lot of this junk is comprised of the remnants of ancient retroviruses, which at some point in our evolutionary history were infectious, but over the course of hundreds of thousands of years became incorporated into our DNA as what had been thought to be harmless pieces of deactivated genetic material.
This is, at best, debatable. Especially, having seen scientists how they tend to flip-flop with different conclusions now and then.
For example, the following is from http://en.wikipedia.org/wiki/Noncoding_DNA
and has been widely reported in science articles to imply that what we call as junk DNA's are those which we have not completely understood.
Some specific sequences of noncoding DNA may be features essential to chromosome structure, centromere function and homolog recognition in meiosis.[19
Another example is the role of DNA and RNA. For decades, it was understood that RNA is mainly an enabler and DNA having most of the intelligence. Some time later, there was an article (I will post if I find it) that turned this whole thing upside down. It said that DNA is nothing but just static information and the intelligence is mainly in RNA, and called it as 'command and control' for protein synthesis.
A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die and a new generation grows up that is familiar with it
- Max Planck