I was watching NOVA the other night, and they were talking about that in large mammals the DNA does not really change too much from species to species. What do change is small portions of the DNA (on-off markers) which turn large portions of DNA on and off.
On off mutations do occur. A well documented example is a mexican family with a mutation that re-activated fur growth. If genes are off they can degrade, blind cave dwellers being a prime example. Their eyes are not just turned off, they are degenerated.
Having said that, not too much difference is relative. Humans and chimps differ less than a horse and a donkey. Horses and donkeys can interbreed, although offspring is infertile. Don't forget that a number of genes are virtually unmodified because they are to a large extent essential and derived through a common ancestor. The spinal chord is a good example. If you talk about large animals in terms of size, also remember that growing big means larger time between generations.
A few scientists think that live on earth was introduced by aliens.
panspermia is the word to google
If this is true the most similar descendents of ancient bacteria should have on-off markers instead of actual changed DNA. Is that so? Or do this microorganisms have there actual DNA simplified?
No, genetics is not as simple as gene on gene off. the plague does not carry genes that, if set to on, would result in a human.