why is evolutionary theory constantly changing?

as the title says why is evolutionary theory constantly changing?
for example, until very recently, apes were considered our closest ancestors with chimps being off on another branch, but now, just apes are being written off as our closest living ancestors and chimps in their place.how can this change take place? why can it be said that our closest living relatives are apes for so long and yet then have it said that now chimps are? what has changed to make evolutionists believe this if research has been taking place on both animals for so long - could it be that the argument that we descended from apes is no longer being accepted by many evolution scientists and so chimps are our new "closest living ancestors?"any views on this would be much appreciated.thanks

"question you might want to ask yourself first is why has christian doctrine changed through time... hmmm..."would it be possible to give an example of this - i've read most types/variations of bible's and they all stick to the original text-there are organisations and printing companies set up who MUSt stick to the original text thoroughly -if anything is unsure that is made clear at some point in the bible. thanksone thing is that some people i guess confuse natural selection and evolution - evolution is creatures evolving in shape and form - natural selection does not make mention of any changing in creatures - it says that those "species" which were able to survive in a certain environment survived and the weaker ones/ones less adapted died out.but i don't know what i'm saying with that - anywayfor evolution changing its ideas - i'm talking about the argument that our closest living ancestors are apes and now they have become chimps - ok so there can be new evidence - but what evidence can there be to suggest we came from apes and chimps are on another "branch" and then suddenly they are similar too - surely that would have been noticed originally......i guess i'm trying to ask - what was the new "evidence" that makes chimps closer to us than they were before?