Humans and Chimpanzees are related.

While mutations occur with pre-existing genetic code, That doesn't mean information cannot be added to the genome, There are multiple ways in which a genome can grow, One way involves Transposable elements which are parts of DNA that move from one place to another in the genome often duplicating themselves in the process, Mutation and natural selection can turn this into information, And sometimes entire genomes can get duplicated leaving a second set of genes via genome polyploidy, Some strawberries have eight duplication's of their entire genome, This is why they are used in the classic DNA biology experiment. Secondly you don't always loose information, The information changes, And if the information for a gene is lost and the loss of that gene ends up being fatal, Then that organism is out of the gene pool, Also if a creature stopped reproducing it would simply not pass its genes on and die, But when two populations of a species become so genetically diverse from each other where breeding with each other is no longer possible, This is called a speciation event. Natural selection is more complicated then your explanation though, Natural selection is basically a process where organisms that are better fit to survive will end up living and passing there genes throughout the species population, While those who are not fit to survive end up eventually dieing out in favor of more successful organisms, Of course this happens because of random mutation which is caused by DNA replication and Radiation, The thing you need to remember is that all of the organisms in a population are subjected to this and are breeding which each other at the same time, Its a whole complex orchestra. There is no assumption, All life on earth today is related back a single cell called "LUCA" although there was definitely forms of life before this cell, This is the last cell in which all life derives its linage back to. Humans and Chimpanzees definitely shared a common ancestor six to eight million years ago because all life is related back to a single common ancestor, Its a huge twisted tree of genetics and diversification, We are able to determine when species split off from a common ancestor, This is why chimpanzees and humans share the majority of DNA with each other and why they split off recently. Measuring mutation rates is pretty simple, For single cells its as easy as comparing successive genomes to previous genomes, For multicellier sexually reproducing organisms the genomes are sequenced from the two parents, The offspring's genome is also then sequenced so we can see what parts of the genome of the father and mother went where, And we can make a perfect sequence of what the genome should read if it is an errorless copy from the parents, This errorless copy is then compared to the offspring's actual genome, With this we can find genetic mutations and determine the mutation rate per generation, The trend is obvious, The larger the genome and more stable the organism, The longer it takes for huge portions of the genome to be effected, This is why it takes thousands of years just for humans to have a slight characteristic change, While things like bacteria and virus's can become immune to our antibiotics and vaccines in less then a single year, Also just for reference the human mutation rate is 1. 1*10^-8/Base-pair/Generation. Secondly yes there is more then enough fossils to determine that humans were not always modern humans as we see today, What you need to remember is however is that every single generation is transitional from the previous because every generation is different from the last, Secondly fossilization is a rare process that requires proper conditions and proper timing, And its not like there was billions of early humanoids running around, There probably wasn't even a million. I'm going to link you to a online fossil gallery where you can the fossils for yourself. http://humanorigins. Si. Edu/evidence/human-fossils/fossils The problem is that Christians and creationists tend to think there should be a ****load of fossils of almost perfect preservation and quantity, This is simply not the case.