The evidence for a finely tuned universe is there in the measurements of over 122 physical constants. The evidence for a multiverse is not there. It is only very flimsy and indirect evidence and in fact some scientists disagree. So that is why I discount a multiverse and not the fine tuning based on the evidence. As for common decent I havnt discounted this. I have disagreed with the conclusions. There maybe common decent for animals to a certain degree in that all dogs can be traced back to an ancestor such as the wolf. The wolf may be traced back to another ancient dog like creature.
But beyond this there is no evidence apart from speculation based on the fossil records which is very fragmented and has many gaps and up for interpretation based on observation of anatomical similarities. As stated before these similarities can be misinterpreted as normal variations between the same species. The genetic evidence does not support a tree like formation for all life being traced back to a common ancestor. It has many trunks and branches in a horizontal formation which indicates a sideways passing of genetic material rather than a vertical one which would be required for common ancestry. There is absolutely no verifiable evidence for the original common ancestor or any evidence for how it came about of how a single celled organism could evolve into a more complex living creature. There is plenty of speculation but no empirical evidence.
In fact there is evidence that life was already complex from the beginning and shared genetic info without any evolution through mutations and natural selection. The evidence points to non adaptive driving forces fro change such as HGT, epigenetics, developmental biology and endosymbiosis. Add to this the evidence already posted that shows that the amount of mutations needed to produce every living creature is beyond the capabilities of evolution. Mutations are mainly harmful or neutral and rarely beneficial. But evidence shows that all mutations have a slight cost to fitness and not an advantage for fitness. The tests show that to even get two small changes through random mutations to add new function would take to much time which is beyond the time needed for evolution to evolve complete living things. In fact the time needed would be more than the earth and universe has been in existence.
So my disagreement is valid and based on the evidence of science. Science is suppose to be testable and verified before it is classed as true. So if anything I am supporting things with the science.
Estimating the prevalence of protein sequences adopting functional enzyme folds.
http://www.ncbi.nlm.nih.gov/pubmed/15321723
The frailty of adaptive hypotheses for the origins of organismal complexity
http://www.pnas.org/content/104/suppl_1/8597.abstract
Some Problems in Proving the Existence of the Universal Common Ancestor of Life on Earth
http://www.hindawi.com/journals/tswj/2012/479824/
Opposing the multiverse
The very nature of the scientific enterprise is at stake in the multiverse debate.
Its advocates propose weakening the nature of scientific proof in order to claim that the multiverse hypothesis provides a scientific explanation.
This is a dangerous tactic.
Two central scientific virtues are testability and explanatory power.
The extreme case is the multiverse proposal, where no direct observational test of the hypothesis is possible. Despite this, many articles and books dogmatically proclaim that the multiverse is an established scientific fact.
http://astrogeo.oxfordjournals.org/content/49/2/2.33.full