Well,' he responds, 'science has never been closed to people who had ideas they wouldn't change. Every scientist has a set of presuppositions and assumptions that he never questions.'
For evolutionists, he says, one of these is the conventional evolutionary assumption that all living things are descended from a common ancestor
There is one, right off the bt, that he is wrong about.
All that I know is said is that, so far, ALL the data that we have is just what we would expect if there was a single common ancestor.
However, (somewhere I can never remember) I read an article that mentioned the possiblity that some other lineages may exist but we haven't uncovered them. That is, some bacteria or virus like organisms that clearly share no common ancestor or at least split off before the LUCA of all the rest of life that we have examined so far. One challenge is to guess where to look for them and what to look for. If they are there we will most likely (if lucky) stumble over them by accident when we are doing a lot of wholesale gene sequencing or find something with life like chemistry that doesn't use our DNA or RNA.
One might say that the existance of them is unlikely. I think that we have only so recently (decade or two) started exploring in a way that could find them that we can't comment on that yet. If nothing is uncovered in a decade or two more than one might make the single tree of life a "dogmatic" presupposition.
It is not a fixed presupposition. It is simply the only thing that the evidence supports right now and supports it more solidly in the last decade or two then ever before.
If he means multicellular organisms, especially anything as complex as a worm and more then it is very nearly (but not yet) a done deal as far as a tested idea. It is sure looking like the only explanation that one can devise.
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