Actually, I'm not sure evolutionary theory strictly requires that, though it traces all life back to very simple forms.
The problem is not the data and the experiments in themselves, but how they are interpreted and sold as the truth. Upon closer inspection, for example, the "simple" forms seem to be far more complicated than the not-so simple forms.
The idea that the ancestor of both ape and human must have been ape-like is now also disputed by some Harvard anthropologist who wrote a book "The Upright Ape". It's not even funny to see how the "experts" on science forums reject any critical thinking concerning the established order and call anyone who asks critical questions a "creationist", an "ignoramus", or an "idiot". Too religious for comfort, really.
In general, they should simply man up and admit they don't know really, but hey: how would they get further funding in such a way? Instead, to fill in the gaps and to cover up the holes in their theories, they think up terms like "junk DNA" (waved off as "insignificant" to the explanation), which is now gradually proven to actually contain the key to the differences and to the answer why some species simply cannot evolve ever into certain other species if there is no possibility of interbreeding and/or if no third party (for example humans) is involved.
If we are not supposed to believe in miracles (untestable hypotheses), then why should we automatically swallow incorrect inferences from correct data?