It's not an assumption. The conclusion of the evolutionary history of life is based on direct investigation, empirical testing and observation from various independent lines of evidence.
It is the explanatory power of evolution (eg understanding how organisms evolve and the mechanisms involved in that process) which gives it its application.
The conclusion that there is life on this planet is based on direct investigation, empirical testing and observation from various independent lines of evidence.
The scientific method in no way proves
ANY theory!
Evolution assumes that organisms exist with all the complexity that they have and how those complexities interact with the environment (i.e. their "application") - all by chance. The fact that life exists does not prove Darwin's theory.
If what is observed, (let's say humans with Down's Syndrome, humans with Primordial Dwarfism and humans with neither). All three of those groups of people technically have different genomes. Two of those groups' genomes are caused by genetic mutations.
Now we know that women who have children when they are older, have a greater chance of having a baby with Down's Syndrome. What causes the mutation that causes primordial dwarfism, they don't know.
Yet if you were to find skeletal remains of a these three groups all in the same community; would you the evolutionist automatically assume they are different species of humans? And if so; why? That's called "observer bias".
Now say in the next layer of fossil, you discover more dwarf and DS skeletons than in the previous layer. Are you now going to assume that these "subspecies" "interbreeding" was the cause of the population increase? When in reality, there were just more DS babies because the women didn't have children until they were older.
Those are all assumptions made by the observer who fails to understand that the current imperial evidence of the time these populations lived, is that those with Down's Syndrome and Primordial Dwarfism do not reproduce. Now theory says that they can; yet in medical literature there's no evidence that any ever have.
So moral of the story is you can not make assumptions on evidence you don't have. Thus the same thing you accuse the ID people of not having - (by the way).
Scientific method DOES NOT PROVE THEORY!