Hello

I really really need your help!
Please, look at "at the expense of" in the below sentence.
The pre-Mendelian "state of confusion" reflected in part the absence of conceptual distinctions that were clarified in the early twentieth century, and then only
at the expense of an oversimplification of the complex relationships between A and B.
What does "at the expense of" mean?
And if possible, can you parapharase this sentence?
Thank you!