Complex sentence: To explain the fact that some cultures no longer had traits from Egypt, they claimed these cultures had simply become "degenerate", i.e. in contrast to the civilized world, the lesser developed peoples had simply forgotten the original ideas borrowed from Egypt.
Split into simpler sentences: To explain the facet that some cultures no longer had cultural traits from Egypt, they resorted trues had simply become "degenerate". That is, in contrast to the civilized world, the lesser developed peoples had simply forgotten the original ideas borrowed from Egypt.