Which primate classification system replaced the older classification based on complexity?

The classification system that significantly advanced the understanding of primate relationships is based on ancestral-descendant relationships. This approach emphasizes the evolutionary lineage and phylogenetic connections among species, allowing researchers to trace how different primate groups have diverged from common ancestors over time. By focusing on these evolutionary histories, this system provides a more accurate and scientifically grounded framework for classifying primates rather than relying solely on their complexity or morphological features.

In contrast, the other options, while relevant in different contexts, do not encapsulate the modern understanding of primate taxonomy as effectively. Genetic classifications certainly play a crucial role in understanding relationships at a molecular level, and behavioral patterns and ecological adaptations provide important insights into how species interact with their environments. However, the foundational system that underpins contemporary taxonomy relies on the evolutionary lineage and relationships among species, marking a critical shift from older, more simplistic classifications based solely on characteristics or complexity.