Final answer:
Palynology is the scientific field that studies pollen grains and is one of the most effective tools for understanding past human sediments and climate conditions at sites.
Step-by-step explanation:
The scientific field that studies pollen grains and is one of the most effective tools for understanding past human sediments at sites is palynology. Palynologists use pollen fossils from sedimentary records to interpret climate conditions in the past. Each plant species produces pollen grains of different shapes and sizes, allowing palynologists to determine which plants were present and track plant migration as climate changes.