System of flattened sacs that modify and package protein

What is the next step in the process after a substrate enters the active site of an enzyme

The process of replication is the same for all living things because

Fossils of ancestors of the modern horses show a small 4-toed almost dog-like browsing animal named eohippus over time the horses got larger lost toes and changed to a grazing diet what is the best explanation of why this change occurred