Final answer:
The earliest transportation mode by humans was by foot. Transportation evolved over time, with significant milestones including horse-drawn vehicles and the advent of railroads.
Step-by-step explanation:
The earliest mode of transportation by humans was traveling by foot. Before the development of horse riding, trains, or cars, humans relied on walking to navigate their environment and to migrate in search of resources. As time progressed, transportation evolved significantly. Horse-drawn carriages, such as the omnibus and horse car, were used extensively before the advent of the Industrial Revolution. New transportation methods emerged with the development of the steam locomotive and railroads, transforming how humans traveled and connected cities. However, the most foundational and earliest form of transportation for humans remained pedestrian mobility.