Final answer:
The three adaptive shifts of early hominins were the development of bipedalism, increasing brain size, and advanced tool use, which were crucial as they migrated and adapted to new environments.
Step-by-step explanation:
The three clear adaptive shifts of early hominins included the development of bipedalism, an increase in brain size, and the emergence of new technologies and tool use. First, bipedalism allowed hominins to move more efficiently and freed their hands for carrying objects and using tools. This shift is evident in the skeletal modifications seen in fossils, such as broader pelvises, longer legs, and arched feet. Second, there was a progressive increase in brain size, starting from around 550 cubic centimeters (cc) in early Homo species to 1,250 cc in Homo erectus. This increased brain size may have been supported by dietary changes, such as the consumption of cooked meat, which allowed for a greater caloric intake. Lastly, new technologies and tool use spread as hominins migrated across various regions. This is reflected in their cultural practices and the archaeological evidence of their innovative toolmaking abilities.