Answer:
The glacial retreat had enabled humans to thrive in a larger area and to be able to engage in agriculture, thus the population went up. One of the major reasons for this has been the fact that there have been more rivers because of the wetter climate.
Explanation:
While the glacial ice sheets still existed, the humans were very limited as to where they can live, and what they can do. This was due to the fact that the climate was colder, drier, there were fewer rivers, and very large areas were not habitable. This all changed with the melting of the glacial ice though.
The melting of the glacial ice led to an increase in the sea levels, which in turn resulted in a wetter and warmer climate. Because the precipitation increased, the rivers became larger, but also many new ones formed, and this enabled the people to settle in much more places, and to engage in agriculture, which eventually led to growth in population in the development of civilization.
