65888
This may have occurred during a drying trend 2.8 – 2.5 mya in the Great Rift Valley , which caused the retreat of woodland environments in favor of open savanna , with forests growing only along rivers and lakes .
 Great Rift Valley
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Great Rift Valley