civilizations (48) | important (36) | planets (46) | really (189) | something (64) |
through (48) | question (54) | 'cause (80) | things (88) | looking (49) |
biosphere (38) | civilization (48) | people (67) | interesting (39) | system (50) |
different (63) | always (50) | planet (92) | problem (38) | because (84) |
experience (99) | probably (40) | science (75) | evolution (44) | actually (125) |