We relate brain size to appearance time for 511 fossil and extant mammalian species to test for temporal changes in relative brain size over time. We show that there is wide variation across groups in encephalization slopes across groups and that encephalization is not universal in mammals. … Encephalization [vs. time] trends are associated with sociality in extant species. These findings … highlight the role sociality may play in driving the evolution of large brains. (more; HT Razib Khan)
The biggest brains have consistently gotten bigger over the last half billion years since multi-cellular life appeared. Big brains seem to be a necessary precondition for human level intelligence and civilization, and human size brains appeared only very recently. These facts strongly suggest that achieving human level intelligence is just not a big component of the great filter. It appeared quickly after big brains, and big brains seem likely given enough time and sociality, and sociality seems likely.
This unfortunately means that it is very difficult to collect data on all steps of the great filter. It is big and real and matters enormously, but we can hardly see it.