At the time, all ecosystems known to exist relied on plants or photosynthetic microbes as the base of the food chain.
(all ecosystems; known; ) (all ecosystems; to exist; ) (all ecosystems known to exist; relied; on plants or photosynthetic microbes as the base of the food chain; T:At the time)
all ecosystems all ecosystems all ecosystems known to exist
known to exist relied
on plants or photosynthetic microbes as the base of the food chain T:At the time