For all ecosystems then known to exist, plants or photosynthetic microbes constituted the base of the food chain.
(all ecosystems then known; to exist; ) (plants or photosynthetic microbes; constituted; the base of the food chain; T:For all ecosystems then known to exist)
all ecosystems then known plants or photosynthetic microbes
to exist constituted
the base of the food chain T:For all ecosystems then known to exist