Complex sentence: Vertebrates were once thought to be unable to perform this cycle because supposedly they lacked its two key enzymes, isocitrate lyase and malate synthase; however research since about 1989 has proven that this pathway exists for at least some if not all vertebrates.
Split into simpler sentences: Vertebrates were once thought to be unable to perform this cycle because there was no evidence for its two key enzymes, isocitrate lyase and malate synthase. However some research has suggested that this pathway exists for at least some if not all vertebrates.