Our multimillion nerve-cell central nervous system has its roots in the scattered nerve cells of tiny, lowly organisms that lived in water half a billion years ago. Nerve cells evidently first appeared in coelenterates —"hollow-gutted" organisms like hydra and the sea anemone. A coelenterate’’s nerve network lacks any kind of centralized control. This probably began with flatworms — the first creatures to possess a head, specialized sense cells help flatworms respond more flexibly than sea anemones to outside stimulus. But like most animals without a backbone, flatworms act mostly by instinct and reflex. Intelligent behavior remained impossible until the appearance of relatively big, complex types of brain — the types we find among the backboned animals, or vertebrates. The tiniest fish has a larger brain than the largest insect. But the development of a fish’’s three-part brain reflects that beast’’s unin-tellectual priorities. Much of the forebrain deals only with smell. The midbrain handles vision, the hind-brain, balance. With early mammals the brain grew larger and more complex. Sense coordination shifted from the midbrain to the forebrain, a developing structure capped by a folded cerebrum to handle memory and learning. Meanwhile the hindbrain gained a large cerebellum to coordinate complicated movements. Advanced mammals such as monkeys, apes, and humans (the primates) have brains derived from ancestors that took to living in the trees, when vision mattered more than smell. Accordingly the once-big "smell" part of the forebrain grew smaller, while the part that handles vision grew much larger. In the last paragraph, the phrase "took to" could be best replaced by which of the following