The poem follows a man named John Phillips, a figure whose work, judging by the poem, involved naming geological eras and the extinctions between them. The speaker's tone is wry and a little rueful: "Knowledge is funny how it can just eat you up." He has spent his life sorting deep time into neat categories, enjoyed a good dinner, and thought about a strange image of a nude girl seen in an operating theater. Then he falls down stone steps and dies, his soft body meeting hard granite.
The second half turns that death into a reversal of evolution. His body becomes "a fish in reverse," and the poem slides backward through the ages, from Mesozoic and Paleozoic to spines, then no spines, then eyes, then no eyes, down to mosses, spores, and algae. The list gets simpler and shorter, like a mind losing detail.
The mood moves from dry irony to tenderness and awe. It ends in bare light: "sunshine, / sun." Everything gets stripped away, and only the source of life remains.