Barry Evans ("Field Notes," Nov. 5) concisely describes Earth's amazing "Unity of Life." But NASA's "Tree of Life" omits some vital connections between its divergent branches: Our mitochondria evolved from an engulfed aerobic bacterium related to Rickettsidae (unrelated to vitamin D-deficient rickets).
Chloroplasts in plants then evolved from an engulfed photosynthetic Cyanobacterium. We owe our existence to these events. The dramatic endosymbiotic origins of chloroplasts and mitochondria required for photosynthesis and respiration was first recognized by Lynn Margulis (married to Carl Sagan at the time) in 1967.
—Don Garlick, Fieldbrook