A century-old theory about the origins of life on Earth may be wrong. New research from the University of Cambridge suggests that comets traveled too fast and struck at unfavorable angles for the fragile molecules needed for life to survive, making successful delivery extraordinarily unlikely. Surprisingly, Mars may have had a better chance of receiving these cosmic ingredients, although even there the odds were slim. The findings point toward asteroids or chemical processes on Earth as more plausible sources of life's building blocks.