Virtually all of the body plans originated in the Cambrian explosion – a hiccup of time in the geologic record. Imaginative Sci-Fi writers propose a host of body plans in their fiction. So if man can think up novel body plans, why didn’t nature - if evolution is random, given all the time passing, some 500 million years? The question is amplified by a number of intervening mass extinctions which would have provided ample environmental opportunity for new body plans to emerge and survive. Notably, the “no new body plans” mystery is a concern among scientists as well. "No new body plans" may be overstating things. The phylum of vertebrates existed in the Cambrian, but the only vertebrates in the Cambrian were fish. Since then, fish have evolved into amphibians, reptiles, mammals and birds. That's not exactly "no new body plans" unless you think we all look like fish.