We thank Nick Barton and Daniel Weissman for helpful discussions and pointing us to relevant literature. References I. Alhrood AC. (irnixinger.IP, Knoll AFL Burch 1W. Anderson MS. et al. (2009) Controls on development and diversity or early anthean ommainints. Pow Nail And Sri USA 106: 9548-9555. 2. SchopfJW