5. The Cambrian explosion

Other research suggests the possibility that the delivery of extraterrestrial matter may have stimulated and enriched the Earth's biosphere in the geological recent past. Culler et al. (2000) conduct an analysis of lunar soil samples returned by the Apollo 14 mission.[82] They found that the rate of meteorite impact declined from the heavy bombardment until about 400 million years ago. This is in accord with standard theory. However, they also find that from 400 million years ago to present, the rate of impact increased by a factor of 4. The impact trend for the Earth would be very similar to that of the moon due to their proximity, except that the Earth would have approximately double the rate since it is a larger target and has stronger gravity. Culler et al. note that their data indicating a pronounced spike in extraterrestrial input is "roughly coincident with the 'Cambrian explosion' of complex life on Earth. ... the correlation is permissible evidence of a causal relation. It is possible that the increased debris influx had a net stimulating effect on biotic diversity ..." The trend they highlight would also indicate a similar increase in the quantity of MM and IPD reaching the Earth. This is because an increased cratering rate could be due to both asteroids and comets, both of which are associated with MM and IPD.