Whether extraterrestrial impacts or drastic climate changes are to blame, mass extinctions are evolutionary events that alter the history of life. Which species or groups survive a mass extinction, while not entirely random, often has little to do with the factors that promote survival during normal times.
Mass extinctions are ecological disasters. Yet they also create evolutionary opportunities by removing once-dominant groups. Some biologists conclude that humans owe our present dominance to a mass extinction -- the K/T event that saw the end of the dinosaurs and cleared the way for mammals to diversify into all the many ecological roles they now occupy. Also, it should be kept in mind that mass exinctions probably account for the disappearance of only five percent of extinct species, the remainder having disappeared through the constant winnowing of natural selection and other continuous processes.
Over hundreds of millions of years, the planet has had five mass extinctions, and in time life has recovered. The process of recovery has been studied far less than the extinction events themselves, but this is changing. Recent research is revealing some of the patterns that can be seen in past recoveries.
Following a mass extinction, biodiversity is greatly decreased, and it stays low during a "survival interval" before beginning to climb again. While some of the species that reappear after an extinction are new, others are pre-existing. Some floras and faunas often survive in small or isolated geographic areas called refugia.
And, as recovery progresses, the resulting biological communities may be very different from those that were wiped out. This is not simply because the organisms that become extinct leave behind empty ecological niches that are filled by creatures from different ancestry than the former inhabitants. Instead, it seems the niches themselves are destroyed, and new niches are created as new species appear and new ecological relationships are established. In this way a positive feedback process occurs, with species creating roles for still more species. "Recoveries require the fabric of the ecosystem to be rebuilt, and it takes a long time," says paleontologist Douglas Erwin.
One recent study analyzed the fossil record for marine animals since the beginning of the Cambrian, about 570 million years ago. Analyzing the record of biodiversity at two different levels -- the number of families and the number of genera -- scientists looked at the relationship between the peaks in extinction of existing families and genera and the peaks in origination of new groups. What they found was that no matter what the size of the extinction peak, there was a time lag of about 10 million years before the corresponding peak in origination.
If life is undergoing mass extinction right now, as the evidence indicates, even if it is slowed, there could be millions of years of an Earth with dramatically reduced diversity.