UC Berkeley Physicist Richard Muller and grad student Robert Rohde have found something odd, and a bit troubling. Looking at the marine fossil record for the last 545 million years -- that is, from the start of the so-called "Cambrian Explosion" of life -- Muller and Rohde found that the diversity of genera drops significantly every 62 million years. (Genera are the level above species in taxonomy.) Some of the best-known mass extinctions in paleontology, including the end of the dinosaurs, occurred on this 62 million year schedule. But what causes it?
Muller and Rohde's article, published last month in Nature, also shows a weaker 140-million year cycle. The full text of the article is behind a subscription wall, but supplementary material -- including details on the statistical math of their study -- can be freely downloaded.
There are a number of possible explanations for the cycle of extinctions, but none fit perfectly.
Muller suspects that it's an astronomical phenomenon, perhaps something periodically disturbing our solar system's distant cloud of comets, with dire results. Rodhe, conversely, is leaning towards cycles of "flood vulcanism" as the culprit, massive volcanic eruptions around the world resulting from slow-moving magma plumes. But no good evidence exists for either of those explanations.
Some of the die-offs are bigger than others, and some groups of species seem to withstand the effect -- whatever it is -- better than others:
In examining their results, Muller and Rohde found that the fossil diversity cycle is most evident when only short-lived genera (those that survived less than 45 million years) are considered. They also found that some organisms seem to be immune to the cycle, while others are exceptionally sensitive. For example, corals, sponges, arthropods and trilobites follow the cycle, but fish, squid and snails do not. In general, longer-lived genera that are more diverse and widespread stand a better chance of resisting the 62 million year cycle.
It's likely that there are multiple forces at play, and that a random event -- like an asteroid strike -- could have its effects magnified by this as-yet unexplained cycle.
The reason that this is more than an abstract bit of scientific speculation is that the last big extinction happened 65 million years ago, wiping out most dinosaur species (with the survivors staggering on as birds). We may be overdue for the next big biodiversity collapse. This possibility certainly gives added impetus to calls for more asteroid and comet-monitoring, and for more exploration of the geophysics of our planet.
Ironically, a collapse of biodiversity appears already to be underway, without asteroids or flood vulcanism to take the blame. Alex has discussed what has been called the "Sixth Extinction," the ongoing present-day die-off of myriad species caused by human activities; up to half the planet's variety may be gone by 2050. We may not be able to stop volcanos or shove away asteroids in time, but this biodiversity collapse is something we can prevent, if we're willing.
(An obvious question arises from this: if there's a ~62 million year cycle, and we're seeing a biodiversity collapse now at 65 million years since the last one, how do we know this periodic effect isn't to blame? The answer is we don't -- but neither do we have evidence of anything other than human activity causing the collapse. For that, sadly, we have plenty of evidence.)
Every 62 million years, the stars are right, and the Old Ones return to feed.
Perhaps a species arises every 62 million years that manages to wipe out most of the other ones?!
Or gods snooze alarm is realy realy long...
No kidding on "Nemesis" - Muller's been pushing this for over 20 years now, for example see this article in space.com:
But the period used to be 26 or 35 million, not 62 million years...
One interesting comment there - a human return to the Moon with the appropriate geologic surveys would once and for all establish whether there have been periodic variations in cratering impact through the solar systems history.