The finding suggests that biodiversity has been strongly influenced by the motion of the solar system through the Milky Way and of the galaxy's movement through intergalactic space.
Mikhail Medvedev and Adrian Melott, both of the University of Kansas, presented their new theory at a meeting of the American Physical Society earlier this month.
Comment: Note that it is just a theory...
The theory offers the first explanation for a mysterious pattern previously noted in the fossil record.
Comment: It is neither the first explanation nor the best fit...
"There are 62-million-year ups and downs in the number of marine animals over the last 550 million years," Melott said.
Until now, however, even the scientists who first discovered the cyclical pattern had not been able to explain it.
A number of possible explanations had been considered-including volcanic activity, comet impacts, and changes in sea level-but none could account for the phenomenon's regularity.
Comment: That's because you left out a solar companion which, by the way, HAS been proposed as the solution to the regularity.
The Kansas researchers discovered that high rates of extinction in the cycle coincide almost perfectly with periodic "excursions" of the solar system outside the central plane of the Milky Way galaxy.
Comment: "Almost perfectly?" That means that it does not coincide perfectly. Back to the drawing board, boys.
"Excursions to galactic north coincide with drops in biodiversity," Melott said.
During these periods, which include some of the largest mass extinctions known from the fossil record, Earth is bombarded with high levels of cosmic radiation.
The radiation may harm biodiversity by causing mutations or by triggering climate change, the researchers said.
Richard Muller is the University of California, Berkeley, physicist who first discovered the 62-million-year cycle with his graduate student Robert Rohde.
"We spent a year searching for possible mechanisms," Muller said.
"I was stunned when I learned that Medvedev and Melott had succeeded where we had failed, and I congratulate them."
Comment: Congratulations may be a bit premature.
Cosmic Cause
Our solar system travels through the disk-shaped Milky Way on a complicated circuit that takes about 225 million years to complete.
At regular intervals, the system's wanderings take it up and down through the thin central portion of the disk. The sun reaches its farthest distance from the central plane every 62 million years.
Comment: Now, wait a minute... the complete cycle is 225 million years. If you divide 225 in half, you have 112.5. If you quarter it, you have 56.25. So, where does the 62 million come in as a "regular reaching of the farthest distance?
The entire galactic disk, meanwhile, is hurtling through the hot gas that surrounds it at about 125 miles (200 kilometers) a second.
"The movement [of the Milky Way] is not edge-on like a Frisbee," Melott noted. Instead, he said, it is flat, "like a pie in the face."
The new theory suggests that cosmic rays are continually generated in a shock wave produced where the galaxy's "northern" or forward side collides with surrounding gases.
As the solar system rises above the central plane it sticks out like a cherry on top of the flying galactic pie - closer to the source of the cosmic radiation.
"We're exposed to the shock front more when we're emerging on the north side of the galactic disk," Melott said.
Comment: Then that means it ought to occur every 225 million years...
At the same time, he explained, the solar system receives less protection from powerful magnetic fields that form a shield from cosmic radiation in the dense, central portion of the galaxy.
Impacts on Earth
Melott said his group applied their model to the largest existing fossil database, which reconfirmed the finding of a 62-million-year fluctuation in diversity.
Comment: But notice that this is only a marine fossil database.
In a paper recently accepted by Astrophysical Journal, the Kansas researchers discuss various possible mechanisms by which cosmic-ray exposure could result in mass extinctions.
One possibility is that organisms receive harmful doses of radiation from high-energy particles known as muons, which are produced by cosmic rays colliding with Earth's atmosphere.
"Cosmic rays themselves are not really that dangerous," said Medvedev. "They create [charged particles] that propagate down through the atmosphere - especially muons that can go below the sea level."
Comment: Oh, now we learn something new. A few paragraphs back, it was "cosmic rays," now it is muons that go down to the bottom of the sea.
Changes in the chemistry of the atmosphere, and accompanying depletion of the ozone layer, may also cause increased mutations, he added.
In addition, charged particles produced by cosmic-ray bombardment may cause greatly increased cloud cover, leading to climate change.
The researchers said their model does not explain all major mass extinctions.
Comment: No, it certainly doesn't.
For example the demise of the dinosaurs, which is thought to have been caused by an asteroid impact, does not fit the 62-million-year cycle.
Comment: They failed to mention a whole slew of other extinctions as well.
As for what lies ahead, the news is mixed. The solar system has recently passed the galactic mid-plane and is on its way up, Melott said, which could mean greater exposure to radiation.
But, he added, "the next cosmic ray effect is about ten million years ahead of us."
Comment: In short, "nothing to worry about, go back to sleep". Funny that they come out with this at the present moment.
It is well known that there are other major extinctions and the cycle is not ONLY every 62 million years! There is also a very strong signal for a 26 million year extinction cycle. The different estimates of the number of major mass extinctions in the last 540 million years are due mainly to what the individual researcher chooses as the threshold for naming an extinction event as "major" as well as what set of data he selects as the determinant measure of past diversity. As it happens, the 62 million event data stems mainly from marine fossil evidence.
The classical "Big Five" mass extinctions identified by Raup and Sepkoski in 1982 are widely agreed upon as some of the most significant. They are:
The late Ordovician period (about 438 million years ago) - 100 families extinct - more than half of the bryozoan and brachiopod species extinct.
78 million years later:
The late Devonian (about 360 mya) - 30% of animal families extinct.
106 million years later:
At the end of the Permian period (about 245 mya) - Trilobites go extinct. 50% of all animal families, 95% of all marine species, and many trees die out.
37 million years later:
The late Triassic (208 mya) - 35% of all animal families die out. Most early dinosaur families went extinct, and most synapsids died out (except for the mammals).
143 million years later:
At the Cretaceous-Tertiary (K-T) boundary (about 65 mya) - about half of all life forms died out, including the dinosaurs, pterosaurs, plesiosaurs, mosasaurs, ammonites, many families of fishes, clams, snails, sponges, sea urchins and many others.
As you can see from the above, using the number "62 million years" and building a theory on it is really a bit misleading.
Raup and Sepkoski are mentioned as identifying the "Big Five", but the fact is that Sepkoski, a University of Chicago paleontologist suggested that the extinction of the dinosaurs 65 million years ago was part of a 26 million year cycle!! However, I would like to mention that if you multiply the 26: 3 X 26 is 78 - which just happens to be the time between the Ordovician and Devonian extinctions; 4 X 26 is 104 which is very close to the 106 million years between the Devonian and Permian extinctions; and 5 X 26 is 130, which (when dealing with these kinds of numbers) is close enough to the gap between the Triassic and K-T extinction to be in the ballpark. So, maybe there is something to this 26 million year thing after all, only each "return" has varying effects based on many other solar system variables. A companion star with a 26 million year orbit might be more stable, since Muller has suggested that a 62 million year orbit is too great to be stable.
As it happens, if we postulate the 26 million year orbit of a Companion Star, we would find that there ought to be a return about 39 million years ago, and then another 13 million years ago, which would put us half-way in the Companion star orbit cycle.
For more details read: Forget About Global Warming: We're One Step From Extinction!