Death of a Death Star

"Luis Alvarez walked into my office looking like he was ready for a fight. 'Rich, I just got a crazy paper from Raup and Sepkoski. They say that great catastrophes occur on the Earth every 26 million years, like clockwork. It's ridiculous.'"

Thus begins Dr. Richard Muller's excellent book, Nemesis: The Death Star. The quotation is in reference to a discovery made by the paleontologists David Raup and Jack Sepkoski. The essence of their argument requires just a little bit of background to understand.

More than 99% of all species that have ever lived are now extinct. This is well established and far from controversial. The precise mechanisms by which these extinctions occurred are less well established. It has long been assumed that extinction could be accounted for simply by natural selection. Due to changing environmental pressures and competition between species, no animals are safe. There is an inherent risk in life that anyone could go extinct at virtually any time. Gradualism seems to be a decent explanation for many--but certainly not all--of these extinctions.

From time to time, however, there is a mass extinction. What this means is that throughout most of our planet's history, the extinction rates have been more or less stable. The number of species that go extinct in a given period of time does not fluctuate significantly. But every once in a while, something happens that causes the extinction rate to noticeably skyrocket. During these periods of mass extinction, the number of species that go extinct in a given period of time is significantly higher than normal.

Causes for mass extinction are less well known. For a while, it was thought that they were explained best simply by a temporary increase in the very same risks that drive normal extinction. But this is not necessarily the case. It is now widely accepted that, for instance, the mass extinction at the end of the Cretaceous 65 million years ago was caused by an asteroid impact (this is probably what killed most of the dinosaurs).

Raup and Sepkoski caused quite a furor when they published a paper suggesting that extinctions happen every 26 million years, like clockwork. Dr. Alvarez's first reaction as quoted above certainly reflected the common thought: it's ridiculous. No one argued that mass extinctions should not happen that often (though not much more frequently than that, as it takes considerable time for the planet to recover from such an event), but the precision of timing is difficult to explain.

The reader might suppose that there could be a possible model to explain such periodicity of mass extinctions. Perhaps some as yet unknown geological feature or process that only takes place every 26 million years or so. Even that would be a huge stretch, but it is theoretically plausible. However, this does not fit the data.

Since the Cretaceous extinction was caused by an asteroid (as were some others), they can be excluded from any hypothesis requiring a terrestrial explanation. Here's the problem. If these extinctions are excluded from the analysis, the 26 million year periodicity, then the whole system crumbles. Without them, the extinctions are so widely spaced that one can't argue for precision timing anymore. However, excluding them seems to be a bit of a cheat, since the data are otherwise so suggestive.

The problem is, you can't have a mix of terrestrial and extraterrestrial explanations for the same phenomenon. You don't get randomly-timed extraterrestrial impacts to fit in with the precision timing of otherwise terrestrial events. Explaining such a phenomenon requires all of the extinctions to be explained by extraterrestrial impacts. But these impacts are generally regarded as random. Just what is happening here?

Explanations came mostly from astronomers and ranged from Planet X (which doesn't exist) to "Nemesis," which is the topic of this discussion. Dr. Richard Muller, a physics professor at University of California, Berkeley, got a look at Raup and Sepkoski's paper and analyzed it at the request of none other than Dr. Alvarez. His first assumption was the same as Alvarez's--it was utter lunacy! But as he began his work, playing devil's advocate, he became somewhat more convinced that they were right, and invented a model to explain it. That model became Nemesis. The hypothesis, in a nut-shell, is that the sun has a companion star, called Nemesis. Its orbit brings it around every 26 million years, and the gravitational disturbance causes the impacts.

Here's the problem. Raup and Sepkoski were wrong. As their data were more closely scrutinized, it became clear that many of the extinction peaks were either too early or too fall into the 26 million year cycle, and many of them were not caused by extraterrestrial impacts.

So could the Sun have a companion star? Well...it's possible, sure. But not very likely. Since we've never found such a star, and since it no longer has any explanatory power, we should not be prepared to accept the hypothesis into the ranks of scientific knowledge (unless, of course, new discoveries save it, but I don't think that's going to happen). The corrected interpretations of Raup and Sepkoski's data could be said to have killed the death star.

Why is this important? Well, I personally think science in general is interesting, but I must confess that I do have an agenda in publishing this particular paper.

In a creationist children's book, It Couldn't Just Happen: Fascinating Facts About God's World by Lawrence O. Richards, the author attempts to use this very hypothesis as evidence against science! After offering a brief description of the hypothesis (and falsely attributing it to Raup and Sepkoski, rather than Muller), he writes: "Yet other scientists think these theories make an important point. No longer should scientists think that everything we find on Earth must be explained by what is happening on our planet now. Something outside our planet, beyond the natural processes geologists can observe today, may be needed to explain the record in the rocks!"

The point he is trying to build up is that scientists have a bias against any sort of cataclysmic event. Even though no scientist denies that extraterrestrial impacts have shaped our planet, he spends several pages trying to prove that cataclysms can have important effects. Why? Why do you think? So he can use it as an argument for flood geology!

Just a few pages later, he writes: "What we can say is this: Today most geologists scoff at the notion of a worldwide flood just as they once scoffed at the notion of other cataclysms. Yet they can no longer deny that important features of planet Earth have been shaped by cataclysms. Perhaps soon more evidence will enable us to tell just what effects are results of the Genesis Flood."

He's only slightly right. It's true that there was a period in time when catastrophes were shunned as explanations for various facts. But no scientist has seriously held such a position for a very long time. In fact, that is currently the most widely accepted explanation for the formation of the moon: an impact (in a glancing blow) with an object with about 10% of Earth's mass.

The actual story behind the Nemesis hypothesis is a perfect example of how real science works. As new facts are discovered, scientists invent hypotheses to explain them. But then they test those hypotheses and see if they really work. In this case, the hypothesis failed, because science has this self-correcting mechanism. Other scientists scrutinized Raup and Sepkoski's work and found where they made mistakes. The false hypothesis didn't last very long, precisely because of the vigilance of scientists.

Creationists like Richards don't understand this. They accuse scientists of rejecting data that don't mesh with pre-existing biases (sometimes this happens, but these individuals are found out and often shunned by the academic community for intellectual dishonesty). But they fail to recognize the mechanisms that prevent this. Meanwhile, they commit the very same offense, but to a greater degree. Richards still thinks all the species that ever lived fit on a single boat for cryin' out loud! This is not a good scientist. This is a dangerous nutcase who certainly shouldn't be writing children's books. No good can come of that.

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