Friday, March 05, 2010

Dinosaurs, asteroids, and the scientific methods.

Artist's Conception of the Asteroid Impact that May Have Killed Off the Dinosaurs
Artwork Credit: NASA / Don Davis

I like space.  I like dinosaurs.  I own a very well-written book called "T. rex and the Crater of Doom".  So you'd think I would be happy about a news story titled "It's official: An asteroid wiped out the dinosaurs".  But I'm not; in fact, I'm quite grumpy about it.  Why?  Because many of the versions of this story that I've read, whether from internet news, traditional media, or even Scientific American, imply that this is a decision and the final word on the subject.  But that's wrong.  That's not how science is done.  And, to be fair, the original press release does not make any such statement.  Let's look into the topic, what happened, why the news outlets got it wrong, and why it matters.

First, the demise of the terrible lizards.  Roughly 65 million years ago, an entire class of large animals, the dinosaurs, vanished from the face of the Earth in a short time.  So did many other animals on both land and sea.  Why did this mass extinction happen?  Many ideas have been proposed, from large volcanic eruptions in India to disease to dark matter (one of the more exotic ideas).  As a kid, I read many books that said that the appearance of mammals caused dinosaur extinction, though that's pretty much disproven, since mammals co-existed with dinosaurs throughout the Mesozoic.  But the hypothesis that has gotten the most attention recently is the giant asteroid impact.  In short, a big asteroid hit off the coast of Mexico 65 million years ago, causing tsunamis, global fires, acid rain, "nuclear" winter, severe global warming, and many other really bad effects.  The book I mentioned above, T. Rex and the Crater of Doom by Walter Alvarez, does a great job of presenting the story of how the origin of the hypothesis, the scientific evidence, and details of what the last days of the dinosaurs may have been like.

In this week's edition of the journal Science, a panel of 41 scientists from around the world published a review of the scientific evidence for the cause of the mass extinction at the end of the Cretaceous period, and they concluded that the asteroid impact hypothesis was the most consistent with the fossil, geologic, and computer model-based evidence.  This is not surprising, in my opinion, as the first line of evidence that led to the asteroid hypothesis, the famous "iridium layer", is both compelling evidence of an asteroid impact and is the line of demarcation between the geologic Cretatious and Tertiary periods wherever it exists, with little fossil species overlap between the two layers of rock. 

What is important about the new paper is that it is a review of a large amount of data and analysis collected by many different scientists, and the review tries to fit these individual pieces together into a coherent story.  All of these studies were scrutinized in the light of as much modern science and collective expertise of the panel as possible.  And, in the end, the collective wisdom of the experts in this particular panel felt that the evidence points straight at an asteroid impact. 

Reviews like this are important in science, especially for big questions where no one person has the expertise required to explore every nook and cranny of the issue.  For the dinosaur extinction, the evidence includes studies from many different fields of science: geology, paleontology, vulcanology, climatology, biology, astronomy, planetary science, physics, chemistry, biochemistry, and many more.  What is the best way to compare results from a paleontological study against conflicting results from an astronomical study?  Lots of smart people covering all the bases who can take as unbiased a look as possible at the data. And that is what happened in this study.

However, reviews like these are not the final word on a subject in science.  These reviews can be very influential, but there is no law that says we scientists must agree with the conclusions reached by such a review.  In fact, reviews like this often inspire scientists who agree with the findings to go off and try and disprove the consensus model! This review essentially is laying out a cogent, coherent argument for an asteroid impact by detailing the most compelling arguments.

This review does not mean that all other hypotheses for the extinction of the dinosaurs are themselves extinct, though.  It is quite possible that the extinction was already underway before the asteroid hit, or that the combination of an asteroid impact and volcanic eruptions was necessary to cause the great extinction, or some other channel of extinction.  It is also possible that another panel of 41 equally distinguished scientists could come to a different conclusion based on their different expertise.

In the end, the asteroid extinction hypothesis will stand or fall on its own merits, not on the conclusions of this review.  The hypothesis makes several concrete predictions, such as that the extinction was a sudden, catastrophic event that happened on the timescale of days, months, and years.  If several triceratops, T. rex, and other late Cretaceous dinosaur fossils are found in fossil beds thousands of years younger than the iridium layer, the asteroid extinction hypothesis is dead and unsalvageable.  If we find that the last dinosaurs died tens of thousands of years before the iridium layer was deposited, then the asteroid hypothesis is gone.  If future climate models find that the asteroid impact off the coast of Mexico would not have affected ecosystems in Mongolia, then the asteroid hypothesis craters.

That is how science works at its most basic level.  Theories make testable predictions.  We scientists go and see if those predictions are right.  If not, the theory is in serious trouble if not completely dead.  If the predictions hold true, then we go and test a different prediction.

In short, the Science review on the asteroid impact hypothesis is not the final word on the subject.  Scientific truths are not decided by review panels but by rigorous testing.  (This is not to say that review panels are unimportant!)  But scientific truth cannot be decided or even affected by any panel of scientists, scholars, or politicians.  Scientific facts are, and can be tested no matter what people say.

1 comment:

  1. Anonymous9:49 AM

    this is bias and, cant be proven....To be a fact it's you'r theorie. Mr PHD