Friday, February 22, 2008

Can we ever be sure about what happened long ago?

Often when I lecture about evolutionary biology, a member of the audience will say, "You don't have a time machine, so you can never be sure about what happened long ago, this is all this speculation."

Well, it is hard work, and often no experiment can conclusively differentiate the hypotheses. However, sometimes one can be as sure as sure can be.

Paleontologists have long recognized a major decline inlarge mammals about 35,000 years ago in the vicinity of the land bridge between Alaska and Siberia. The cause has now been identified.

Firestone and colleagues at Berkeley hypothesized that a meteor impact killed off the mammals.
To test this, they examined 8000 mammoth tusks for the presence of metal particles. Seven tusks and a bison skull had magnetic particles embedded, and the levels of nickel and titanium were substantially different from those found in terrestrial iron samples. One more remarkable observation is the clincher. In all of the tusks with multiple particles, the particles were all on the same side of the tusk.

So, can we be sure? I find it convincing. But let's see the publication and give it a year for comments.

1 comment:

Rob said...

It is ironic that the "no time machine argument" is often found in the mouths of biblical scholars and paleontologists.Both of these claim access to a time machine- a revealled text or fossils. As I write this I a fending off a kitten that is now part of my life with its attendant fleas and fascination with my keyboard. Why do I put up with this? Because its so darned cute, of course. Its vey cuteness is testament to our shared anestry of care-elicting behaviour. Another bit of time machine evidence?