Friday, August 01, 2008

Septentrion

The Big Dipper hangs over the hill behind the house. So familiar is this particular configuration of stars that I have sometimes wondered if it might be part of our genetic inheritance.

This idea is not as bizarre as it sounds. It has been demonstrated experimentally that certain migrating birds orient themselves by observing the stars. In a classic series of experiments, Stephen Emlen placed captive indigo buntings in cages under a planetarium sky and tested their response to stellar cues. The birds were able to find north by observing the rotation of the stars about the fixed pole. Some researchers believe that migrating birds might innately recognize the pattern of the Big Dipper.

Might not humans also have evolved the ability to recognize a prominent pattern of stars that would help them find their way? Several things argue against this hypothesis.

First, the place of north among the stars is not fixed. The Earth's axis wobbles under the stars, like a top that wobbles as it spins. Every 26,000 years, the axis sweeps out a circle in the sky. Today, the axis conveniently points close to the bright star Polaris, but this coincidence is temporary. Fourteen-thousand years ago, the axis pointed towards Vega, and it was the North Star. At that time, the Dipper was farther from the pole than it is today.

Second, the relative positions of the stars on the sky are not permanent. Every star has some tiny motion relative to the others. These motions do not significantly change the appearance of constellations over many generations; however, over longer periods of time, the appearance of constellations is considerably altered. One-hundred-thousand years ago, when Neanderthal humans inhabited the Old World, the Big Dipper looked more like a straight-handled spade than a dipper. One-hundred thousand years in the future it will look like a duck.

The rate of genetic change would have to be extremely rapid for natural selection to keep an inborn star map up-to-date on a time scale of tens or even hundreds of thousands of years. This may be why some birds fix on a center of stellar rotation rather than on a particular pattern of stars.

Moreover, the selective pressure of evolution would seem to have been less for human ancestors, who presumably didn't range far from home, than for migrating birds.

All of which makes an inborn map of the Big Dipper exceedingly unlikely.