The Demon-Haunted World_ Science as a Candle in the Dark - Carl Sagan [152]
They scrutinized the shape of the depressions. The footprints of a fast-moving animal display a more elongated symmetry. A slightly lame animal favours the afflicted foot, puts less weight on it, and leaves a fainter imprint. A heavier animal leaves a deeper and broader hollow. The correlation functions are in the heads of the hunters.
In the course of the day, the footprints erode a little. The walls of the depression tend to crumble. Windblown sand accumulates on the floor of the hollow. Perhaps bits of leaf, twigs or grass are blown into it. The longer you wait, the more erosion there is.
This method is essentially identical to what planetary astronomers use in analysing craters left by impacting worldlets: other things being equal, the shallower the crater, the older it is. Craters with slumped walls, with modest depth-to-diameter ratios, with fine particles accumulated in their interiors tend to be more ancient, because they had to be around long enough for these erosive processes to come into play.
The sources of degradation may differ from world to world, or desert to desert, or epoch to epoch. But if you know what they are you can determine a great deal from how crisp or blurred the crater is. If insect or other animal tracks are superposed on the hoofprints, this also argues against their freshness. The subsurface moisture content of the soil and the rate at which it dries out after being exposed by a hoof determine how crumbly the crater walls are. All these matters are closely studied by the IKung.
The galloping herd hates the hot Sun. The animals will use whatever shade they can find. They will alter course to take brief advantage of the shade from a stand of trees. But where the shadow is depends on the time of day, because the Sun is moving across the sky. In the morning, as the Sun is rising in the east, shadows are cast west of the trees. Later in the afternoon, as the Sun is setting toward the west, shadows are cast to the east. From the swerve of the tracks, it’s possible to tell how long ago the animals passed. This calculation will be different in different seasons of the year. So the hunters must carry in their heads a kind of astronomical calendar predicting the apparent solar motion.
To me, all of these formidable forensic tracking skills are science in action.
Not only are hunter-gatherers expert in the tracks of other animals; they also know human tracks very well. Every member of the band is recognizable by his or her footprints; they are as familiar as their faces. Laurens van der Post recounts,
[M]any miles from home and separated from the rest, Nxou and I, on the track of a wounded buck, suddenly found another set of prints and spoor joining our own. He gave a deep grunt of satisfaction and said it was Bauxhau’s footmarks made not many minutes before. He declared Bauxhau was running fast and that we would soon see him and the animal. We topped the dune in front of us and there was Bauxhau, already skinning the animal.
Or Richard Lee, also among the IKung San, relates how when briefly examining some tracks a hunter commented, ‘Oh, look, Tunu is here with his brother-in-law. But where is his son?’
Is this really science? Does every tracker in the course of his training sit on his haunches for hours, following the slow degradation of an eland hoofprint? When the anthropologist asks this question, the answer given is that hunters have always used such methods. They observed their fathers and other accomplished hunters during their apprenticeships. They learned by imitation. The general principles were passed down from generation to generation. The local variations - wind speed, soil moisture - are updated as needed in each generation, or seasonally, or day-by-day.
But modern scientists do just the same. Every time we try to judge the age of a crater on the Moon or Mercury or Triton by its degree of erosion, we do not perform