Science of fear
Aged 5,200, the mummy called Iceman emerged
from an alpine glacier near the Italy-Austria border at 3,200 meters
above sea level. Photo: Courtesy Alex
Susanna, South Tyrol Museum of Archaeology
shows Iceman's discovery location, and his apparent home range,
based on the isotopes found in his body.
Map: Wolfgang Müller, Science
With an arrowhead in his back, the Iceman
had reason to grimace! But where did he call home? Photo courtesy
Alex Susanna,South Tyrol Museum of Archaeology
When the nicely preserved remains of "Iceman" were
discovered at about 3,200 meters above sea level in an Alpine glacier
in 1991, the well-mummified -- actually freeze-dried -- stiff
opened a window into European life 5,200 years ago when copper was
displacing stone in tools.
Clad in leather and grass, shlepping a flint dagger, a medical kit, a copper-bladed ax, and a backpack, Iceman was well-prepared for high-country life.
Iceman was likely grazing animals when he was killed by an arrow in the back. Recent studies of DNA, found the blood of four people on Iceman's clothing. Tom Loy of the University of Queensland, Australia, suspects that one was a companion who tried unsuccessfully to yank out the arrowhead, while three others were probably enemies Iceman had injured or killed.
Despite his excellent state of preservation, most of his DNA was too degraded to analyze. But DNA in the mitochondria -- cellular bodies with their own inheritance patterns -- indicated a central or northern European origin.
This week in Science, researchers described a project that used isotopic analysis to find Iceman's home address. Isotopes are atoms of an element with different numbers of neutrons and thus different masses. Isotopes are godsends to archeology: While isotopes cannot be separated chemically, they sometimes are modified according to mass due to physical processes such as evaporation and condensation. Because different isotopes have different patterns of radioactive decay, the ratio of the "daughter" isotopes betrays the age of rocks or bones they are found in.
Wolfgang Müller, a post-doc in the Research
School of Earth Sciences at Australian
National University, and colleagues looked at oxygen isotopes
in bones and teeth. In bones and teeth, both the common oxygen isotope,
oxygen-16, and the rarer isotope, oxygen-18, come from drinking
water. But while these isotopes are chemically identical, the heavier
O-18 rains out faster from clouds, so the isotope ratio in rain
and drinking water reflects distance from the ocean, the source
of moisture in clouds.
One key question about Iceman was whether he lived north or south of the highland where he became a mummy. Areas to the south are closer to the Mediterranean Sea than areas to the north are to the Atlantic Ocean. Müller's research group found that the oxygen isotope ratios in Iceman's bones reflected today's ratio in surface water to the south.
A second set of analyses concerned isotopes in teeth and bones that originated in radioactive decay of heavy elements. "Different rocks contain different amounts of elements like uranium and rubidium that are naturally radioactive (at very, very low levels) and decay to strontium and lead isotopes," Müller emailed us. "Thus the amount of decay products varies between rock types. Plus, different rocks have different ages, so the 'radioactive' clock started to 'tick' at different times. This is why we see small but characteristic differences in strontium and lead isotopes" in the four major categories of rock in the area.
Because soil forms from the underlying rock type, soil
-- and whatever eats food grown in the soil -- contains isotope
ratios similar to those in the big rock bodies.
From strontium in tooth samples, the researchers concluded that Iceman could not have spent his childhood on land above limestone or basalt. (Teeth represent childhood conditions because they mineralize early in life.)
Next, by looking at argon isotopes in tiny flakes of mica from Iceman's gut, which they assumed he'd recently ingested, the researchers concluded that he'd spent his adulthood on sites above rocks called gneiss, which exist south of the discovery site.
For his childhood, the researchers matched the oxygen-isotope ratios in tooth enamel to waters in a few valleys about 60 kilometers southeast of the discovery site. This area includes a copper-age archeological site Feldthurns, where soils closely match the strontium-lead isotopes in the Iceman's teeth. Feldthurns also features a menhir -- a one-piece stone carving -- typical of copper-age Europe, and it is here, Müller says, that Iceman may have called home, at least while a young sprout.
Although the study demonstrates the precision of isotopic analysis, it only paints a part of Iceman's life, Müller wrote. "Our data are consistent with the Iceman spending one to two months per year (decreasing over his life) as a shepherd herding sheep or goats at pastures at high altitudes, above the timber line. This 'transhumance' has been practiced for about six millennia in the Alps, as deduced from the pollen spectrum in bog profiles, and it is still performed today. The shepherd hypothesis has been a favored hypothesis of the Iceman's occupation for a few years, but it is not certain, of course."
-- David Tenenbaum
Origin and Migration of the Alpine Iceman,
Wolfgang Müller et al, Science, 31 October, 2003.