Blood from Stones: Tests for Prehistoric Blood Cast Doubt on Earlier Results
Kaiser, Jocelyn, Science News
Forensic experts aren't the only scientists who mine bloodstains for clues. For more than a decade, archaeologists have been borrowing crime-lab techniques to hunt for ancient blood on scraps of stone.
Using antibodies to detect blood and the species it came from, some researchers have seemingly obtained astonishing results. Margaret E. Newman of the University of Calgary in Alberta and her colleagues reported finding buffalo blood on stone knives at a 5,600-year-old butchering spot in Canada. Thomas H. Loy's team picked up human blood in paint dating to 20,000 years ago on a cave wall in Australia. And at an Iraqi site, Loy says, he detected 180,000-year-old blood spilled by a man whittling wood.
It seems that dirt stuck in the grooves of a stone scraper or a dark spot on a rock slab can reveal such secrets as what creatures early peoples sacrificed and when they turned from hunting to farming.
But just as discoveries of ancient DNA have met with skepticism, researchers' zeal for archaeological blood tests, known as residue analysis, has begun to fizzle. In a recent spate of papers, scientists question not only one another's findings, but whether it's even possible for traces of buried blood to survive thousands of years.
"People are getting very capricious and puzzling and different results," says Christopher Chippindale, editor of Antiquity, a journal on whose pages the debate is unfolding. "There's something in the biochemistry that is giving false positives. That really puts quite a question mark on the various studies."
Loy, now at the University of Queensland in Australia, leads the field in archaeological blood claims, having reported ancient blood on more than 1,000
tools since 1983. Initially, he identified prehistoric hemoglobin, a protein in blood, by crystallizing it. That test has come under heavy criticism, but Loy stands by his results.
When he and others began using immunological tests, they seemed to move to firmer ground. These tests, which detect blood proteins, date back more than 40 years. (Archaeological DNA tests, used since the 1980s, decode genetic material.) To devise a test for, say, deer blood, scientists inject fresh deer blood into a rabbit, which makes millions of antibodies to the blood. The antibodies in rabbit serum, called antiserum, can then be used to search for deer blood.
To test a stone tool for traces of such blood, a researcher would generally wash the tool, then pour the washing extract onto a solid to which the blood proteins stick--a plastic membrane, for example. At that point, he or she rinses the solid with deer antiserum, then with a second antibody that sticks
to the antiserum. Because this second antibody is tagged with a fluorescent molecule or some other marker, it flags any deer blood in the sample.
In practice, the assays are more complicated. Because closely related species have similar blood proteins, the antiserum for, say, elk can react with blood from a deer or cow. So it's necessary to test each antiserum against many other species' blood for cross-reactions and to be aware of these reactions when testing a piece of stone.
The test itself varies from one laboratory to the next. Some people buy commercial antisera, while others make their own. Some testing methods are a thousand times more sensitive than others. An antiserum can be made to react with a single protein, such as albumin or hemoglobin, or even with one region
of a protein instead of the many proteins in whole blood.
A chemist for 27 years, Judith A. Eisele had these things in mind 4 years ago when she began looking at blood residues on tools for an anthropology master's project at the University of Nevada at Reno. Working with biochemist
Roger A. Lewis, she used a dozen antisera, from turkey to bear, to test for blood on more than 150 flaked stone tools from the Southwest.
When only seven tools tested positive for blood and these results proved ambiguous, she tried another experiment. …