Probing Molecule Clues to the Past
Scientists analyze ancient biochemical remains for evidence of stone-age people, animals
LIKE an Indiana Jones among the molecules, a new breed of archaeologist is tracing ancient humans through the biochemicals they left behind.
Blood residues on stone tools and in rock art, proteins in fossils and mummified tissue: Such molecular remains hold clues to the identity and activity of stone-age people and of the animals with whom they shared their world. "That gives us a window back into the past that we've never had before," says Thomas H. Loy of the Australian National University at Canberra.
He explains: "We can get back to [specific] moments in time. The way a person held a tool. What they worked on. How the tool was used.... We can examine the blood of the animal and make conclusions about evolution and animal migrations.... We can also do that with human beings." Dr. Loy adds, "Having human blood preserved as residues on stone tools from the distant past ... raises the possibility of direct investigation of the genetic differences and similarities between modern humans and Neanderthals."
The evidence of the molecules may be the only way archaeologists can get at some of this information. Biochemical residues may persist where other relevant evidence fades away. David C. Hyland of Mercyhurst College in Erie, Pa., says, "In those situations where you have acid soils or the like, and you have very little bone preservation so that the faunal situation is unknown, blood residues eluted from stone tools or other artifacts give us another way to get at the issue of what types of fauna were bein g utilized or living in a particular locale at a particular time."
Such is the new science of molecular archaeology. It has emerged so fast over the past decade that Dr. Hyland says the session he organized to discuss it during the American Association for Advancement of Science annual meeting in Chicago in February was "the first time that all of us have sat down in one place at one time."
Its rapid emergence is also a personal triumph for Jerold M. Lowenstein, a specialist in nuclear medicine at the University of California at San Francisco.
Dr. Lowenstein became interested in studying fossil protein 15 years ago when he realized that anthropologists would never sort out the relationships among human ancestors using the few bones and teeth they could find. He wanted to get down to the biochemical details that mark the differences among related species. So he applied to the National Science Foundation for funds.
He explains: "Everyone knew that there were no [protein] fossils left ... therefore, my proposal was rejected. But I continued anyway. And now, of course, this is a very exciting field. And you find all the time that there are, of course, abundant [protein] fossils if you only know how to do them."
Fossil biochemicals generally are present in minute quantities. "Doing" them requires ultrasensitive analytical techniques familiar to molecular biologists but foreign to traditional archaeologists. These include methods for identifying and analyzing the DNA (deoxyribonucleic acid) that carries the genetic information in cells. Some of the most sensitive techniques are based on subtleties of the animal immune system.
In a human or animal body, that system deals with foreign material by making antibodies that neutralize the invader. These antibodies are molecules that seek out and latch onto alien molecules. There are specific antibody molecules for every type of protein molecule.
BIOCHEMISTS use this phenomenon to identify unknown proteins. For example, antibodies to human blood will seek out any residual human blood that may be part of a stain on a stone tool. Likewise, deer-blood antibodies would find any deer blood on the tool. Molecular archaeologists like Lowenstein and Loy, who use these immunoassay techniques, may keep several hundred different antibodies on hand to work with.
Lowenstein points out that, in addition to dried blood, the body's main structural protein, collagen, and the main serum protein, albumin, also are important archaeological molecules. He says they "are, usually, pretty well preserved in many fossils."
What Lowenstein calls "one of the most interesting" of his studies so far involved the Piltdown Man hoax. Paleontologists had accepted a skull unearthed at the Piltdown site in Sussex, England, in 1912 as a "link" between humans and apes. In 1953, it turned out to be a fake made up partly of ape bones. But physiologists could not decide whether the hoaxer used chimpanzee or orangutan bones. Lowenstein tested the residual proteins and found the bones to be orangutan.
One of the oldest objects studied so far is a blood-stained flint tool from a site called Barda Balka in northwestern Iraq. It may be as old as 200,000 years. Loy has identified the blood as human, and Lowenstein has confirmed that identification.
Speculating from that fact, from the presence of bits of wood in the blood, and from the general shape of the tool, Loy suggests the following scenario:
"The people who were using the tool made it [quickly]. It's a hastily made tool, whacked out very simply. They were scraping wood with it - a conifer of some kind. And because both edges were sharp and the way one would hold the tool, they obviously cut themselves while they were working with the tool. And [they] then continued to work while the blood dried long enough to let little bits of wood get in and dry in the blood residue. And then [they] threw it away. And here we are 200,000 years later just p icking it up."
This is not the end of Loy's work with that tool. He notes that there is a deposit of the dried blood that has been sealed away from subsequent handling. He says he hopes to extract complete cells from that sample and study their DNA.
In fact, ancient DNA may become an important marker in tracing migration and development of ancient people. Rock paintings in Australia and many parts of the world are a likely reserve of ancient blood samples, because the artists often mixed their own blood with the pigments.
Loy observes: "The further into the past you go, the less human skeletal remains there are to work with.... But there's rock art spread all over.... And virtually all of the rock art samples that I've looked at have some blood component in them."
Biochemical residues on stone tools, in rock art, and in dry old fossils now are important archaeological evidence.
Lowestein notes that molecular archaeology has already produced much valuable new information. And, he says, "All of this is arising from a field in which all the experts 'knew' there were no fossils to be found, that no proteins were to be found in fossils."