The Art of Digging

When it’s potatoes you’re looking for, the best way to get started is to grab a shovel and dig. But when it comes to mining ancient artifacts, a shovel is an archaeologist’s worst enemy.

Over the decades, archaeologists have refined the art of digging. They traded shovels for chopsticks, burlap sacks for fine mesh screens, and measuring tapes for computers. The improvements in their excavation methods result in far better data, and more of it.

During the 1930s and 1940s, most archaeologists subscribed to the potato-digging philosophy of excavation.

“They went in with picks and shovels,” says Curtis Marean, a scientist with the Institute of Human Origins and professor of anthropology at Arizona State University. “They destroyed a lot of stuff. It’s terribly frustrating.”

Klasies River in South Africa is one archaeological site mauled by early excavators. Because its soil layers span many thousands of years, the site is one of South Africa’s most important. Most of its artifacts, however, are lost for good.

“They were looking for hominids,” says Marean. “They were chucking the stuff out. They only kept the very best bones.”

Shovels and other large tools are too big to pick apart finer materials. Metal tools, even dental picks, can scratch and break bone and other objects. Marean uses tools so harmless they couldn’t even hurt a child.

“We use toy shovels,” he says. “You know, the little plastic things. That’s the biggest tool we use. We use chopsticks because the wood doesn’t scar the bone. We use spoons and brushes.”

This more delicate approach reduces the likelihood that researchers will damage any artifacts during the process of unearthing them.

Digging at a finer scale also makes it possible to collect smaller pieces of evidence. Many of these would have been overlooked in earlier days. But sometimes the littlest bits of soil are the most revealing.

“We find beads, little pieces of ochre, the tiny teeth of mice and voles. Micro-mammals are very specific to vegetation types, so their changing frequencies can tell us a lot about how the vegetation is changing,” says Marean. Vegetation and climate are crucial elements of the environment in which modern humans evolved.

Archeological artifacts are meaningful only if their age is known. Often the best indicator of a found object’s age is its location. Like tree rings, new layers of soil accumulate over time. Objects found in deepest strata are therefore the oldest. Items near the top are the newest.

Decades ago, an artifact’s location might not have been recorded at all. Marean now maps the three-dimensional coordinates of every object larger than one centimeter.

Instruments called electronic total stations take the needed measurements. A laser beam emitted by the instrument zaps and maps each item. A computer then directly captures the data.

“It allows you to get very precise measurements very quickly on all your artifacts,” Marean says. “In the past, when you found a bone, you’d get out three tape measures and a plumb bob. Then you’d measure its position in three directions. That’s about a 15-minute enterprise. With the new technology, I shoot that in a second.”

Originally designed for land surveyors, these instruments are a boon to archaeologists.

“In archaeology, we’re scavengers of techniques,” Marean says. “Physics, geochemistry, engineering, whatever it is, we go out and grab it. Then we cannibalize it for our own uses.”

Marean is hoping for the development of new methods to date very old samples. The bone tools, ochre, and soil from the depths of Blombos Cave are too old to be accurately dated using current techniques.

“Ten years from now, we might have a technique to date whatever is in that soil. But radiocarbon dating only goes back 40,000 years,” Marean says. “Otherwise, we could date the bone directly.”

Using radiocarbon dating, scientists can estimate an object’s age from its content of carbon-14, a radioactive, heavy form of carbon.

Until he can directly date very old bone and soil, Marean can measure the age of individual grains of sand. A relatively new technique called luminescence dating detects charged particles called electrons in the sand grains.

Sunlight striking the sand causes it to emit electrons. When a grain of sand is sheltered from the sun, such as in a cave, it is never stimulated to release its electrons. Instead, the electrons accumulate in minute cracks within the sand grain. Over time, more and more electrons build up.

“As soon as it’s exposed to light, those electrons are released again,” Marean explains. The longer a sand grain has been buried, the more electrons it will give off.

Sand samples must be kept in the dark until the moment they are ready to be measured, Marean says. “We pound in a big tube to collect the sand. No sun gets on the sand grains.”

The scientist then sends the sand samples to a laboratory that specializes in luminescence dating. “They are carefully taken back to a dark lab. The electrons are released in a closed environment,” Marean adds.

Blombos Cave’s dune layer proved to be an invaluable dating tool. Using luminescence, the sand was dated to 70,000 years ago. Scientists used this layer as a benchmark for relative dating of the other layers. The sediments below the sand horizon could not be dated directly. However, their position below the sand layer indicates they were older than the sand itself.

One criticism of relative dating is that it is only accurate for undisturbed soils. Breaks in the earth might offer passageways for newer artifacts to fall through from above. The age of such intrusive objects would be overestimated by relative dating.

Marean says that the conditions in Blombos Cave made relative dating very reliable. The dune sand looked much paler than the dark Middle Stone Age soils below, and any breaks in the sandy layer were easy to see.

Only one small area near the back of the cave showed any such disturbance. The two bone tools found below it were excluded from the scientists’ final analyses. The remaining findings were all safely sealed by an unbroken sand ceiling.

Other archaeologists have suggested that formal bone tools were produced in Africa long before they were in Europe. But, unlike Marean’s group, the credibility of those researchers was widely questioned.

“They got a lot of heat,” says Marean. They were criticized mainly because the dates on their findings were equivocal. And old ideas aren’t likely to be let go without a fight.

“There’s very much a Eurocentric bias that’s occurring in the record,” Marean admits. “It’s very hard to knock those things down in science because they become entrenched. People’s careers become based on those ideas. So they’re not willing to give up very easily.”

Marean credits the thoroughness of the excavations and analyses at Blombos for keeping his group’s data resilient to similar criticism. Even some of the staunchest proponents of a European origin for behavioral modernity are now beginning to concede, he says.

“What we’re seeing now is kind of a rear guard action in the process of an orderly retreat. You fight as you back up. But I think we’ll probably get them now.”—Danika Painter