Replicating the History of Human Evolution

Discovering a multi-million year old human fossil may represent a truly one-of-a-kind look into the history of the human species, but even the most well preserved specimen is not much use in a vacuum. Fossils must be compared to each other to give scientists insight into where they fit in the long progression to humans of today, and here, the rarity of such finds are a real obstacle.

Janet Monge has a solution. As the associate director and manager of the Casting Program at the University of Pennsylvania Museum of Archaeology and Anthropology, she and her volunteer assistants make replicas of these priceless fossils that are identical at up to 4,000 times magnification.

The Penn Museum has more than 3,000 casts of fossilized human remains, more than any institution on the planet. Such casts have become vital tools for anthropological research, because original artifacts and fossil remains no longer leave the countries where they are discovered.

"Those fossils are considered to be national treasures of their countries of origin," Monge said. "The only way I could compare Neanderthals in Croatia to Neanderthals in France is if I have an accurate casting."

The casting process is more complicated than simply making one copy; the objective is to make a "backup" of the fossil that can be substituted for the original when making future molds.

"Our objective isn’t to simply reproduce fossils but to reproduce them over and over again," Monge said.

The first step is to capture all of the fine detail of the original specimen in special metallic clay, which can be worked into the fossil’s nooks and crannies and then be removed without damaging its surface.

Casters then make silicone molds from the clay impressions. These molds are flexible and tough but can only make a few copies of the fossils before breaking down. The casters therefore first make a durable plasticine "pattern" from small sections of the silicone mold. Once completed, the patterns enable the casters to reform such silicone molds as necessary. The final casts are made from epoxy, which is poured into the silicon molds, clamped shut in plaster blocks.

The process is time-consuming and painstaking, since microscopic fidelity to the original is necessary for the cast to be considered a success.

"We make 10 rejects for each final cast, and we’re really good at this," Monge said. "You have to be a bit of an anatomist, a bit of a biochemist and a bit of an artist, all at the same time."

The final casts are painted and sold to other museums or anthropologists, though a large amount remain in the Penn Museum for use in research and exhibits. The museum’s "Human Evolution: The First 200 Million Years" exhibit has more than 100 such casts on display.

And because these specimens can simply be recast if they are ever damaged or destroyed, they don’t need to be kept behind glass. Museum visitors can feel with their own hands the changes in the size and shape of the human skull through the millennia.

Video by Kurtis Sensenig
Photos by Steven Minicola