Mudflats in the shadow of the Ol Doinyo Lengai volcano captured a huge trove of ancient human footprints.
Photograph by Robert Clark, National Geographic Creative
Nine miles from the volcano the Maasai call the "Mountain of God," researchers have cataloged a spectacularly rare find: an enormous set of well-preserved human footprints left in the mud between 5,000 and 19,000 years ago.
The more than 400 footprints cover an area slightly larger than a tennis court, crisscrossing the dark gray mudflat of Engare Sero, on the southern shore of Tanzania's Lake Natron. No other site in Africa has as many ancient Homo sapiens footprints—making it a treasure trove for scientists trying to tell the story of humankind's earliest days.
Some of the tracks seem to show people jogging through the muck, keeping upwards of a 12-minute-mile pace. Other prints imply a person with a slightly strange, possibly broken big toe.
Yet more tracks suggest that around a dozen people, mostly women and children, traveled across the mudflat together, striking toward the southwest for parts unknown. The mud tracked it all—including the dirty droplets that fell from their feet with each step.
"The first time we went out there, I remember getting out of the vehicle, and I teared up a little bit," says Cynthia Liutkus-Pierce, the Appalachian State University geologist and National Geographic grantee who led the research.
"Human origins is a huge interest of mine: where we came from, and why we are who we are. It was definitely emotional to see our own history in this."
The Engare Sero tracks add to an incredibly exclusive catalog of human footprints that have stood the test of time. Australia's Willandra Lakes site, for instance, has 700 fossil footprints made about 20,000 years ago. And two sites on the South African coast have Homo sapiens tracks dated as far back as 120,000 years ago.
Laetoli—a site in Tanzania some 60 miles southwest of Engare Sero—even has 3.6-million-year-old footprints possibly made by the human ancestor Australopithecus afarensis, a discovery partially funded by the National Geographic Society.
Engare Sero is exciting because of the abundance and diversity of prints, which offer a strikingly detailed snapshot of what life was like for our ancestors in Africa.
"It's a very complicated site," says William Harcourt-Smith, a paleoanthropologist at the City University of New York and a member of Liutkus-Pierce's team. "There's one area where there are so many prints, we've nicknamed it the 'dance hall,' because I've never seen so many prints in one place. It's completely nuts."
Picking Up the Trail
The Engare Sero site—and the researchers who excavated it—owe a great deal to Ol Doinyo Lengai, the volcano that looms over Lake Natron. The 7,650-foot-tall peak, known for its bizarrely thin, silvery lava, is a place of pilgrimage for the pastoralist Maasai, who travel there to entreat their god Engai for rain, cattle, and children.
Fittingly, the modern pilgrimage site also captured the wanderings of ancient humans. Liutkus-Pierce and her team think that the ash-rich mud containing the footprints originally washed off Ol Doinyo Lengai's flanks, making its way downhill to form the mudflats.
In a matter of hours to days, the mud's surface dried out, preserving the prints in a cracked crust. Another flow of debris then buried the footprints at least 10,000 to 12,000 years ago, entombing them for millennia.
Local villager Kongo Sakkae found some of the footprints prior to 2006, but the site didn't reach scientists' attention until 2008, when Pennsylvania-based conservationist Jim Brett happened to be staying at the Lake Natron Tented Camp, just a few hundred yards from the footprints.
Stunned by what he saw, Brett snapped as many pictures as he could and resolved to pass them along to a scientist he knew he could trust: Liutkus-Pierce, whom he had met when she was a postdoctoral researcher.
The trouble was, Brett picked the worst possible day to call.
"It was April Fool's Day, I kid you not," says Liutkus-Price. "He called me and said, 'I think I have found some really cool hominid footprints.' And I said, 'Jim, can you call me tomorrow, so I know that this is not a joke?'"
The sheer number of footprints that were all created at essentially the same time allows us to directly investigate social aspects of the lives of these ancient humans.
Several days later, Liutkus-Pierce saw Brett's photographs and was awestruck by the quality of preservation. In short order, she recruited a diverse team of scientists to the cause, including her friend and former classmate, paleoanthropologist Briana Pobiner of the Smithsonian Institution's Human Origins Program.
"I was thrilled to learn the scope of the project—human footprint sites are so rare, and it has been exciting to be involved in working on one of them," Pobiner wrote in an email. "The sheer number of footprints that were all created at essentially the same time allows us to directly investigate social aspects of the lives of these ancient humans."
Surprisingly Rocky Road
But unlocking Engare Sero's secrets—particularly, pinpointing when humans walked across the mud—proved to be an enormous challenge, as the team explains in a study recently published in Palaeogeography, Palaeoclimatology, Palaeoecology.
Originally, Liutkus-Pierce and her team thought that the mud that captured the footprints began as ash that had rained down after Ol Doinyo Lengai erupted. If that were true, the ash would be essentially the same age as the footprints themselves—an approach that initially suggested the prints were about 120,000 years old. The team announced this possible age at a conference in 2011, raising excitement but also stirring up some debate over the interpretation.
However, once the team realized that the ash had been carried to Engare Sero by water, determining the footprints' age morphed into a punishing game of finding the mud's youngest crystals, which would put a cap on how old the site can reasonably be.
A shell found in the mud above the footprints helped the team determine the youngest possible end of the range, ultimately placing the prints at a more conservative age somewhere between 5,000 and 19,100 years old.
"It's not a smash, easy type of solution to the problem [of dating the site], but at least they have some things they can work with," says Craig Feibel of Rutgers University, an expert on sedimentary geology who wasn't involved with the study. "[Volcanic sediments] can be very, very tricky, and I think they did a nice job of it."
Still, delays in getting the site's dates had frayed the team's nerves, and even now that the geologists have narrowed down the dates, the anthropologists' analysis remains stalled.
Then, in early 2016, the researchers were blindsided by reports that team member Brian Richmond was under investigation for allegedly assaulting a female research assistant at a 2015 conference. The American Museum of Natural History, Richmond's employer, opened an independent investigation of the allegation that is still ongoing, the museum confirmed in a statement. In previous reports, Richmond has maintained his innocence, and he did not respond to multiple requests for comment for this article.
Liutkus-Pierce says that Richmond has never harassed her, and that the field work at Engare Sero was "a wonderful experience" that went without incident. Richmond remains a listed co-author on the newly published study.
Taking the Next Step
Now that the researchers have published their study of Engare Sero's enigmatic geology, Pobiner and Harcourt-Smith are hopeful that the paleoanthropology analysis isn't far behind. The researchers say that it will resemble a 2012 presentation of their early results, which identified at least 24 tracks, some possible joggers, and a group of more than a dozen people traveling together.
Liutkus-Pierce is also looking into ways to preserve Engare Sero in the long term—a need driven home by the tire tracks that crisscrossed the site when the team first arrived.
For now, the Tanzanian government has cordoned off the site with barbed wire. But even in the worst-case scenario, future scientists will be able to see what Liutkus-Pierce saw eight years ago. With the help of the Smithsonian, the team has created 3-D scans of the entirety of Engare Sero, dance hall and all.
"God forbid anything happens to that site," says Liutkus-Pierce, "[but] we essentially have the ability to replicate it with 3-D printing."