Fossils show two types of ancient human ancestors lived at the same place and time. One was possibly an unknown species

Ancient, fossilized teeth, uncovered during a decades-long archaeology project in northeastern Ethiopia, indicate that two different kinds of hominins, or human ancestors, lived in the same place between 2.6 million and 2.8 million years ago — and one of them may be a previously unknown species.

The discovery provides a new glimpse into the complex web of human evolution. Ten of the teeth, found between 2018 and 2020, belong to the genus Australopithecus, an ancient human relative. Meanwhile, three teeth, found in 2015, belong to the genus Homo, which includes modern humans, or Homo sapiens. The results were published Wednesday in the journal Nature.

Such an overlapping of two hominins in the fossil record is rare, which had previously led scientists to believe that Homo appeared after Australopithecus, rather than the two being contemporaries. Australopithecus species walked upright much like modern humans, but had relatively small brains, closer in size to those of apes. The emergence of Homo species, with their larger brains, is easy for people today to view as some sort of evolutionary upgrade on a path to modern humanity.

But the coexistence of the two demonstrates that hominins developed, and lived, in multiple varieties at once. “This new research shows that the image many of us have in our minds of an ape to a Neanderthal to a modern human is not correct — evolution doesn’t work like that,” said study coauthor Kaye Reed, research scientist and president’s professor emerita at the Institute of Human Origins and emeritus professor at the School of Human Evolution and Social Change at Arizona State University, via email. “Here we have two hominin species that are together. And human evolution is not linear, it’s a bushy tree, there are life forms that go extinct.”

Since 2002, Reed has been a codirector of the Ledi-Geraru Research Project, which is focused, in part, on searching for evidence of early Homo species. In 2015, the team announced the discovery of the oldest known Homo jawbone at 2.8 million years old. It has also searched for later evidence of Australopithecus afarensis, which first appeared 3.9 million years ago, but there is no sign of these ancient human relatives in the fossil record after 2.95 million years ago — suggesting they went extinct before Homo’s first appearance.

Australopithecus afarensis is best represented by the famed fossilized remains of Lucy, discovered in 1974 in Ethiopia. Lucy was shorter than an average human, reaching about 3.3 feet (1 meter) in height, had an apelike face and a brain about one-third the size of a human brain. Her fossil showcased a mixture of humanlike and apelike traits and provided proof that ancient human relatives walked upright 3.2 million years ago.

When the team discovered the Australopithecus teeth during two separate digs in 2018 and 2020, it compared them with species such as afarensis and another hominin group known as garhi, but they didn’t match up. Instead, the scientists believe the teeth belong to a previously unknown species of Australopithecus that walked the Earth after Lucy — and alongside an early Homo species.

“Once we found Homo, I thought that was all we would find, and then one day on survey, we found the Australopithecus teeth,” Reed said. “What is most important, is that it shows again, that human evolution is not linear. There were species that went extinct; some were better adapted than others, and others interbred with us — we know this for Neanderthals for sure. So anytime that we have another piece to the puzzle of where we came from, it is important.”

Cracks in Earth’s surface

The teeth were found in Ethiopia’s Afar region, a key place for researchers seeking answers about human evolution. A variety of preserved fossils have been found there as well as some of the earliest stone tools, Reed said.

The Afar region is an active rifting environment — the tectonic plates beneath its earth are actively pulling apart and exposing older layers of sediment that shed light on almost 5 million years of evolution, Reed said.

“The continent is quite literally unzipping there, which creates a lot of volcanism and tectonics,” said study coauthor Christopher Campisano, associate director and associate professor at the Institute of Human Origins and associate professor at the School of Human Evolution and Social Change at Arizona State, in a video the school released. “At 2 1⁄2, 3 million years ago, these volcanoes spewed out ash that contain crystals called feldspars that allow us to date the eruptions that were happening on the landscape when they’re deposited.”

The Australopithecus teeth documented in the new study were dated to 2.63 million years ago, while the Homo teeth are from 2.59 million and 2.78 million years ago. But the team is cautious about identifying a species for any of the teeth until it has more data and more fossils.

“We know what the teeth and mandible of the earliest Homo look like, but that’s it,” said Brian Villmoare, lead study author and associate professor in the department of anthropology at the University of Nevada, Las Vegas, in a statement. “This emphasizes the critical importance of finding additional fossils to understand the differences between Australopithecus and Homo, and potentially how they were able to overlap in the fossil record at the same location.”

The Australopithecus teeth broadly resembled those of the afarensis species in contour and the size of the molars, but features of the cusps and canine teeth had not been previously seen in afarensis or garhi teeth, Villmoare said. The teeth were also different in shape than those of any Homo species, or of the ancient human relative Paranthropus, known for its large teeth and chewing muscles.

“Obviously these are only teeth,” Villmoare said, “but we are continuing field work in the hopes of recovering other parts of the anatomy that might increase resolution on the taxonomy.”

Even just finding the teeth was a complicated task, according to Campisano.

“You’re looking at little teeth, quite literally, individual teeth that look just like a lot of other of the little pebbles spread on the landscape,” he said in the video. “And so, we have a great team of local Afars that are excellent fossil hunters. They’ve seen these things their entire lives walking around the landscape.”

A blip for evolution

The new study is important because it provides insight into a time frame from 3 million to 2 million years ago, a mysterious period in human evolutionary studies, said Dr. Stephanie Melillo, paleoanthropologist and assistant professor at Mercyhurst University in Pennsylvania. Melillo was not involved in this research, but she has participated in the Woranso-Mille Paleontological Research Project in the Afar Triangle of Ethiopia.

Part of the problem in learning about this stretch of prehistory is how ancient layers of dirt were deposited over the course of history in eastern Africa.

“Erosion in rivers and lakes were at a low level and only a little bit of dirt was deposited in the Afar,” Melillo wrote in an email. “That deposited dirt contains the fossils — of our ancestors and all the animals that lived with us. When there is little deposition, there are few fossils.”

A key feature helping archaeologists to understand humanity’s evolution are structural basins, or “bowls” on Earth’s surface that naturally collect layers of sediment better than the surrounding landscape does — like the Turkana Basin stretching across southern Ethiopia and northern Kenya, Melillo said. Previous research has found evidence to suggest that Homo and Paranthropus coexisted there 1.5 million years ago.

The new study focuses on the Afar Depression, a basin to the north of the Turkana.

“This contribution by Villmoare and colleagues demonstrates that in the Afar there was also some other species around with Homo — but it isn’t Paranthropus,” Melillo said. “Instead, they identify this ‘non-Homo’ genus as Australopithecus. They do a very convincing job of demonstrating why the new fossils are not Paranthropus.”

The study adds to growing evidence that Australopithecus was not roaming the Afar Depression alone, she said.

A mysterious coexistence

When Australopithecus and Homo were alive, the Afar Region, now mostly a semidesert, had much more seasonal variation in rainfall than it does today, Reed said. Millions of years ago, the environment there was still dominated by a dry season, but it was interrupted by a brief wet season. Rivers that carried water across the landscape existed for only part of the year. Few trees grew near the river, and the environment nearby was largely wetlands and grasslands.

“We have a fossil giraffe species that was eating grass, which probably indicates they were stressed as they eat trees and bushes almost every place else,” Reed said. “Were the hominins eating the same thing? We are trying to find out by examining isotopes in their teeth and microscopic scratches on their teeth.”

Understanding whether or not Homo and Australopithecus had the same food sources could paint a portrait of how our ancient ancestors shared or competed for resources, Reed said. The team also wants to try to identify which hominin made the stone tools found at the site.

At the moment it’s impossible to tell exactly how the two hominins coexisted, but Reed said she is hoping that future findings will provide more answers.

“Whenever you have an exciting discovery, if you’re a paleontologist, you always know that you need more information,” Reed said. “You need more fossils. More fossils will help us tell the story of what happened to our ancestors a long time ago — but because we’re the survivors we know that it happened to us.”

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