Fossilized teeth unearthed in northeastern Ethiopia over a lengthy archaeological investigation suggest that two different types of early human ancestors lived side by side between 2.6 and 2.8 million years ago—one possibly being a yet-undiscovered species.

This finding offers a new perspective on the complex paths of human evolution. Ten of the found teeth, collected between 2018 and 2020, are associated with the ancient genus Australopithecus, while three teeth recovered in 2015 come from the Homo genus, which includes modern humans. These results were published in the journal Nature.

It's relatively uncommon in fossil evidence to see such an overlap of hominin types. Previously, researchers believed the Homo genus emerged after Australopithecus had disappeared. Australopithecus species walked upright but had brain sizes similar to apes. Homo species, by contrast, are characterized by significantly larger brains, which is often interpreted as evolutionary progress.

However, this discovery suggests that various forms of early humans existed simultaneously. “This research shifts our thinking—evolution isn’t a smooth transition from ape to modern human,” said Kaye Reed, a coauthor and professor emerita at Arizona State University’s Institute of Human Origins. “Here we find two distinct hominin species in the same environment, reinforcing the idea of human evolution as a branching tree rather than a straight line.”

Understanding Lucy’s World

A key figure in this study, Kaye Reed, has co-led the Ledi-Geraru Research Project since 2002, focusing on spotting early Homo fossils. The group's 2015 discovery of the oldest known Homo jawbone, dated at 2.8 million years, was a significant milestone. Notably, while researchers have tried finding later specimens of Australopithecus afarensis—first appearing around 3.9 million years ago—there's no evidence they survived past 2.95 million years ago. This suggests they went extinct before Homo emerged.

Australopithecus afarensis is best known through a famous fossil nicknamed Lucy, discovered in Ethiopia in 1974. Lucy stood just over 3 feet tall, had a small brain comparable to that of apes, and displayed a mix of primitive and modern features, including walking upright. She lived roughly 3.2 million years ago.

The teeth found in 2018 and 2020 didn’t fit existing Australopithecus species like afarensis or garhi. Researchers now believe they may have discovered a previously unrecognized hominin species that lived after Lucy and shared space with early Homo individuals.

“We assumed Homo was all we’d find, but then we came upon these Australopithecus teeth,” Reed explained. “It reinforces the idea that human evolution had many parallel paths, some of which didn’t survive. We even know some of these ancestors interbred with us. With each new find, we get closer to understanding our origins.”

Shifting Earth Reveals History

The teeth were recovered from Ethiopia’s Afar Region, a central location for evolutionary studies. This landscape has yielded numerous fossils and some of the oldest known stone tools.

Afar is in an area where tectonic plates are gradually pulling apart, exposing ancient sediment layers that preserve traces of nearly 5 million years of evolutionary history.

“The continent is literally pulling apart here, creating volcanic and tectonic activity,” said Christopher Campisano, a coauthor and associate professor at Arizona State University. “Around 2 to 3 million years ago, volcanic eruptions spread ash across the land. This ash contains crystals that help us determine the age of these layers.”

The recently studied Australopithecus teeth were dated to about 2.63 million years ago, while the Homo teeth date from 2.59 to 2.78 million years ago. Scientists are taking a cautious approach, avoiding precise species identification until more fossil evidence is collected.

“We understand what early Homo teeth look like, but that’s a narrow picture,” said Brian Villmoare, lead author and professor at the University of Nevada, Las Vegas. “Finding more bones is essential to distinguish how Homo and Australopithecus differed and how they may have lived side by side.”

The Australopithecus teeth resemble afarensis in shape and molar size, but the detailed features of the canines and cusps show unique traits not seen in afarensis or garhi, Villmoare noted. Additionally, their differences from Homo and Paranthropus teeth suggest they come from a distinct group.

“Yes, these are only teeth,” he added, “but we continue excavating in hopes of discovering more skeletal remains that can help clarify their classification.”

Finding these teeth was no simple task. “They’re small and can easily be mistaken for pebbles,” Campisano remarked. “We rely on local Afar residents, who’ve grown up here and have excellent fossil-spotting expertise.”

A Window into a Forgotten Era

This period, stretching from 3 million to 2 million years ago, remains one of the more mysterious parts of human history, said Dr. Stephanie Melillo, paleoanthropologist and professor at Mercyhurst University in Pennsylvania. Though not part of this research, she’s worked on related projects in the same region.

In that ancient time span, only a small amount of sediment accumulated in eastern Africa, limiting the number of preserved fossils.

“Most natural processes like rivers and lake deposition were minimal in the Afar during this era,” Melillo explained. “Less sediment means fewer fossils of our ancestors and their surroundings were preserved.”

Some regions, such as the Turkana Basin, collect sediment effectively and have helped researchers discover that Homo likely coexisted with Paranthropus about 1.5 million years ago.

This study turns focus to the Afar Depression, just north of the Turkana. “This work shows that not only Homo lived in the Afar back then, but so did another genus — Australopithecus,” Melillo stated. “They make a strong case that the new fossils are not from Paranthropus.”

The findings deepen the view that Australopithecus wasn't alone in the Afar Region during this era, she added.

Life in a Shared Landscape

During the time Australopithecus and Homo coexisted, the Afar landscape was quite different from today’s arid terrain. According to Reed, it experienced alternating dry and brief wet seasons. Rivers flowed only part of the year, and the surrounding environment offered mostly wetlands and open grasslands.

“We’ve even found a type of giraffe that was likely struggling to find food, as it was grazing on grass rather than browsing leaves,” said Reed. “We’re now examining whether the ancient hominins were competing for similar resources by analyzing the chemical and microscopic features of their teeth.”

Understanding the dietary habits of Australopithecus and Homo could reveal how they coexisted, whether through direct competition or separate ecological niches. The team also aims to link specific stone tools found at the site with their respective makers.

While it is not yet possible to determine exactly how these hominins interacted, Reed is optimistic that ongoing and future discoveries will help piece together the broader story of human origins.

“When you make a remarkable discovery, you always know it’s just the beginning,” said Reed. “We rely on more fossils, more data, to uncover the truth about our past. We’re here today because one line survived—ours.”