Many people picture human evolution as a straight line—an ape slowly standing taller, becoming Neanderthal, and finally evolving into modern humans. That’s just wrong, and newly discovered fossils in Ethiopia may help to illustrate this fact.

An international team of researchers has discovered nearly 2.8-million-year-old teeth, which reveal that our earliest human relatives (members of the genus Homo) lived side by side with a completely new species of Australopithecus.

“We report the presence of Homo at 2.78 and 2.59 million years ago and Australopithecus at 2.63 million years ago. Although the Australopithecus specimens cannot yet be identified to species level,” the researchers note in their study.

These findings show that human evolution (and evolution by natural selection in general) is not a straight march forward but a tangled story with multiple species belonging to different branches sharing the same landscape.

Volcanic ash revealed the age of teeth

The discovery happened in the Ledi-Geraru region of Ethiopia’s Afar desert, an area that has become famous for producing some of the most important fossils in human history. In 2013, the same site yielded a 2.8-million-year-old jawbone, the oldest fossil ever assigned to our genus Homo. 

Now, researchers have uncovered 13 teeth that shed even more light on this critical period of human origins. At first glance, such fossil teeth might not seem extraordinary, but to paleoanthropologists, they are treasure troves of information. 

The researchers carefully compared the newly discovered teeth with known specimens of Australopithecus afarensis (Lucy’s species) and early Homo. They found that these teeth belonged to both early Homo and a completely new kind of Australopithecus, never seen before. However, determining the age of these fossils was important to confirm this. 

The Afar region is riddled with volcanoes, past and present. When ancient eruptions blanketed the land in ash, they left behind tiny crystals of feldspar that can be dated very precisely. By analyzing the volcanic ash layers above and below the fossil beds, the study authors pinned down the fossils’ ages between 2.6 and 2.8 million years old. 

The known specimen A. afarensis had already vanished by this time. There is no evidence of Lucy’s kind younger than 2.95 million years ago. Therefore, this further confirmed that the teeth belonged to a completely new Australopithecus species. Moreover, the dating method gave researchers confidence that early Homo and the new Australopithecus truly coexisted at the same time and place. 

“These specimens suggest that Australopithecus and early Homo co-existed as two non-robust lineages in the Afar Region before 2.5 million years ago, and that the hominin fossil record is more diverse than previously known,” the study authors added.

The fossils also hint at the environment these early hominins lived in. Today, Ledi-Geraru looks like a dry, broken landscape of cliffs and gullies. 

However, according to the study authors, 2.7 million years ago, it was a patchwork of rivers, grassy areas, and shallow lakes that expanded and shrank with time. Such a shifting landscape could have encouraged diversity, allowing multiple species to carve out different ecological niches.

Human evolution is twisted and has many dead ends

This discovery reshapes how we think about our evolutionary story. It shows that early humans were not the only hominins walking the Afar plains. Instead, several branches of the human family tree were growing at the same time, competing for resources, or perhaps finding ways to share them. 

Evolution is often described as a tree rather than a straight line with many branches sprouting, but only some species survive. Possibly, there are many other extinct Homo species that remain unknown to us. 

For now, the study authors have yet to find the answers to many questions related to their discovery. For instance, they don’t yet know what to call the new Australopithecus species, because teeth alone aren’t enough to define it fully. 

They also don’t know how these early humans and their cousins interacted. Were they eating the same foods or different ones? Did they compete directly, or simply pass each other by? The study authors are now studying the microscopic structure of tooth enamel to find out what these species ate, which could answer some of these questions.

The study is published in the journal Nature.