In 1974, paleoanthropologist Donald Johanson and his team found the partial remains of a 3.2 million-year-old hominin skeleton in Ethiopia. This famous discovery, which they nicknamed “Lucy,” was later classified as part of a new family of ancient human ancestors called Australopithecus afarensis. For years, scientists have believed that Lucy and her ilk were the only bipedal creatures living during the middle Pliocene, but a recent fossil find has shown that another ancient hominin was also traipsing across the African plains. Even more amazing, the two species were neighbors.

The breakthrough comes courtesy of a team led by Dr. Yohannes Haile-Selassie of the Cleveland Museum of Natural History. In a new article in the journal Nature, they detail their discovery of jaws and teeth belonging to a previously unknown species of ancient human they call “Australopithecus deyiremeda.” The fossils were found in March 2011 in a region only 20 miles from where Lucy once lay, and they have since been dated to be between 3.3 and 3.5 million years old. That means that Lucy’s species and A. deyiremeda once lived alongside one another and may have even crossed paths.

“The new species is yet another confirmation that Lucy’s species, Australopithecus afarensis, was not the only potential human ancestor species that roamed in what is now the Afar region of Ethiopia during the middle Pliocene,” Dr. Haile-Selassie said in a statement. “Current fossil evidence from the Woranso-Mille study area clearly shows that there were at least two, if not three, early human species living at the same time and in close geographic proximity.”

Australopithecus deyiremeda—a name meaning “close relative” in the Afar language—differs from Lucy and other afarensis specimens in several key ways. “This new species has very robust jaws,” Haile-Selassie told the BBC. “In addition, we see this new species had smaller teeth. The canine is really small—smaller than all known hominins we have documented in the past.” A. deriyemeda’s teeth also boast a thicker enamel than other species from the time, which suggests it may have consumed a richer and more human-like diet than Lucy.

While it’s long been agreed that modern humans and Neanderthals lived at the same time, such variation was not believed to have been present in their more ape-like ancestors. A. deyiremeda now joins an ever-growing list of hominin discoveries that challenge that understanding of human evolution. The new finds show that Lucy’s species may have been just one of several that existed at the same time. They include the flat-faced Kenyanthropus platyops, found in Kenya in 1999, and Australopithecus bahrelghazali, which was unearthed in Chad in the mid-1990s. Haile-Selassie, meanwhile, has previously found 3.4 million-year-old foot bones in Ethiopia that don’t match those of any known species.

“Historically, because we didn’t have the fossil evidence to show there was hominin diversity during the middle Pliocene, we thought there was only one lineage, one primitive ancestor—in this case Australopithecus afarensis, Lucy—giving rise to the next,” he told the BBC. “That hypothesis of linear evolution has to be revisited.”

Not all scientists are eager to embrace such a crowded family tree. Since male and female fossils can look very different, some have argued that A. bahrelghazali and other finds are merely examples of variation within Lucy’s A. afarensis species. Dr. Tim D. White, who worked with Donald Johanson in analyzing Lucy, believes the new discovery could be a similar case of mistaken identity. “Lucy’s species just got a few more new fossils,” he told the New York Times. “A piece of a mandible doesn’t tell you much. Whenever you have small samples, you run a very real risk of mischaracterization.”

Haile-Selassie says he welcomes debate, but he maintains that the new fossils are incontrovertible evidence that more than one species existed simultaneously. “This new species from Ethiopia takes the ongoing debate on early hominin diversity to another level,” he said. “Some of our colleagues are going to be skeptical about this new species, which is not unusual. However, I think it is time that we look into the earlier phases of our evolution with an open mind and carefully examine the currently available fossil evidence rather than immediately dismissing the fossils that do not fit our long-held hypotheses.”

Haile-Selassie agrees that more research is needed to shed light on what life might have been like in an ancient Africa populated by several potential human ancestors. “What remains intriguing,” he and his co-authors write in their paper, “and requires further investigation, is how these taxa are related to each other and to later hominins, and what environmental and ecological factors triggered such diversity.” With the help of further fossil discoveries, the researchers expect to learn if the various species interacted with each other or competed for food and resources. More importantly, they hope to find out which one in particular is humanity’s direct ancestor. “The question that is going to come up is which taxa gave rise to our genus, Homo,” Haile-Selassie said in an interview with “Nature.” “That’s going to be the 64-million-dollar question.”