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By Alan Boyle

Lucy and her kin weren't the only human ancestors to live in modern-day Ethiopia 3.3 million years ago, scientists say. They report that a different species, similar but not identical to Lucy's, was living nearby during the same time frame.

The study, appearing in this week's issue of the journal Nature, claims to provide "incontrovertible" evidence that multiple species of human ancestors, known as hominins, co-existed in east Africa in the age before the rise of our own species.

The newly named species is called Australopithecus deyiremeda, and is considered a close relative of Lucy's species, Australopithecus afarensis. In fact, the species name "deyiremeda" comes from the local Afar language's terms for "close" (deyi) and "relative" (remeda).

"Some colleagues might be skeptical about this species," lead author Yohannes Haile-Selassie, a paleoanthropologist at the Cleveland Museum of Natural History, told NBC News in an email. "However, in terms of the presence of multiple hominins during the Middle Pliocene, their doubts end."

That's because Haile-Selassie and his colleagues previously found the traces of yet another hominin species in Ethiopia's Afar region — the foot bones of a creature that was unlike Lucy, with opposable big toes. The researchers were following up on that discovery when they came upon the fossil teeth and jawbones that they now attribute to Australopithecus deyiremeda.

"When you are out in the field, you don't look for something specific, although you go out with some research questions in hand," Haile-Selassie said. "The specimens now assigned to the new species were found during the last two days of our 2011 field season. They were found in a sedimentary patch we never surveyed before."

The discovery site was just 22 miles (35 kilometers) from the place where Lucy's skeleton was found in 1974.

The team reconstructed the shape of the skull and determined that the creature's face would look different from Lucy's. "A. deyiremeda would have a less projecting muzzle," Haile-Selassie said.

He said the facial characteristics overlapped not only with those of Lucy, one of the world's best-known fossils of a human ancestor, but also with several other species found in east Africa — such as Kenyanthropus platyops and early specimens from our own genus, Homo. That means some of the characteristics associated with Homo may have arisen earlier than previously thought.

Yohannes Haile-Selassie of the Cleveland Museum of Natural History compares casts of fossil bones.Laura Dempsey / CMNH

The picture that emerges from this and other discoveries is far more complex than the stereotypical evolutionary chart that shows a linear progression from one human ancestor to the next. The traditional "tree" of human origins is looking more and more like a tangled bush, with a wide spectrum of similar species living alongside each other.

"Finding such taxonomic diversity raises the question of how multiple species could have co-existed over a long period in a stable ecosystem, particularly when they live in close geographic proximity," Fred Spoor, an evolutionary biologist who was not involved in the latest find, wrote in a commentary published in Nature.

Spoor suggested that the species might have been set apart in slightly different environmental niches, due to contrasting preferences in diet or habitat — for example, meat vs. vegetables, or forest vs. grassland.

He also cautioned that the claim that Australopithecus deyiremeda is truly a new species, distinct from previously identified types of hominins, "must be confirmed by further study and fossil finds."

Just last week, a different research team reported the discovery of 3.3 million-year-old stone tools at a site known as Lomekwi 3, on the western shore of Lake Turkana in northern Kenya — and scientists can't yet determine which species made those tools. Spoor says it could be Australopithecus deyiremeda, while Haile-Selassie thinks that Kenyanthropus platyops is the likelier suspect.

The fact that fresh finds — and fresh mysteries — are coming to light in east Africa 40 years after the discovery of Lucy's skeleton demonstrates that the story of human origins is far from finished.

In addition to Haile-Selassie, the authors of "New Species From Ethiopia Further Expands Middle Pliocene Hominin Diversity" include Luis Gilbert, Stephanie Melillo, Timothy Ryan, Mulugeta Alene, Alan Deino, Naomi Levin, Gary Scott and Beverly Saylor.