Ever since Darwin, the researchers who study human evolution have been preoccupied with the characteristics that set humans apart from the other great apes: our large brains, bipedalism, hunting, abstract thought, and the use of technology.

It’s long been assumed that these features came bundled together, according to William Kimbel, Director of Arizona State University’s Institute of Human Origins. Kimbel conducts fieldwork studies in East Africa in search of the origins of our species, and he thinks that shedding the assumption of the “package deal” can open up new clues about the origin of our lineage.

Without the benefit of a fossil record, it’s reasonable to think that these features evolved in tandem, but “it’s a model that remains firmly entrenched in paleoanthropology today,” Kimbel told Ars Technica—even though a picture is starting to emerge that looks very different.

As one intriguing fossil discovery after another has made headlines over the past year, our understanding of our species’ history has started to shift, and a new story is emerging: one where our extinct relatives share many of the traits we had thought were uniquely human, and our own species is not that special after all.

The dark, foggy dawn of Homo

One of the biggest question marks hanging over our understanding of human evolution has been the emergence of the genus Homo. This is the group of hominids (great apes) that includes our own species, Homo sapiens, as well as other, extinct species of human like Neanderthals, Homo erectus, and Homo habilis.

In March of this year, the journal Science published a paper describing the discovery of a 2.8 million-year-old jawbone in Ledi-Gararu, Ethiopia, that shed some light on this dark patch in our history. “It is part of a jaw; it’s not a whole skull, and it’s not a skeleton,” said Kimbel, one of the authors of the paper. But “in this time period, we celebrate the smallest advances, because we know so little.”

Fossil records in Africa between 2.5 and 3 million years ago (mya) are extremely limited. Earlier than 3 mya, the record is pretty good, with a substantial body of evidence contributing to our knowledge of the genus Australopithecus (best known as the genus including the famous Australopithecus afarensis Lucy). By around 2 mya, there’s “evidence of several species that most people would attribute to Homo: H. habilis, H. erectus, and H. rudolfensis,” Kimbel explained.

In between, there are some Australopithecus fossils, but there’s been very little evidence discovered that could explain where and when the first few Homo species appeared. The blank period is so large that it’s not even clear what we can expect it to contain. Bipedalism is already accounted for, having emerged around 4 mya, said Kimbel. This was the first feature to be removed from the “package deal” of distinctive human characteristics identified by Darwin.

Maybe we could expect to see a difference in brain size, but we know that some early Homo specimens still have notably small brains, Kimbel added. He had expected that the most promising evidence to be found in the blank period would appear as changing shapes of jaws, teeth, and skulls, and that we shouldn’t focus too much on looking for brain size, technology use, and increased meat consumption evolving in lockstep.

The Ledi jaw was found in a rock layer that was placed at 2.75 to 2.8 million years old by radiometric dating, and there’s little debate about its age. Rather, the debate centres around the interpretation of the characteristics of the jaw, and what they mean for placing it on the human family tree.

The region and the time period are exactly where we’d expect to find the emergence of Homo. Tim White, a researcher in human evolution and integrative biology at the University of California, Berkeley who wasn’t involved in the research, told Ars that the interpretation of the existing evidence has been that something like A. afarensis lived in that region and gave rise to two different lineages: one became later species of Australopithecus, and the other led to the genus Homo.

The authors of the Science paper argue that the jaw provides exactly the evidence you’d expect to find in an early Homo fossil—some characteristics associated with later Homo specimens, and other characteristics associated with Australopithecus.

One of the main distinguishing features, Kimbel explained, is the shape of the molars. “In Australopithecus—Lucy as well as other species—the molar teeth tend be very bulbous. The flat tooth surfaces give way to bulging sides that sort of puff out; they look like they had a hard night of drinking.” Homo molars tend to have straighter vertical walls, and that’s what the Ledi jaw shows.

A possible explanation of the puffy Australopithecus teeth is that they maintain a larger surface area for chewing as the teeth are worn down. A jaw with slimmer teeth suggests a change in chewing behaviour, which is in line with the overall picture of Homo having smaller chewing muscles and less heavily built jaws. The chin region of the Ledi jaw, on the other hand, looks more like Australopithecus.

For Kimbel, this combination of characteristics is what makes the jaw so exciting: “Isn’t that what might be expected in a very early representative of a lineage? You would expect a higher number of primitive, ancestral characteristics than you would later on.”

This also makes it difficult to place the species. Does the jaw fall under Homo, because of its molars, or Australopithecus, because of its chin? Kimbel and his co-authors favoured an approach that emphasises the more modern traits, leading them to tentatively place it within Homo. A different group of researchers has contested this, arguing that many of the jaw’s features overlap with various Australopithecus species. It’s difficult to decide on how to classify a species based on isolated remains, the authors write: “because at present we cannot be certain what the rest of the dentition, skull or skeleton of [the Ledi jaw] might have looked like.”

It’s not actually vital to answer this question, Kimbel argued. “What’s important is not what you call it; what’s important is what it tells us about the early evolution of the human lineage.” What it seems to tell us is that in Ethiopia around 2.8 mya, there existed a species that had some—but not all—of the early characteristics of Homo.

Listing image by Smithsonian's Human Origins Program