This jawbone pushes back the evolutionary origins of our genus by nearly half a million years, researchers reported today.

Photo: Brian A. Villmoare

Paleontologists working in Ethiopia have uncovered the lower jaw and five teeth of an unknown hominid that lived 2.8-million years ago. Remarkably, the fossil exhibits a blend of primitive and modern features, leading the researchers to declare it the oldest example of the Homo genus, and the earliest-known ancestor that led exclusively to modern humans.

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The discovery of this yet-to-be named hominid pushes back the appearance of the Homo genus by some 400,000 years. This fossil strongly indicates that Homo — the genus to which we ourselves belong — first emerged in East Africa approximately 3-million years ago when a select group of hominids transitioned away from the more ape-like Australopithecus.

The mandible and teeth, catalogued as LD 350-1, were uncovered at Ethiopia's Ledi-Geraru research area of Afar Regional State in 2013, on an expedition led by Arizona State University's Institute of Human Origins.

Fossils of this nature are exceptionally rare. Specimens from this particular time period, between 2.5- and 3-million years ago, are few and far between, and those that are found are typically not very well preserved. Consequently, we know very little about this critical era of human evolution, and there's considerable disagreement as to when the Homo genus first emerged.

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But the discovery of this fossil, dated to 2.75- to 2.8-million years ago, is helping to narrow the evolutionary gap between Australopithecus and early Homo, while at the same time offering an excellent example of a transitional fossil in a crucial time period of human evolution

A Species in Transition

Analysis of the fossil, led by Brian A. Villmoare of the University of Nevada, Las Vegas, produced fresh insights into the jaws and teeth of early Homo. While the specimen shares some characteristics with Australopithecus afarensis (the most famous fossil of which being the partial skeleton dubbed "Lucy"), the researchers observed a number of differences, as well. The mix of physiological traits, the researchers say, is indicative of a species in evolution transition.

Photo: Villmoare et al/Science

"Much of the anatomy of the mandible shows a close relationship with later Homo — the teeth and shape of the bone," Villmoare told io9. "However the front of the mandible — the symphysis — is similar to Australopithecus afarensis. Also, it is similar in size to A. afarensis."

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The researchers dated the sample by analyzing various layers of volcanic ash using argon-40–argon-39 dating. Using this technique, they were able to measure the different isotopes of argon to establish the age of the eruption that created the sample.

"This discovery places the origin of Homo in East Africa," says Villmoare. "It also extends the record of Homo back a further 400,000 years, as previously the oldest fossils attributable to Homo were 2.3- to 2.4-million years old." The specimen, in other words, shifts the evolutionary appearance of our genus to almost 3.0-million years ago. "This mandible," Villmoare says, "is the oldest fossil that can be attributed to our genus."

Broadly speaking, Villmoare says the the shift from Australopithecus to Homo marks "a major adaptive transition from a more ape-like existence to one more dependent on tools, large brains, and meat."

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Indeed, the left side of the lower jaw and the five teeth exhibit definite Homo-like characteristics, including slim molars, symmetrical premolars, and an evenly proportioned jaw. At the same time, however, the Ledi-Geraru jaw has a "primitive" sloping chin, resembling that of the more "ape-like" Australopithecus. Taken together, it's these characteristics that are helping paleontologists distinguish between early species of the Homo lineage – such as Homo habilis – from Australopithecus.

A Potential Ancestor of H. Habilis

That said, the researchers are reasonably confident that the Ledi-Geraru jaw is not an example of Homo habilis, an early human estimated to have lived between 2.3- and 1.4-million years ago. This newly discovered species, though still undefined in the literature, would appear to predate H. habilis by several million years. The new fossil, though limited, exhibits features inconsistent with those of H. habilis.

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In an unrelated but highly relevant study also released today, a research team led by Fred Spoor from the Max Planck Institute for Evolutionary Anthropology in Leipzig have created a state-of-the-art computer reconstruction of the original fossil of Homo habilis. Spoor's team's work shows that the evolutionary roots of H. habilis go further back in time than previously thought. Their analysis indicates that at least three different species existed between 2.1- and 1.6-million years ago, namely Homo habilis, Homo erectus, and Homo rudolfensis. What's more, Spoor believes that the Ledi-Geraru hominid makes a good candidate ancestor for Homo habilis and other species of early Homo.

"By digitally exploring what Homo habilis really looked like we could infer the nature of its ancestor, but no such fossils were known," noted Spoor in a statement. "Now the Ledi-Geraru jaw has turned up as if 'on request', suggesting a plausible evolutionary link between Australopithecus afarensis and Homo habilis."

Climate Change and the Emergence of the Homo Genus

One theory that's used to explain the sudden appearance and disappearance of early hominids around this time is global climate change. The subsequent increase of African aridity some 2.8-million years ago may have directly contributed to the appearance of Homo.

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In an accompanying study published in this week's issue of Science, Erin DiMaggio of Pennsylvania State University and colleagues sought to investigate the role of climate change in human evolution by analyzing the remains of various animals that lived alongside the Ledi-Geraru hominid.

Image: Erin DiMaggio

"By studying the geology and fossil record at Ledi-Geraru we have found that the Ledi Homo was living in an extremely open environment, similar to that of the Serengeti Plains today," DiMaggio told io9. "This environment was much more open and probably more arid than the environments reconstructed for Australopithecus afarensis."

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Her team's work shows that the Ledi hominid lived alongside animals that dwelled in more open habitats such as grasslands and low shrubs — animals like prehistoric antelope, water-dependent grazers, prehistoric elephants, crocodiles, fish, and a type of hippopotamus.

"Various lines of research suggest African climate and climate variability changed between 3- and 2.5-million years ago," says DiMaggio. "Some studies suggest this was a gradual change, while others suggest it may have been more marked around 2.8-million years ago."

Photo: Erin DiMaggio

The environment in which the Ledi hominid lived was relatively open and possibly arid, especially compared to earlier times when Lucy's species existed.

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"So yes, we can say that at this particular time and place, our data fits well with those hypotheses suggesting a drying of the east African environment from 3- to 2.5-million years ago," DiMaggio told io9. "However, it is important to note that not all fossil records in east Africa at this time say the same thing, which is why it is important for both the Ledi-Geraru project, as well as other research projects... to continue looking for and studying sediments and material from this critical interval."

Indeed, as noted by study co-author Kaye Reed, "We need a larger sample of hominin fossils, and that's why we continue to come to the Ledi-Geraru area to search."

But Is It An Ancestor?

At a press conference held Wednesday morning, William Kimbel – a paleoanthropologist at Arizona State University and co-author of the jaw analysis – emphasized that it would be inaccurate to describe the Ledi jaw as the earliest human fossil ever discovered. The word "human" is a colloquial expression that means different things to different people. For example, many paleontologists and anthropologists consider Australopithecus to be human. What's more, when many of us think of "human," we think of Homo sapiens, our own species that goes back some 200,000 years.

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As to whether or not we can say the Ledi hominid is an ancestor of Homo sapiens, Kimbel says it's not a direct ancestor; but the researchers behind the specimen argue that this particular population ultimately led to modern humans. It's a part of our lineage, albeit a deep part of that lineage. In fact, it is now the oldest fossil on the evolutionary branch that leads to us.

It's possible, of course, that other hominids were around between Australopithecus and Homo, but fossils supporting their existence have yet to be discovered.

Lastly, the researchers don't have enough evidence to formally declare the Ledi homind a distinct species.

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"I think we need to know a lot more about the population it represents before we can be sure as to whether it represents an already known species or perhaps even a new one," said Kimbel during this morning's press conference. "We have a jaw with teeth that preserves enough anatomy to be quite confident that it does represent an early part of the Homo lineage, but I would suggest that we need to know more about it before we can make any firm decisions about what species it might belong to."

You can read the two studies published in this week's issue of Science here and here, and the relevant H. Habilis paper at Nature.

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