By John Timmer, Ars Technica

The story of the evolution of modern humans can be a bit confusing, species-wise, with many early hominins co-existing without an obvious linear succession. But, geographically, all the action has appeared to take place in Africa, at least until the appearance of Homo erectus, which left Africa and spread globally, only to be replaced by later species of African origin: us. Over the past year or so, however, our history has become a bit more complicated, with evidence that our ancestors interbred with earlier human relatives that had already dispersed throughout Asia. Now, earlier events are also looking a bit more confused, as archeological finds in the nation of Georgia are being promoted as evidence that Homo erectus didn't even get its start in Africa.

[partner id="arstechnica" align="right"]The site of the new finds, Dmanisi, is only about 50km away from Georgia's capital Tblisi, and has been making waves in the field of human origins for a while. Skeletons uncovered there date from about 1.75 million years ago, and are a confused mix of features that place them right near the base of the Homo genus. These include very ancestral skulls and upper bodies, but lower bodies and legs that appear far more modern. A number of features in these skeletons are shared with various other early Homo skeletons, including ergaster, habilis, and erectus. Confusing matters further, the skeletons were extremely small relative to the Homo erectus individuals that later spread throughout the globe—these make their first unambiguous appearance in Africa right about this time.

There have been a number of responses to these findings. The least disruptive is that these just represent a very early form of Homo erectus, and provide an indication that the species was very mobile right from its start. In this view, the Dmanisi individuals represent a side-branch that was later swamped when their larger cousins appeared on the scene. Others have placed these individuals in their own species, Homo georgicus, without necessarily clarifying its relationship to other contemporary hominins.

But the most radical interpretation has been that, if the Dmanisi skeletons look like the earliest form of Homo erectus, that's simply because they are. In this scenario, the species originated in Asia, and evolved its larger form there before going mobile, eventually returning to the Africa that its more distant ancestors left. In this view, the Dmanisi skeletons go from being a side show in the drama of human evolution to a starring role in the main show.

The latest finds, detailed in a paper released by PNAS, don't involve any new skeletons that will shed light on the relationship among the earliest members of our genus. But the stone tools that have shown up indicate that Dmanisi was occupied for tens of thousands of years, which suggests a significant and stable local population. And, in the authors' view, that strengthens the case that the population could have served as the launching point for the global spread of Homo erectus.

The site itself contains a series of flaked tools that extend the site's history nearly to the base of the sediments in the area. There's a solid layer of basalt underneath it all, and that's covered by some looser layers of volcanic ash immediately above that. But, just about as soon as there are indications of more abundant plant life in the area, stone tools also become apparent. And the evidence of these tools is much deeper (and therefore much older) than identifiable skeletal remains, indicating that the site was occupied for at least 80,000 years, and back as far as about 1.85 million years ago.

So, there was probably a significant population at the site. Was it old enough to be ancestral to the rest of Homo erectus? The authors clearly think yes, writing "The initial occupations of Dmanisi are possibly older than the first appearance of Homo erectus in East Africa." It's possible to make these comparisons fairly well, since the Dmanisi site is well dated, and the dates of occupation flank a reversal in the Earth's magnetic field.

According to the authors, a more central site of origin like the Caucasus also makes sense given that finds are turning up in Asia as early as 1.7 million years ago. The problem is that there's absolutely no evidence that any species, either Homo or Australopithecus, was anywhere outside of Africa before Dmanisi. And there's clearly not going to be anything older at the site, given that the oldest finds are already just above solid rock.

So, although the new data strengthen the case of those working at Dmanisi, who have been arguing for an origin outside of Africa, they're not definitive, and the site won't be able to provide any earlier evidence. We may be stuck waiting to see whether another site, either in Africa or elsewhere, coughs up older remains.

Image: Matt Celeskey/Flickr

Citation: PNAS, 2011. DOI: 10.1073/pnas.1106638108

Source: Ars Technica

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