The recent discovery of a fantastically complete early Homo skull from Dmanisi, Georgia (dated to about 1.8 million years old) has set off a proverbial "bomb" in the paleoanthropological community (PDF here).
Over the past two decades paleoanthropologists have discovered several different early human fossils from Dmanisi, which provide researchers with the rare opportunity to compare numerous individuals from the same site and from the same time period. The variation of skulls found at Dmanisi over the past few decades have proven that intraspecies phenotypic variation is similar to that of phenotypic variation in the genus Pan and modern humans. In fact, the range of phenotypic variation is comparable to the variation observed in East African Homo fossils from the same time period. This raises the possibility that all early Homo species (e.g., Homo habilis, Homo rudolfesnsis, Homo ergaster, Homo erectus) discovered throughout East Africa 1.8-2 million years ago were actually one species.
From the article itself:
When seen from the Dmanisi perspective, morphological diversity in the African fossil Homo record around 1.8 Ma probably reflects variation between demes of a single evolving lineage, which is appropriately named H. erectus.
Before I continue, let me first state that paleoanthropology has always been a field of study fraught with endless taxonomic debates. These debates are usually unresolvable because it is inherently difficult to rigidly classify a continuum of species variation, especially with a lack of empirical data.
Let me also add that this discovery does not re-write any textbooks on human evolution. Current introductory textbooks discussing early human evolution acknowledge that the "first" species in the genus Homo (e.g., Homo habilis) is considered a "junk taxon". A junk taxon means that we acknowledge that it may have been multiple species, or it may have been one species... we just don't have enough data to conclusively "lump" or "split" it. So most evolutionary anthropologists already openly acknowledge that early human species are difficult to classify with current evidence.
Nevertheless, most discussion on the recent fossil discovery are focusing on classification. Maybe the first members of the genus Homo were one genetically variable species, or maybe they were several closely related but distinct species. From my perspective it is more important to acknowledge that we are consistently finding fossil remains that exhibit uniquely human phenotypic variation and remains that indicate uniquely human behavioural characteristics. These early humans were hunting with increased frequency, experimenting with fire, and designing complex technology. All of these characteristics are completely absent in the australopithecines. A remarkable transition was occurring here.
I don't think it is necessarily important whether we classify these species as Homo erectus, Homo ergaster, or Homo rudolfensis.
While discussing this new discovery, I would instead like to direct everyone's attention to the remarkable fact that this site at Dmanisi is the first known human site outside of Africa. These species (however we classify them) had (knowingly or unknowingly) ventured into Eurasia and had started adapting to foreign environments. With surprising rapidity they spread into Asia and Europe, undoubtedly encountering wildlife, seasons, and ecological niches that would have been completely novel.
These species were displaying the remarkable ecological flexibility characteristic of modern humans. They were the first human experiments with continental-scale migration and global diffusion. We may be unable to reliably classify them, but that doesn't have to stop us from reflecting on these amazing first human journeys.
High-res photography of the new discovery and artistic reconstructions of the hominid can be found here.