Scientific American is carrying this somewhat technical summary of recent fossil discoveries in Ethiopia's Middle Awash region. Some scientists are arguing that a handful of fossil teeth provides evidence of a much more diverse set of hominids during the late Miocene than was previously thought.

Ok, "real people" explanation: apes evolved during the Miocene epoch (a nice overall summary is here). Somewhere between 5 and 10 million years ago (mya) a set of apes evolved fully bipedal locomotion, for reasons that are still unclear*. When climate change destroyed the gigantic Euro-African forests that apes evolved in, some of these bipedal forms (species) struck out onto the savannah to make a living.

Exactly how many species made it out is also unclear and the subject of a growing debate. Regardless, the various species created what seem to be only two strategies for survival: scavenging and grazing. Both strategies were quite successful at first. The grazers grew larger and larger over time, eventually ending up with a species currently known as Paranthropus bosei (I learned it back when it was Australopithecus bosei), a creature with a huge bony crest on its skull and molars more than an inch long.

The scavengers also became more specialized, growing taller and more lightly built (compared to the grazers). The protein content of their diet was presumably what allowed the development of ever larger brains, gradually changing a scavenging model to one of hunting.

At this point, about 1.5-2 mya, there were at least two, perhaps as many as four, different families of erect walking apes on the savannahs of Africa. Homo (us), and Paranthrapus (the huge grazers) are the ones I learned about. Again, for reasons poorly understood, the grazer forms died out at around this time while the hunter forms exploded (perhaps several times) out of Africa and into the Eurasian landmass. They eventually turned into, well, us.

So what these scientists are arguing about is just how many species of erect walking ape there actually were before the forests receded, and which one evolved into us. The big problem is the complete lack of fossils covering this period. They're arguing over six teeth because six teeth is just about all that's been found in Africa from around this time (6-10 mya). However, the discovery is in and of itself a very promising sign, because until recently as far as anyone knew there were no places to find Hominid fossils from this time period. The Awash region is an extremely remote and primitive area, and dangerous to boot with various rebel and bandit clans roaming it at will. Future discoveries will be hard won, but hopefully will come anyway.