The New York Times has an article up reiterating the fabled "bushiness" of hominid phylogenetic trees:
Scientists who dated and analyzed the specimens -- a 1.44 million-year-old Homo habilis and a 1.55 million-year-old Homo erectus -- said their findings challenged the conventional view that these species evolved one after the other. Instead, they apparently lived side by side in eastern Africa for almost half a million years.
My knowledge of bones is not strong, so I leave it to John Hawks or Kambiz to decompose the details. That being said, the big picture is that this is another strike against anagenesis for the human lineage, which basically is a scientific concept expressed in the famous t-shirts showing a progression from ape to ape-man to Neandertal to modern human. Anagenesis easily slots into human cognitive biases such as the "Chain of Being," and I don't know how revolutionary it is to bury it again for the thousandth time. All that being said, it is a little irritating to see this bit of pop-evolutionary thinking crop up near the end of the piece:
Susan Anton, an anthropologist at New York University and one of the report's authors, said that the small skull pointed up a significant variation in the sizes of erectus specimens, particularly differences between the male and female of the species, or sexual dimorphism.
Such a characteristic is thought to be a primitive stage in evolution. In humans, males average about 15 percent larger than females, and the same is true for chimpanzees. Sexual dimorphism is much more striking in gorillas, and apparently also in erectus.
1) If you are writing about evolution you had best be cautious about throwing out the term "primitive." Ancestral and derived would be more precise and less loaded, if a bit more opaque to less verbally fluent readers.
2) Sexual dimorphism isn't "primitive" in any case. Just as chimpanzees and gorillas are no more "primitive" than human beings. Man is not the measure of evolution. Additionally, gibbons have very little sexual dimorphism, and if man was the measure of a beast then they would be far more primitive than gorillas or chimpanzees.
When I read this story, my immediate reaction was to think that Homo Erectus much have evolved in Asia, from some earlier Home lineage, may be like those from Dmanisi, and then back migrated to Africa!
my impression is that terms like 'homo erectus' or 'achaic homo sapiens' are very much grab-bags.
Take a loot at "Handy Man" material on the web sometime. last I took a gander I found two distinct types, listed as belonging to the species Homo habilis, but radically different in appearance; the second type being larger and more ape-like in appearance. We simply don't have the workers to adequately evaluate what we have in the way of specimens.
Fossils simply don't give you the contextual clues living specimens do. We currently have three sub-species of chimpanzee; t. troglodytes, t. schweinfurti, and t. verus. P. t. verus is currently the focus of a debate regarding the taxonomical status of an eastern population, which some would see raised to sub-species in it's own right. P. t. schweinfurti is under consideration for species status, which could mean the Bili Ape (currently members of the sub-species) would be raised to sub-species status (Pan schweinfurti leomortis is my recommendation :) ).
Then you have the late Oliver, a chimpanzee who had an easier time than standard chimps at walking bipedaly, and possessed a head that was shaped differently than that of standard chimps. He also preferred humans as potential sexual partners over standard female chimps. Last I heard Oliver is considered a member of P. t. verus, though he was more robust and had a differently shape head than that sub-species. Again, it's a matter of not having the personnel needed to do the research.
It comes down to this, thanks to a shortage of personnel and other resources, plus gaps in the fossil record, we have no real idea of just how bushy the hominid tree was. There are more species out there waiting to be discovered, and a number of them are represented by stray bones filed away in university and museum storage lockers. Paleoanthropology will always be a field in flux.
Fascinating times for evolutionary paleo-anthropologists, I reckon. My interest, while stricktly amateur, continues to provoke curiosity by asking the question: if genetically controlled behaviors are selected due to some change in the environment, such as would have happened during the period in question, as forest-mosaic was changing over to a mixed savannah, lacustrine mosaic, wouldn't those primates who's genetic behaviors were tolerant of other primates and who cooperated with others in their social structure be different from those who by dint of their genetics showed little capacity for socialization and instead of concentrating at water or food sources, spread out to take advantage of the drier, sparser resources, by keeping their population lower.
When a similar situation was observed in breeding foxes, the results showed that the genetic propensities as displayed by behavior did indeed have surprising physical and behavioral traits become evident in susequent generations: floppy ears, spotted colors, puppy like skulls (neotony),and of course, an increased tolerance for the proximity of others of their kind. Would a more gracile form of h. erectus arise among the bands of hominids that banded together (self-selected) because they had a greated genetic tolerance for the presence of their own species? I suspect that might be the case. A recent article seemed to think this mechanism was at work when domestication of the dog occured, i.e: that the wolves which displayed the greatest degree of neotony regarding tolerance for others (in this case people around their camp and food supply)self-selected and subsequent generations carried these traits to even greater degrees which humans then further specialized to become the breeds of dogs we all know.