December 23, 2011 | 5
During a recent reporting trip to South Africa for a forthcoming feature article on a new fossil human species called Australopithecus sediba, I asked readers to submit their questions about this dazzling find. Inquiries about the nearly two-million-year-old hominin–which has been held up as a possible ancestor of our genus, Homo–came in via Twitter, Google Plus and the comments section of this blog. I put them to paleontologist Lee Berger of the University of the Witwatersrand in Johannesburg, who found the Au. sediba remains at a site called Malapa, some 30 kilometers north of Johannesburg. Reader questions and Berger’s answers follow below.
@milst1 on Twitter asked: Was Australopithecus sediba polygamous? Polyandrous?
Lee Berger: We don’t know. However, primate models can offer some clues. Living primates tend toward polygamous, male-dominated societies. As a general rule of thumb, the higher the level of observable physical difference—or sexual dimorphism–between males and females of a species, the more patriarchal their social structure. (Orangutans are an exception to this rule, living more solitary lives.) We expect early humans were troupe animals with lower degrees of violent interaction between males competing for females than are seen among most modern mammals. Clues come from features like the reduction the size of male canine teeth, which in other creatures are used for fighting. Australopithecus sediba appears to have an even lower level of sexual dimorphism than is seen in other early human species, and is more like modern humans in this regard. It also has very reduced canines, like we do. So members of this species probably lived in polygamous societies, just as humans in many societies do today, possibly with groups of related males cooperating to a greater degree than we see in even chimpanzee troops. One of the great things about Malapa is that we have every reason to believe that we will one day have a good representation of the troop structure of Au. sediba, which of course may eventually turn this relatively speculative answer into one with more concrete evidence behind it.
@zzace on Twitter asked: Were Homo habilis and Australopithecus sediba contemporaries? If so, is there any evidence that they interacted? Were they rivals or did they interbreed?
LB: There is in our opinion no convincing fossil evidence that Homo habilis existed much before about 1.8 million years ago. That does not mean the species did not exist prior to this, but Au. sediba, with its mosaic of australopithecine-like and Homo-like traits, shows us that you can’t use isolated areas of anatomy to identify species in the way we paleoanthropologists have been doing. Extraordinary claims take extraordinary evidence, and the claim for the earliest evidence of the genus Homo is an extraordinary one. The isolated teeth and bits and pieces that have been attributed to early Homo older than about 1.8 million years ago do not meet the criteria for extraordinary evidence. At the same time people have criticized Au. sediba saying it’s too young [to be a candidate ancestor of Homo]. Since the evidence for early Homo of an equal age or older is so poor, we don’t think it is. But we are also not claiming in any way, shape or form that this is the first nor last existence of this species. It certainly existed before Malapa and probably after.
As for the question of whether the two species interacted, a lot will depend on how we eventually sort out the relationship between the two species. Were they genetically close enough to interbreed in same way we now know modern humans and Neandertals hybridized? A few years ago that might have seemed like an unanswerable question, but the fossil record is now producing extraordinary things that might actually allow us to answer such questions in the future. The first order of business here would be to establish that they overlapped in time and space.
Calvin Phuong on Google Plus asked: Is Australopithecus sediba a descendant of Australopithecus afarensis (Lucy)?
LB: Good question! Most present models have put Australopithecus afarensis (Lucy’s species) as the ancestor of all later hominid species. However, certain aspects of the anatomy of Au. sediba do draw into question whether this is true or not. Of what has been published so far the ankle, and in particular the heel bone, in sediba appears to be more primitive than that seen in the more-than-a-million-years-older afarensis, which has a more humanlike heel bone. Thus in this one area of anatomy it does beg the question of whether an already derived foot give gave rise to what appears to be a more primitive foot. If the answer is no, then Lucy’s species did not give rise to sediba and there’s something else out there [that did]. If the answer is yes it did, then it raises the intriguing question of why a primitive heel–and not just any primitive heel, but one that looks more apelike–would evolve once one has already become a terrestrial biped.
Dredd on ScientificAmerican.com asked: Are there any inquiries yet as to whether or not microbes had become symbiotic to that species as they are to Homo today?
LB: We quite simply don’t have the answer to that question at this time. As we are working on the possible organic material–and I stress the word “possible” here as we have yet to prove it is organic tissue–we will have to first establish, and prove to the scientific community that it is what it appears to be: fossil skin. If it is fossil skin, and the level of preservation is fine enough to preserve structures at the micron level such as hair and pore structures, then all bets are off as to what we can find. It is just these sorts of questions that emphasize why we are involving the entirety of the scientific community in this analysis and not just one lab, as there are so many potential questions one could ask if such fossil material were available.
Sauce23 on ScientificAmerican.com noted that you have hinted at the preservation of soft tissue and tools at the site, and wants to know more about this.
LB: If we were to find soft tissues preserved from a fossil hominin, particularly skin, a myriad of important questions once thought unanswerable could possibly be answered. Did early hominins have hair or fur? What was the distribution pattern of the hair or fur? Did they have sweat gland distribution like a human or had this not evolved yet? The answer to that question would tell us something about how they moved about the landscape. We might even get a sense of color–after all, researchers have identified color in dinosaur feathers. And there are probably questions that we as scientists have simply never thought to pose given that we really just didn’t conceive that one day we might actually be looking at fossil skin from two million years ago. One of the important things about this research is that it now makes us at least explore the possibility that organics exist in the hominin fossil record and I would bet that, just like dinosaur feathers and skin, we will now find that such preservation is more commonplace than we once thought.
Concerning the presence of tools or not, certainly sediba’s hand looks like the hand of a toolmaker–it has all the critical criteria we would typically associate with one: a long thumb, reduced curvature of the finger bones and other features. Tools have been found [elsewhere] pre-dating sediba by almost 600,000 years. However, with sediba’s small brain, the [discovery of associated tools] would require some remodeling of hypotheses about the necessary changes in this area of anatomy that were needed for complex stone tool manufacture. The presence of stone tools in direct association with sediba would be of major importance. What did the tools look like? How complex were they? Do they fit our present ideas of what tools should look like at two million years ago. All these are critical questions that we simply cannot answer until we excavate, and the answers are too important to speculate on in the meantime. Unfortunately, we will just have to wait, but Malapa is certain to hold more surprises as we dig into this remarkable assemblage.