Our ancestry may not be so straight and narrow

  Photo: Nature

BBC News- New human species identified from Kenya fossils

When I first read this article I was suprised by the implications, I haven't really ever thought about the possible braching off from our ancestral linage that could have happened on the route to the humans we have today. Based on talking to a few of my classmates, I think that the linear ancestry idea is a popular perception, and other theories aren't discussed much in public education. However, a major discovery in human evolution that has grabbed tons of media attention lately is adding to the growing case against linear evolution of our species.  The discovery of fossils in Kenya at Koobi Fora by Dr. Meave Leakey and associates (1) was referenced in a recent BBC new article (title) stating that it provides evidence for a new species of human. The discovery consisted of three human fossils with faces, two jawbones, and teeth that were considered to diverge enough from other homo erectus fossils from the time to suggest a new species. The researchers suggest that there were actually multiple species of humans in coexistence in Africa in addition to Homo erectus around 2 million years ago.

When I looked at some of the background on this topic of linear versus branched evolution, I found that in the scientific community many do accept the branched lineage of our species and think that it makes sense that we would have various species within homo that later died out, which is seen throughout most of the animal and plant kingdom. These findings by Dr. Leakey and associates are not the first to suggest a branched evolution pattern.  In 1964 Dr. Leakey and associated published a paper suggesting the existence of homo habilis, a species within the homo genus but not belonging to homo erectus or the another closely related genus at the time, Australopithecus (4). Then again in 1972, Dr. Leakey and associates found a skull (KNM-ER 1470) believed it to be a different species (Homo rudolfensis) again within the homo genus but not related to homo habilis or homo erectus because it was larger than other specimens of the time, had a different face morphology (3). However, these were the only fossils known to be in existence at the time, which is why they were widely considered abnormal/outlier specimens. 

 

Enter the new fossils found in Koobi Fora in Kenya (above left mandible and mandibular gragment, right facial bones). In a paper published in Nature 2012 (1), Dr. Leakey states that the new fossils discovered in Kenya share similarities with Homo rudolfensis, and are similar enough to give more evidence for the species. KNM-ER 1470  skull stood out from other fossils of the period because of it's much larger size, a flatter face shape than previously seen, and more anterioraly placed zygomatic bones (1). The new fossil face (KNM-ER 62000) is smaller than the KNM-ER 1470, but shows the same flat-faced morphology and teeth structure. A complete mandible (KNM-ER 6000) and mandibular fragment ( KNM-ER 62003) were also considered to be similar enough in morphology to be possibly considered part of the species homo rudolfensis, although they caution that a more in-depth look at the original homo habilis fossil is needed to rule that out. Either way, the Leakeys state that their evidence proves the existence of another human species living at the same time as homo erectus.   

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The idea that multiple species of humans coexisted challenges the previous theory of distinct linear evolution from chimpanzees to modern humans. This evidence has now convinced most scientists that  we have had a more branched lineage of humans, each with different mutations and adaptations, similar to the evolutionary track taken by other animal species. There are still some critics, not necessarily to the branched evolution theory, but to the evidence that we can confidently classify these new fossils as a new species or even all as part of the same species. Personally, I think that the limited number of fossils is not enough to fully prove the existence of a new species, we need more specimens. Also the fact that they have all been found in the same area, while it could be explained because they were a relatively short lived species that didn't migrate as other species did, could also suggest a set of outliers all related to each other. When considering the mutations and diseases seen today that can cause deformmities and physical abnormalities I don't think it's impossible that these specimens could represent mutations or genetically abnormal specimens of an already known species. 

Another concern that popped up was the amount of variation with the homo habilis species itself. Andrew Kramer and associates actually concluded in 1995 that the variation within the specimens being called homo habilis had different patterns of variation than typically seen among species when compared by statistical analyses (5). Phillip Rightmire also stated that depending on how you group different fossils found, certain femurs with certain skulls for example, the similarity or difference to certain species and specimens changes, and more needs to be found to link individual discoveries (2). Both of these studies were done prior to the discovery of the new fossils this past year, but even within the paper by Dr. Leakey, there is a picture comparing the mandibles found to the mandible usually associated with KNM-ER 1470,  KNM-ER 1802 (picure below). The lower part of KNM-ER 60000 is on the left and KNM-ER 1802 is on the right, the black  outlines represent the reconstructed piece of KNM-ER 62000. This proves that the newly found face matches the new mandible pieces, but not necessarily mandibles found previously.

When considering the variation that has been found among fossils classified as homo erectus and other found to be concurrent with that period, it does suggest that there was at least some branching off within our ancestry. However, I also think that more fossils need to be located and more ways to relate fossils to existing specimens should be explored to get a more complete picture of the evolution of the homo genus, and how our ancestors became the species that eventually gave rise to us. Also, if there were other homo species, what made homo erectus so capable of enduring and surviving to eventually give rise to us compared with our relatives? Did our species interact or intermingle giving rise to even more genetic variation? The questions about our human evolution are already great, but with the existence of other homo species, they will become even more complex.

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Cited Sources:

1) Meave G. Leakey, Fred Spoor, M. Christopher Dean, Craig S. Feibel, Susan C. Antón, Christopher Kiarie & Louise N. Leakey. (2012) New fossils from Koobi Fora in northern Kenya confirm taxonomic diversity in early Homo. Nature 488, 201–204

2) G. Philip Rightmire. (1993) Variation Among Early Homo Crania From Olduvai Gorge and the Koobi Fora Region. American  Journal of Physical Anthropology 90: 1-33. 

3) R.E.F. Leakey. (1973) Evidence for an advanced Plio-Pleistocene Hominid from East Rudolf, Kenya. Nature 242: 447-450. 

4) L. S. B. Leakey, P. V. Tobias, J. R. Napier. (1964) A new species of the genus homo from Olduvai Gorge. Nature 202: 7-9. 

 5) Andrew Kramer, Steven. M. Donnelly, James Kidder, Stephen Ousley, and Stephen Olah. (1995) Craniometric Variation in large-bodied hominoids: testing the single-species hypothesis for homo habilis. Journal of Human Evolution 29: 443-462.