Look Ma, No Bones!


June 29, 2010

For most of the twentieth century what we knew about human evolution derived from the finding and analysis of fossilized bone.  Reconstruction of bone fragments, comparison with previous finds and contemporary ape and human anatomy kept pushing our understanding of human origins deeper in time.  A dramatic example of this point was the 1974 discovery of AL-288-1, the partial Au. afarensis skeleton nicknamed Lucy and the subsequent anatomical analysis demonstrating that she was our earliest bipedal ancestor. 

In the mid 1980s two molecular biologists on the Berkeley campus of the University of California, Allen Wilson and Vincent Sarich, concluded from their research into mutation rates that the last common ancestor of Cchimpanzees and humans lived between six and eight million years ago. A startling, revolutionary hypothesis at the time, this seems to be confirmed with fossil discoveries in the past decade, in which bipedal, or nearly bipedal predecessors of Lucy resemble quadrupedal apes even more closely than she does.

Wilson and Sarich paved the way for DNA research being done by scientists at the Max Planck Institute in Germany, who have presented two papers this year producing controversial conclusions based wholly on genetic, not anatomical analysis.  These researchers announced in the journal Nature in April the existence of a new hominin, closely related to but distinct from both us,and the Neanderthals

The middle joint of  the left pinky finger of a juvenile was the only hominin remains found in a cave called Denisova in the Altai Mountains of Southern Siberia. The extremely cold temperatures prevailing within the cave preserved genetic material and impeded fossilization. The Max Planck team pulverized the bone fragment, analyzed the genetic material and found a sufficient difference in the DNA to conclude the individual was Homo but neither H. sapiens nor H. neanderthalensis.

The abstract from the April8, 2010 article in Nature, under the headline “The complete mitochondrial DNA genome of an unknown hominin from southern Siberia“ reads, 
“With the exception of Neanderthals, from which DNA sequences of numerous individuals have now been determined, the number and genetic relationships of other hominin lineages are largely unknown. Here we report a complete mitochondrial (mt) DNA sequence retrieved from a bone excavated in 2008... It represents a hitherto unknown type of hominin mtDNA that shares a common ancestor with anatomically modern human and Neanderthal mtDNAs about 1.0 million years ago. This indicates that it derives from a hominin migration out of Africa distinct from that of the ancestors of Neanderthals and of modern humans… “

The authors of the paper are not claiming the discovery of a new species; they confine their conclusion to saying the genetical material is sufficiently distinctive from ourselves and Neanderthals. If this is confirmed by future findings and analysis, it will establish the existence of four closely related species around 40,000 years ago in the Old World: anatomically modern humans; Neanderthals, this new, genetically distinct form, and Homo floresiensis, the “Hobbit” fossils from Indonesia.

An accompanying article can be found in Nature.

Two weeks later, at AAPA meetings in Albuquerque, a team headed by Jeffrey Long from the University of New Mexico reported their genetic analysis of more than 1900 living individuals showed two periods of interbreeding among Neanderthal and human populations, sixty thousand and forty thousand years ago in the Middle East and Central Asia.  Shortly thereafter the evolutionary genetics team from Max Planck Institute seemed to confirm this observation with their paper, a draft of the Neanderthal genome.

Whereas the UNM team had analyzed DNA from living humans, Paabo et al. from Germany were comparing DNA from three extinct Neanderthals in Bosnia to five contemporary humans.  See the commentary in a Nature story entitled “Neanderthals may have interbred with humans” by Rex Dalton.

The Max Planck Institute group has been looking for evidence of interbreeding for some time, until now without success.  Now they claim to have it, and Long et al. seem to confirm it.  And this was all done without reference to anatomy or fossils, except to the extent Neanderthal bone from the Balkans was the source of DNA for MPI.  The conclusions were reached after computer powered, statistical analysis of extensive processing of data.  Paleontologists are skeptical of conclusions reached without reference to the morphological evidence.

This is nopt a case of competing techniques, however, each vying for dominance. The search for fossils and artifacts will continue, just as there will be refinement and increasing application of analysis of genetic material.