Sunday, May 11, 2014

Svante Paabo, Neanderthal Man: In Search of Lost Genomes (2014)

If the prospect of future climate change poses difficult problems for estimating the impact on the extinction of species (see previous post), retroactively looking at the causes of the extinction of a species long dead before humans recorded history is not that easy either.  We can probably make some educated guesses in a few cases based on examination of the geological record and what we can find in the chemistry and perhaps biology of dated samples of earthen material and fossils.  But we remain hard-pressed right now to figure out why the species homo neanderthalensis became extinct.  They overlapped in time and habitat with our own species homo sapiens sapiens, and now we know thanks to the research of Svante Paabo and his colleagues at the Max Planck Institute that the two species interbred.  A small piece of the DNA of homo neanderthalensis lives on in anatomically modern humans.  Anatomically modern humans are believed to have emerged approximately 100,000 years ago, probably in southern Africa.  (See September 25, 2013 post).  This species is believed to have migrated out-of-Africa approximately 50-60,000 years ago (id): first to the Middle East and then to South Asia.  But this was not the first species of the genus homo to migrate out-of-Africa.  Modern humans were preceded by homo erectus, homo heidelbergensis and perhaps homo neanderthalensis. 

"According to the fossil record," says Paabo, "Neanderthals appeared between 300,000 and 400,000 years ago and existed until about 30,000 years ago.  Throughout their entire existence their technology did not change much.  They continued to produce the same technology throughout their history, a history that was three or four times longer than what modern humans have experienced.  Only at the end of their history, when they may have had contact with modern humans, does their technology change.  Over the millennia, they expanded and retracted with the changing climates in the area that lived in Europe and western Asia, but they didn't expand across open water to other uninhabited parts of the world.  They spread pretty much as other large mammals had done before them.   In that, they were similar to other extinct forms of humans that had existed in Africa for the past 6 million years and in Asia and Europe for about 2 million years. *** All of this changed abruptly when fully modern humans appeared in Africa and spread around the world in the form of the replacement crowd.  In the 50,000 years that followed --- a time four to eight times shorter than the entire length of time the Neanderthals existed --- the replacement crowd not only settled on almost every habitable speck of land on the planet, they developed technology that allowed them to go to the moon and beyond.  If there is a genetic underpinning to this cultural and technological explosion, as I'm sure there is, then scientists should eventually be able to understand this by comparing the genomes of Neanderthals to the genomes of people living today."

Until this statement, late in Neanderthal Man, Paabo's story has been about his personal and scientific journey from a young man in training to be a physician who takes an interest in the DNA of dead humans and ancient species to become director of the Department of Evolutionary Genetics at the Max Planck Institute who dissected the Neanderthal genome. The memoir reads a bit like a detective story. Now Paabo is trying to give meaning to what he has found.

Who is this Replacement Man?  It is the anatomically modern human (homo sapiens sapiens), but not the anatomically modern human that chronologically and immediately replaced the archaic human (homo sapiens) approximately 100,000 years ago.  This is the anatomically modern human who began spreading across the earth "shortly after 50,000 years ago."  (See also September 25, 2013 post). The oldest modern human bones found in the Levant date back are approximately 100,000 years old.  Further evidence indicates that modern humans and Neanderthals mixed here in the Middle East for about 50,000 years, but there is no evidence that either was dominant.  Their stone tools appear to be the same.  But "shortly after 50,000 years ago," co-existence was no longer the norm.  When humans appeared in an area, Neanderthals disappeared either immediately or shortly thereafter. These modern humans "replaced" Neanderthals.  Modern human tools and weapons were more advanced than Neanderthal technology and the Aurignacian culture, as it is described, produced the first cave art and first figurines of animals, including mythical creatures. "The 'replacement crowd,' says Paabo, "thus exhibited behaviors that were only occasionally or not seen at all among Neanderthals and among the earlier modern humans" who occupied the Levant the previous 50,000 years.  "We don't know where the 'replacement crowd' came from.  In fact, they could have been the descendants of the same humans who had already been living in the Middle East, simply accumulating the cultural inventions and proclivities that enabled 'replacement,' but it is more likely that they came from somewhere in Africa."  Paabo does not report the evidence in support of this thesis, but it would be interesting to know.

As a result of Paabo's work, we now know the modern human genome contains small amounts of Neanderthal DNA, and thus we know interbreeding occurred between homo neanderthalensis and homo sapiens sapiens.  As a result of Paabo's work, the Neanderthal genome has been published and the work is in its incipiency to identify crucial differences between the Neanderthal genome and the human genome.  it is estimated that the total number of DNA sequence positions at which Neanderthals and humans differ is roughly 100,000.  One goal of this research is to identify genetic changes that might be relevant for how humans think and behave --- a possible clue to what makes us human --- a subject that has reappeared in this blog more than once.  (See September 27, 2009, June 12, 2011, November 21, 2012March 28, 2013October 26, 2013 post).  There will be more to come.

No comments:

Post a Comment