Saturday, 23 February 2013

Genetics and the Last Human Common Ancestor

I thought I was on to something yesterday, but it actually just highlighted the fact I need to learn a bit more (a lot more) about human genetics.  It involved a lot of rough sketching of human phylogenies, and confused questions to my husband.  I think sadly, the breakthrough I was working towards is not actually anything really interesting.

I was thinking about supporters of macromutations for the origin of grammatical language (as I do).  Many of these theories also posit a late emergence of language, between 50,000 and 100,000 years ago (it's become quite normal to talk of some kind of 'consensus' that language evolved roughly 100,000 years ago - I'm sorry, did I miss that meeting?).  Some examples would be research by Norbert Hornstein, old work by Davidson and Noble, and basically anyone with a conservative view of language emergence who is smart enough to drop the 'human revolution' ideas of the 80s and 90s.  I'm guessing this date is reached because of accepted material culture proxies seen in the archaeological record taken as undoubtably 'symbolic' - 90,000 year old beads at Qafzeh cave and the like, and the dating of Out of Africa 2.
"Through random drift or selection lineage will
trace back to a single person. In this example
 over 5 generations, the colors represent extinct
 matrilineal lines and black the matrilineal line
 descended from the MRCA." - Wikipedia

My thought process went like this.  If humans require a robust neural substrate in order to be linguistic beings, this substrate must have evolved before the last common human ancestor, otherwise a portion of humans living today would not have this genetic endowment for language.  And since all human populations are language capable, this 'macromutation' or whatever is being proposed, is very old indeed.

Or so I thought.  mtDNA allows us to trace back our last maternal ancestor, our 'mitochondrial eve', and this human lived around 200,000 years ago.  Y-chromosone DNA can also be traced back to a last common ancestor, which if I'm remembering correctly dates to around 50,000 years ago.  Our ancestral... Adam we could say.  But for the sake of this argument, I wanted to know how long ago the most recent person that was ancestral to all living humans lived.  So I did some internet trawling and ended up with a big surprise.

The date of the most recent common ancestor is a limit on the minimum recency a genetic macromutation for language could have feasibly spread to all modern humans.  And I was expecting this date to resemble the date for mitochondrial eve, or at least farther back than the conservative estimate for the evolution of human language (though if I was thinking about the Y-chromosone dates I would have realised it wasn't going to be earlier than that).  But, dating back a single gene will necessarily go back further than the most recent common ancestor, which turns out to be a much more recent date that all humans can trace back to (Wikipedia):

The identical ancestors point for Homo sapiens has been estimated to between 15,000 and 5,000 years ago.[4] 

Woah, really?  I wasn't expecting a date that recent for a common human ancestor between all humans living today (You can click on the Wikipedia citation in the quote for a link to a Nature article on this subject).

So, my idea about the dating of the last common ancestor of humans doesn't force back the limit on macromutationist accounts of a recent origin of human language like I thought it might.  But it was an interesting exercise.

So you're safe for now, macromutationists.  But you're still biologically unlikely.

1 comment:

  1. Thank you for sharing such wonderful information!In my opinion, Keep a healthy life by consuming healthy food and doing exercise regularly is the best healthy formula.

    Nursing Jobs


Related Posts Plugin for WordPress, Blogger...