Friday, February 04, 2011

New Roots On The Family Tree

The salient features
Of human-ish creatures,
Our sisters and brothers and cousins,
Across generations
Shows slight variations
And species, it seems, by the dozens
The picture’s still muddy,
As scientists study,
Concerning our relatives’ species
The task is colossal
Inferring from fossils
Of footprints, or bones, teeth, and feces
The clues, though, are leading
To claims of cross breeding
In hominids once thought distinct
With two populations
In different locations
With whom we’re genetically linked
It’s really exciting—
You see, we’re re-writing,
As only hard evidence can,
The view, now outdated
That claimed that we mated
With only the sapiens clan

In NPR's 13.7 blog, Ursula Goodenough writes of recent research on hominid genomes, and the increasingly varied story of our family tree.  To take just one tidbit, it appears that the locals of Papua New Guinea are "roughly 92.5% African, 2.5% Neanderthal, and 5% Denisovan"--that is, this population (with which the rest of us are interfertile, of course) has genetic material belonging to two extinct hominid species, whom we had previously believed were distinct from (read: non-interbreeding with) H. sapiens.  We knew we shared common ancestry with them, but the notion that they are among our direct ancestors (for some modern populations, at least) is new.

As I tell my students, Darwin's Origin of Species was ironically titled, because evolution by natural selection makes obsolete the concept of "species" as it was used at the time (or as creationists still use it).  Rather, as populations vary across time and geography, black and white distinctions simply do not exist.  Take the example of ring species, for instance, where neighboring populations can interbreed just fine, but populations a bit further apart (geographically, or chronologically, it works both ways) cannot, despite a continuous line of interbreeding populations linking the two.  Where is the species line to be drawn?  Are these one species, or two?

Frankly, it's a bit like Pluto.  Pluto is what it is, whether it is called a planet or a planetoid; our linguistic handle on it is for our sake, and simply allows us to talk about it.  The concept of "species" is a similar abstraction; tremendously useful in some cases, impossibly vague in others.  

H. sapiens, H. erectus, Neanderthal, Denisovan, and more... which are "us"?  It depends on the context.  

This, like Pluto, may take some getting used to.  Human exceptionalism (not to mention the historical influence of creationism) and ego ("what a piece of work is man...") have expressed themselves in a history of dehumanizing our ancestors.  Neanderthals are still brutes in the public eye (so easy a caveman could do it), despite recent attempts to update their image.  It would be difficult to maintain our belief that we are the pinnacle of creation, the top of the evolutionary ladder (yes, I know the metaphor is wrong), the final product of nature, if we must also recognize that by some measures we are far more closely related to Neanderthals than we had thought.   


cpbm said...

I agree. I don't think that the line between species should be decided by inter-fertility. You can get into all kinds of paradoxes that way. For example, there is a chain of inter-fertility between any two species going through their common ancestor. Since there is a global common ancestor, is there only one species? Perhaps this is where the irony of the Origin of Species comes from?

Nice poem, btw.

(Should the comma in "Papua, New Guinea" be there?)

Cuttlefish said...

(what comma? *innocent look*)

Good point; in my classes, I have confused people sometimes by using the first person plural a bit oddly. "We--and by 'we', I mean mammals", or "We--and by 'we', I mean vertebrates", or "We--and by 'we', I mean mammals", or "We--and by 'we', I mean eukaryotes"...

It all depends on how far you extend "extended family".

Anne Hedonia said...

Great article! Thanks for sharing! I had remembered reading something about how the natives of Papua New Guinea had at least 2.5% Denisovan in their DNA and I couldn't remember where I had read it for the life of me. A quick search into Google with the words "Papua New Guinea Denisovan 92.5%" and BAM! There was your page with all the info I needed to help further my point :) Do we know if it is JUST the Papua New Guineans that contain Denisovan DNA? And if so, is that the case because of the close proximity to where the Denisovans had lived? Forgive me if my questions seem...stupid. I have to admit I don't know much about genetics or anything like that, but I do find this incredibly fascinating and I would like to know more. Who knows what % of our DNA belongs to our non-Homo-Sapien ancestors...I imagine it would take some sort of global DNA sequencing for local populations to ever get close to an accurate answer to that question. Maybe other populations around the world have genetic ties to OTHER long-extinct primate ancestors that we never thought had inter-bred with Homo Sapiens. As cliche as it is, it's pretty amazing that those "species" have long been extinct and yet their DNA lives on among the existing human population. It really makes you question your own "humanity" and leads one to wonder what exactly that means...