Wednesday, November 17, 2010

Venema's Genesis and the Genome

This week, I'm discussing a series of articles on human origins from the
September issue of PSCF (see parts one and two). Today's article is Dennis Venema's Genesis and the Genome: Genomics Evidence for Human-Ape Common Ancestry and Ancestral Hominid Population Sizes. This is the only scientific entry in the series (the other three are biblical/theological papers), and the first to specifically advocate an evolutionary creation perspective.

The bulk of the article was spent discussing the similarity between human and animal genomes, especially focusing on similarities that indicate that humans and animals share a common ancestor. I'm sure some of my readers are hoping for an answer to this part of his presentation, but I'm going to disappoint those readers. I've addressed very similar issues (not exactly the same) in my 2006 chimp genome paper. Since that paper, my assessment of the issue has not changed, and despite my explanation that common design is (to borrow a phrase from Venema's paper) "enormously strained and severely ad hoc," creationists continue to pretend that common design explains homology. Nevertheless, I remain confident that a satisfactory creationist explanation will be found. Naive? Maybe.

I'm actually far more interested in Venema's other subject in this paper: ancestral population sizes estimates. This work suggests that the original human population was much larger than just Adam and Eve, and this clearly makes an impression on the theologians' papers. This is a subject that I'm not as familiar with as the straightforward comparative genomics data, but after pondering the issue for a while, I'm prepared to make these preliminary comments:

First, I would say that estimating ancestral sizes by comparing the human genome to genomes of chimps and gorillas is examining evidence of human/animal genomic similarity, which falls back under the question of biological similarity that I discussed above. Long story short: I don't know how to answer this right now.

Second, the evidence of population size from SNPs and linkage disequilibrium (LD) is quite different, in that it is an evaluation of human genetic diversity only. Once again, though, we find that the ancestor of modern humans is a population and not just two individuals. This evidence presently does not bother me too much, and the reason is quite simple: If young-age creationists are correct that Neandertals and Homo erectus are also descendants of Adam and Eve (and therefore human), then these studies are only looking at part of human genetic diversity. To get an estimate of the true ancestral human population size, you'd have to include genes at least from Neandertals and erectus, if not other species. I'm not saying that this solves the problem by any means, but it would at the very least render this SNP/LD argument inconclusive until such time as a more comprehensive evaluation could take place (skimming through the Neanderthal genome paper, I don't see an estimate of the population size of the Neanderthal/sapiens ancestor).

The evidence of population size from SNPs and LD would seem to be far more devastating for the RTB origins model, which posits that only Homo sapiens younger than 50,000-70,000 years are actually human descendants of Adam and Eve. The SNP/LD evidence is inconsistent with an origin of modern Homo sapiens from two individuals 50,000-70,000 years ago. Ironically, just as I was getting ready to write this post, RTB posted their latest New Reasons to Believe e-zine, which has an article by Fuz Rana titled "Were they real? the scientific case for Adam and Eve" (get the full PDF here, Rana's article begins on p. 4). In this article, Rana discusses population size estimates:
Did humanity originate from a single pair? Even though population estimates reveal that humanity originated from several hundred to several thousand individuals based on mathematical models, it could well be the case that these models overestimate the original numbers for the first humans.
OK, well I guess that's no more handwaving than my reaction to the similarity evidence, so I don't have much room to talk.  On the other hand, Rana seems to be evaluating this evidence under the mistaken impression that "It is possible to estimate the effective population size of any ancestral group from genetic diversity of present-day populations if the mutation rate is known." That's not how LD works. It's actually a lack of mutation that indicates LD. My fellow creationists (old earthers and young earthers alike), if we're going to formulate a response to LD-based population size estimates, we first need to understand how the LD evidence works.

I found one other comment in Rana's article quite revealing:
As Hugh Ross and I discuss in Who Was Adam?, numerous studies indicate that humanity originated: (1) recently (around 100,000 years ago, plus or minus 20,000 years or so)
That's 80,000-120,000 years ago, but on p. 248 of Who Was Adam?, they state,
The RTB model views Adam and Eve as historical individuals - the first human beings - originating by God's miraculous intervention approximately 70,000 to 50,000 years ago.
Those dates don't even overlap. I've previously documented RTB's (both Ross and Rana) inability to quote and discuss scientific papers accurately. It's fascinating to see this inability to read in action again, only this time, Rana seems to be unable to read the very book he wrote!

But enough about RTB. This is supposed to be a discussion of Venema's paper, and my final judgment on that is that Venema has raised some extremely important issues that need to be addressed. We can't just shrug this stuff off and act like a common designer explains everything. While I certainly disagree with Venema's conclusions about human evolution, I am always happy to be confronted with evidences that need to be explained.

And on a personal note, I am glad to have Dennis as a Christian brother with which I can respectfully discuss these issues.

Feedback? Email me at toddcharleswood [at] gmail [dot] com.