Any geneticists here?

I think the following line of reasoning, if substantiated with solid proof, would constitute a serious disproof of the flood story in Genesis. Serious enough that no telescoping genealogies pushing it back by thousands of years are going to do anything. It goes like this:

1) If the flood happened, humans and animals would have a genetic bottleneck at the exact same time, sometime in the last 20,000 years say.

2) Human genetics shows we don’t have a bottleneck within the last 50,000 years at all. For many species of animals, there is no bottleneck in the last 100,000 years. Certainly not at the same time.

3) Therefore all humans and animals aren’t descended from the pairs on Noah’s ark.

4) Therefore, the tradition that a flood killed everyone outside the ark, including all the nephilim (giants) is wrong.

If this is the central point of the flood story, then we have entire chapters devoted to fictional stories in genesis. If Noah was fictional, why is there a lineage from fictional characters to people in Genesis?

This would seem to show that Genesis was not given to humans by an all knowing God. The tradition that the Pentateuch is totally true cannot be relied on.

I am completely not sure about #2. Can anyone please do some research on how we conclude about bottlenecks from genetic evidence? How strong are our conclusions — one part in how many? Or is it just a weak conclusion right now?

Posted: June 8th 2011

George Locke

Two basic ideas go into inferring the time since the last bottleneck. The first is that if you have a large sample of genomes from the present day, you can infer what the ancestral sequence was. The second is that you can measure the rate of mutation.

Start with a large set of human genomes and focus on one part. It’s particularly easy to work with mitochondrial DNA (or the Y-chromosome), because it’s inherited only from your mother (father) and doesn’t get shuffled during meiosis. Now, you line up your set. At most locations, all the sequences will be the same, but there will be some spots where there will be variants. Even at these spots, you will typically find that one version of the sequence is the most common, and the common one is inferred to be the ancestor. Using this technique on each position in the sequence produces your guess for the original sequence.

So you have an estimate of the original sequence, and you know how many mutations have occurred. You can figure out how frequently mutations occur, so, basically, you just divide the number of mutations by the rate of mutation, and that’s the time since the last bottleneck.

This is a very rough portrait of the process. There are complications in mutation rates: different loci mutate at different rates for chemical reasons. You typically assume no selection pressure (i.e. neutral evolution or pure drift), which may not be true in some cases. The really hairy bit is dealing with correlations due to family trees, but these issues are are not intractable. You can assess how far off your estimate is likely to be (“error bars”). This is a well studied problem and the results are pretty robust.

For some background, see here or here.

Posted: June 15th 2011

See all questions answered by George Locke

SmartLX www

You’re right on all counts.

They identify bottlenecks by deducing that the entire population is descended from individuals who lived well after the origin of the species. In the case of humans, we’re confident there was a bottleneck about 70,000 years ago, but “bottleneck” in this case means about 15,000 people, not just eight.

I thought of the same thing not long ago, and looked up the creationist solution by Answers in Genesis. It goes something like this: humans’ physical condition was perfect up until the Fall of Adam and Eve, and even after that it was in a sense superhuman for many generations. (That’s why some Biblical figures lived for 200-900 years.) Noah and his family benefited from this, such that their DNA contained more variety and “good stock” than any modern human. The extreme bottleneck of the Ark only served to bring them down to what we would think of as “normal” longevity and other genetic properties. The same goes for all the other animals.

The whole thing is simply a fresh layer of unsupported assertions on top of the already ludicrous story, but it doesn’t matter. The important thing for the creationists is that they have an answer, any answer, so that they can reassure any believers who might be starting to doubt. That’s the major goal, far more than actually converting non-believers.

Posted: June 15th 2011

See all questions answered by SmartLX


The biblical flood story is fictional. There is no need to be an geneticist to figure that out. However, there are geneticists who blog on the net, why don’t you ask them your genetics question?

Posted: June 15th 2011

See all questions answered by logicel

brian thomson www

I am not a geneticist, but I don’t need to be to have an opinion on this. Several evolutionary biologists, using DNA, have come to the conclusion that there have indeed been population bottlenecks – one, about 140,000 years ago, another about 70,000 years ago.

There have been catastrophes in the Mediterranean and Middle East all throughout history. The Atlantis myth, for example, may be related to the volcanic eruption of Santorini and a tsunami resulting from that. The Mediterranean Sea itself may have been created in catastrophic fashion, since it was a dry basin before water flooded in from the Atlantic past Gibraltar. If that really happened (which is uncertain), it would have seemed as if the whole world was flooding. But only people in that region would be affected – what about e.g. sub-Saharan Africa, where life originated?

Such stories would be passed down through generations, subjected to amplification and distortion. The time scales involved would become totally unreliable; with no recording of dates in any fashion, standardised or not, 100,000 years is as “long ago” as 10,000 years. You simply can not trust biblical dating on its own.

You ask “if Noah was fictional, why is there a lineage from fictional characters to people in Genesis?”. Do we know that Noah was fictional? As with Jesus, the basic existence of a person is a separate question to that of the validity of the stories told about them. There could have been a Noah who told tall tales about himself, or about whom such tales were told. Or, it could simply be the case the supposed lineage is a work of fiction too. Forget the stories, biblical or otherwise: what does the evidence tell us?

Posted: June 15th 2011

See all questions answered by brian thomson


Is your atheism a problem in your religious family or school?
Talk about it at the atheist nexus forum