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City of Brass

Partly because I just went to Africa and flew over (but did not have a chance to actually visit) the Great Rift Valley (also known as the Cradle of Mankind), I’ve been thinking a bit about evolutionary theory of late. I am a scientist, but not a biologist – my field is medical physics. Still, my background makes me biased towards the scientific establishment and I am an ardent believer in the scientific method. While technically as a deeply religious person I do believe in “intelligent design” (in the abstract) I don’t believe in Intelligent Design as promoted by the various evolution-denialists in the political arena. I am quite strictly against introducing religious theories such as ID into the science curriculum.

All that said, I still am unable to accept the blind assertion that genetic mutation is the sole source of speciation. Note that I am not talking about the origins of life, but rather the evolution of life afterwards from species to species. It strikes me that the evolutionary dogma can be reduced to the idea that DNA is “read-only”. Contrast this with the (discredited) ideas of Lamarck who argued that the environment can introduce changes to an organism that are then heritable by its progeny; given that DNA is indisputably the mechanism by which species reproduce, that implies that DNA is “read/write”.

From an engineering and aesthetic perspective, I have trouble with the idea that a system so complex as DNA and gene expression can be so rigid. My intuition is that DNA does not posess enough degrees of freedom to “encode” life as we know it. But how can I test that intuition without getting a PhD in genetics? I think I’ve come up with a way, though of course it is crude and rife with bland layman assumptions. Still, bear with me (and I hope to attract some attention to this from experts so we can refine it).

Let’s take some basic numbers. There are about 20,000 genes in the human genome, with an average size of 50 kilobases (ie, 50,000 base pairs. Remember DNA is a double-helix, unlike RNA). Also, we are often told that humans and chimpanzees differ in their genomes by only 1%. Actually, that figure is only for genes where humans and chimps totally differm but there are some genes where the variation between teh species might not be so absolute. I’ll use 5% instead. Finally, we know that according to best estimates, humand and chimpanzees diverged from their common ancestor about 5 million years ago.

Taking these bits of data, we can actually estimate the required rate of evolution in terms of point mutations in DNA needed to turn a chimp into an ape. Of course, humans did not evolve from chimps, so we would then halve the rate we calculate to get the change from the common ancestor of both to humans (or chimps). So, let’s do the math.

20,000 genes x 50,000 base pairs = 1 billion base pairs
5% difference between humans and chimps = 50 million base pairs
rate of change = 1/2 * 50 million base pairs / 5 million years = 5 base pairs per year

So, to go from the common ancestor of humans and chimps, to modern humans, the average rate of mutation required would be 5 base pairs per year. This seems like a very high rate to me; if we discretize into generations of 25 years each, then we are talking about 125 base pairs every generation.

It seems that this is a number that can be tested over time. You’d need to collect DNA data from thousands of people, over a few centuries, to get an idea of the actual rate of change. But it is definitely something that can be tested (and the exact number of people you’d need to test, and how many generations to test, is something that can be estimated by statistical theory to ensure that the results have statistical significance).

I am sure there are many objections to the methodology above – one that is immediate is that humankind has historically had very small populations, unlike other species like insects or rodents etc. That means that other species have a lot more raw genome floating around. But that kind of supports the contention that human evolution must be very rapid indeed to support the observed evolution over the past few million years, given our far smaller gene pool.

The implication of such rapid evolution is that we should actually notice it on human timescales. And that there is actually is some mechanism of action that is actually driving the mutations themselves – cosmic rays? transcription errors? normal statistical variance?

It seems that if we aren’t mutating at a rate comparable to above, then some sort of alternative mechanism must also be operating to accelerate the changes in DNA required to evolve from one species to another. There’s certainly some evidence that there are such “neo-Lamarckian” processes at work, The case of the humble water-flea certainly is not explicable by normal Darwinian processes.

You could go even further and compute the total information content of the human genome, and then try and see whether that is sufficient to describe a human being. But that is a task I’ll leave for later, or someone else. I think I’ve ventured far enough out on this limb for now 🙂

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