I came across an interesting paper in the BiorXiv yesterday. It was written by a group from the University of Wisconsin and concerns using DNA alignments between species as evidence for common ancestry between us and, indeed, every other organism to have ever lived. The reason this caught my eye is that this is one of the areas that Intelligent Designers and Creationists often try to pick a hole in. Let me give you some background.
The idea of evolution by natural selection inexorably leads to the concept that all life on earth evolved from one common ancestor. For some reason this upsets a lot of people. One of the many lines of evidence we have to back up this conclusion is that when you compare the DNA sequences of organisms that you would expect to be closely related they are more similar than those that you would expect to be less closely related.
So, human DNA is more similar to chimp DNA than it is to dolphin DNA and so on. This is explained by the fact that humans and chimps had a common ancestor more recently than humans and dolphins did and so there has been less time for human and chimp DNA to diverge from each other. So far, so good.
No matter how far back in evolutionary time you go, or which two organisms you happen to be comparing, there are certain similarities shared across all life that has ever been studied. I, as someone who accepts the scientific consensus, am happy to conclude that this is evidence for common ancestry. Doubters argue otherwise. They claim that the reason there are similarities in the DNA codes of life is that there are only so many combinations out there that will work. Put another way, it is the requirement of the proteins that our DNA encodes to function properly that necessitates the similarities. If those similarities weren’t there then you wouldn’t have a living organism.
On the face of it this isn’t a terrible argument and, dare I say it, there is a kernel of truth in there. As ever, though, that kernel has been abused beyond all recognition. There are certain parts of proteins that do have to be absolutely just so, a single little mutation in their functional domains could render them completely useless. These wold be strongly selected against by natural selection and so no diversity would be allowed to take root there. We say that these areas are highly conserved.
There are plenty of areas in proteins that don’t need to be kept just so, though; and, more importantly, there are all the bits between the genes that are pretty much free to mutate as they please as natural selection doesn’t give two squirts of urea what happens to them. So the argument really doesn’t hold up.
For a long time we weren’t able to scientifically refute the argument when it came to the functionally important parts of our genome. We just didn’t have the data. Now, though, with well over a decade of experience of working with our genome we now have a much better handle on which bits are the vital ones, what tolerances they have and how to compare them.
This new analysis does exactly that. They limited their analysis to primates as they specifically wanted to tackle what they called ‘controversial among a large fraction of the general public.’ It will come as no great surprise that, even allowing for the constraints of maintaining the function of a protein, they found ‘overwhelming evidence against separate ancestry and in favor of common ancestry… [and] overwhelming evidence that humans share a common ancestor with other primate species.’
When someone has tried to use evidence and logic to come to their conclusion they will, hopefully, accept that they are wrong and change their opinion if they have been shown to be so. Obviously this will not be what the Creationists and IDers do as they are not interested in evidence, they just want everyone to believe their dogma. They have had their legs kicked out from under them once again, but usually this just provides them with a lower vantage point whence to drag you down to their level.
That would be the wrong way to look at this, however; we have added another strut to the wheel of evolution by natural selection and helped it roll methodically on towards the horizon of our knowledge. I’m glad to be along for the ride.