In this post, I will expand upon the language analogy I was able to present only briefly during the debate. The main message is this: the very same reasoning applies to both languages and the biological world to provide conclusive evidence for one thing being descended from another. If that reasoning is correct for languages (as everyone including the creationists admit), then it is correct for the biological world as well. By the same token, if a young-earth creationist decides to tilt at windmills and say that this reasoning is incorrect in order to deny the obvious fact of common descent in the biological world, then they are also forced to accept absurd claims about language, e.g. that French is not descended from Latin.
OK, here goes. During the Roman Empire, the territory covering Italy, France, and Spain all spoke Latin. There were local variations, but all these Latin speakers could understand one another. They had to, as there were extensive trade connections among them, requiring successful communication.
When the western Empire fell, however, these trade connections gradually disappeared. Communication was no longer important, and without this pressure to remain the same, the local variations increased in number and size. (Imagine if North Americans, Brits, and Aussies stopped communicating for a few hundred years!) Eventually, they diverged so much that the separate populations could no longer understand one another on the rare occasions they came into contact. They were not speaking Latin anymore; they were speaking its descendent languages: Italian, French, and Spanish. (This is the equivalent of descendent species in biology, which have become isolated and no longer “communicate” – or interbreed – with one another.)
Even if we knew none of this history, we could easily see that Italian, French, and Spanish have a common ancestor just by looking at their vocabulary, grammar, and sound. For example, “cat” is “gato”, “chat”, and “gato”; “tree” is “alberro”, “arbre” and “árbol”; and “tower” is “torre”, “tour”, and “torre”. The similarities are not coincidence: they indicate common ancestry. By contrast, the Swahili words for “cat”, “tree”, and “tower” are “paka”, “mti”, and “mnara”. This difference indicates that Swahili is much more distantly related, if it is related at all. Of course you have to look at the languages as a whole to be sure, and linguists have done just that – reconstructing the evolution of languages often without knowing the slightest thing about the people who spoke them.
In biological organisms, the things that change gradually are of course the genetic codes. These are even richer sources of information than languages – the human genome, for instance, is 3 billion letters long, about three times the length of the Encyclopedia Britannica. When we see that the chimp genome and the human genome are 98% similar, we (of course!) conclude that this isn’t a coincidence: they had a common ancestor. This is exactly the same reasoning as in the language case.
Linguists and biologists have both used this simple reasoning to build massive trees (or webs) of descent for most languages and for most organisms (in varying detail). They’ve been able to trace the geographic travels of both. They’ve been able to reconstruct both extinct languages and extinct genomes. They’ve been able to estimate the dates of these extinct languages and extinct organisms, based on rates of change measured today. They’ve been able to trace instances of “borrowing” terms (like the Swahili for “plow” is “plau”), and the biological equivalent (see Mackay’s “Family Trees” post & comments). In the biological case, this has all received ample confirmation in the fossil evidence. (In the language case, of course, the “fossil” evidence is ancient writing.)
Linguists know that common descent is a proven fact for languages. Biologists have even better evidence for common descent among organisms. Case closed – even we base our conclusions only on modern similarities and differences.
I’ve only lightly touched on this rich analogy, and I haven’t given any specific biological examples beyond the Meredith et al. paper I mentioned in my comment on Mackay’s “Family Trees” post. So I will gradually follow up on this and more in the comments, in the coming week and a half.