I learned a fantastically cool thing about biology recently.
Just about everything living organism in the known universe has DNA in it, a sort of code made up of strings of tiny molecules. DNA forms into building blocks called genes, and it’s these genes which determine how the living organism in question – whether that’s a tulip, or a mosquito, or your mum – will be built.
(That’s not the fantastically cool thing. I mean, it is a fantastically cool thing, but I didn’t only just learn it.)
There are certain packets that this DNA comes bundled in, called chromosomes. Humans have 46 chromosomes, which come in 23 pairs, each of which contains a whole bunch of different genes.
Other species can have wildly different numbers of chromosomes, usually varying from around a dozen to around a hundred. The number often doesn’t have much relation to what we might consider an organism’s complexity or evolutionary advancement; there’s a type of fern plant, for instance, which has over 1,200 of them.
Anyway, what’s interesting is that chimpanzees, orangutans, and gorillas all have 48 chromosomes (24 pairs), and this is a potential problem for the idea that humans and other animals share a common ancestor. The theory of evolution states that chimps are the closest living relatives to humans, and you’ve probably heard from various sources how we share around 96% of our DNA with them. So why is our chromosome number different?
Biologists had to come up with an explanation. They guessed that, although our fairly recent common ancestor with the great apes had 24 chromosome pairs, two of those chromosomes must have fused together in the line of descent that led to modern humans, leaving just 23.
The thing that stops this from being just a desperate rationalisation to rescue a shaky theory plagued with inconsistencies, though, is a little thing we like to call science, bitches. Because they didn’t just assume that this fusing thing must have happened for the sake of preserving the theory. They went and checked. They looked for evidence.
There are two chimpanzee chromosomes, 2A and 2B, which show clear indications of being equivalent to the two which merged in our own line of ancestry to form the human chromosome 2. The two chimp chromosomes look strikingly similar, when laid end to end, to the single human chromosome. You can see a sequence of “telomeres” in the middle of the human chromosome – those are the bits that normally come on the end of a chromosome to protect it from deteriorating, and they’ve ended up just where you’d expect if the ends of two chromosomes fused. The “centromeres” also line up perfectly – that’s the middle bit where the two copies of chromosome DNA join up.
As is observed on the Evolution Pages:
Not only is this strong evidence for a fusion event, but it is also strong evidence for common ancestry; in fact, it is hard to explain by any other mechanism.
Unless God is seriously messing with us, and going far out of his way to make it look like our genetic material is inextricably linked to that of other animals in the geologically recent past, this is one of the most profound and succinct demonstrations of the truth of evolution that I’ve met.
And remember, we might have found a conspicuous lack of these remnants of chromosome fusion. We honestly didn’t know if we were going to find what we were sure must be there, and if we didn’t, it was clear that the theory of common descent would have been in trouble. This prediction was made at a time when we had no data on whether this evolutionarily crucial evidence was actually present.
Good science often involves sticking your neck out, and accepting that there’s something wrong with your ideas if the proof you’re looking for isn’t where it should be. You don’t get that from proponents of creationism or intelligent design. There are numerous observed facts about the history of life for which they have to assume that God intervened in some way to arrange things just so – but it has to remain an assumption. They can’t then test their hypothesis by checking his day planner.
Anyway. I thought this was cool and I wanted to talk about it.
Also, Chromosome Fusion is what I’m going to call the microbiology-themed nightclub I’m opening.