So having acquired a horse’s leg, what do you do with it?
Ignoring some of the cruder suggestions that I know some of you are thinking (yes, I mean you Bill), here’s how I use it in lessons with my Year 11s/12s…
I start the lesson with the story I wrote last week. It’s a good introduction for what follows, if only because it grabs their attention. Huh? here’s this going?
I then ask, what have we got in common with horses?
It doesn’t take much prompting to unleash a deluge of similarities – we’re both animals, both mammals, have two eyes and ears, a mouth, a nervous system. We have a digestive system and a back bone…
Ah! Yes, both vertebrates. And, in fact, both possessors of a fine skeleton.
What do all these points tell us about our respective family trees?
Again, they’re very happy with the idea that at some point in the past, we shared a common ancestor. Which is why we both have, for example, legs….
I fish out the first bone. It’s a whopper. Which leg?I ask.
For some reason, they always think it’s the back leg. But no, it’s actually the front.
Which brings me to the Curious Incident of the Badger in the Night. Because ideally I would not start with the enormous horse humerus, but the horse scapula, because it’s so glaringly obvious what it is and so glaringly similar to our own scapula, that great flat sheet of bone for powerful muscle insertion. It’s perfect for setting the stage of homology, which is of course where this is all heading, as it establishes more clearly than any picture or discussion that their bones are our bones.
So why don’t I have one?
Well, I did. The leg so generously deposited on our drive all those years ago was in full possession of a scapula, and a scapula was certainly deposited, along with all the other bones, into the dustbin of horse-stock that I described last week. But at some point in the Neutrase X-tremo-Stench digestion that followed, some wretched animal –that must have found the smell appealing… – found the dustbin, nosed off the lid, and made off with the top-most bone, a nice juicy scapula. A badger? The neighbours’ dog? We’ll never know. And I never found it, despite extensive searches. Grrrrr….
Still, it adds another nice little detail to the story – they think it’s funny – and we’ve established, via a rather roundabout route, that this is a horse front leg. We’re also standing next to the lab human skeleton.
OK, if this bone was attached to a horse scapula, what is it? What’s the equivalent bone in a human?
Yes, that’s right, it’s a humerus. How are they different? And why?
It’s a striking contrast. The two bones are pretty much the same length, but the horse one is massively thicker and heavier. Of course it is. Horses don’t use their front limbs for swinging through the trees or picking fruit or grooming other horses. They need load-bearing supports that can hold up half a tonne of equine beast.
OK, if this is a humerus, what’s next? They generally know their skeletal anatomy – yes, we should have a radius and ulna.
Quick aside – which one’s which? This they don’t know. Or just guess. There are groans for my handy aide-memoire – the ulna articulates with the ulbow, while the radius articulates with the rist.
So what’s going on here? Because the next horse bone seems to bear no resemblance whatsoever to the human equivalent. It appears to be a single, massive load-bearing bone again.
But wait, look more closely. There are two bones, one articulating with the ulbow, the other with the rist, but they have fused together. It’s a beautiful illustration of the line from the Evo-Devo song – “safer the mutation aimed at regulation, keep the building blocks but switch their activation…”
It’s almost my favourite moment. As evidence for a shared ancestor, it’s compelling. As evidence for evolution itself, it’s hard to beat. No divine creator would make such a bodge job if designing the thing from scratch. The students think it’s seriously cool. Look! A horse embryo develops exactly the same bones that we do, and then glues them together because it actually only needs one, and for a completely different function.
But it gets better. If the broad base to this bone makes it the radius, what bone should come next? A quick cross-check with the human skeleton, and they realise it’s the wrist, not just one bone, but lots, the carpals.
And here they are, a really pleasing set of bones with an intriguing jigsaw puzzle for anyone sufficiently interested and motivated to try and piece them together.
But how odd! The horse’s wrist is half way up its leg. Again, only evolution can explain the idiocy of having a cluster of blobby bones half way up a load bearing limb. Intelligent design? Give me a break…
On we go. What’s next? The finger bones! The meta-carpals! Again, something seriously weird is going on.
There only seems to be one meta-carpal, but look closely and there are the others, shrunk and withered and, again, fused either side of the central one.
We put them out in order, ending with the pleasingly shaped “hoof-secreting” bone.
And I ask them to demonstrate their understanding by giving me a simply visual indication of what a horse runs around on. … Yes, that’s right. It’s their one and only chance at school to safely give their teacher the finger…