If you have men who will exclude any of God's creatures from the shelter of compassion and pity,
- St Francis of Assisi
I'm outta here!
Source: c1940 - photographer unidentified; from the R P Kingston collection
This kitten, born with two faces, only lived for two days.
The Nature of Normal Human Variety: A Talk with Armand Leroi
If you go to teratology museums - literally "monstrosity museums" - in places such as Amsterdam and Philadelphia, you can see rows of babies in bottles. These infants, usually stillborn, are deformed in ways that are truly hideous, that really represent the kinds of monstrosities that you might expect from Greek myth. I mean this quite literally. They include children born with a single eye in the middle of their forehead, and who look exactly like the monsters of Greek myth - Polyphemus in The Odyssey, for example. Indeed, it's sometimes suggested that the monsters of Greek myth were inspired by deformed children, and this seems to be a fairly remarkable correspondence, at least with some of them.
These infants, when you see them, are truly horrific. But very quickly, after you look at them, a sort of intellectual fascination takes over because it's clear that these children tell us something very deep about how the human body is built. Take, for instance, these children with a single eye in the middle of their foreheads. The syndrome is called, appropriately, Cyclopia. Cyclopia is caused by a deficiency in a gene called Sonic hedgehog. Sonic hedgehog is named after a fruit fly gene which when mutated causes bristles to sprout all over the fruit fly larva, hence "hedgehog". When the gene was found in mammals, some wit called it Sonic hedgehog after the video game character. If you get rid of this gene, bad things happen. You lose your arms beneath the elbow and legs beneath the knee. The face collapses in on itself, such that you get a single eye in the middle of the forehead and the rest of the face collapses into a long, trunk-like proboscis. The forebrain, which is normally divided such that we have a left and a right brain - the left and right cerebral hemispheres - is fused into a single unitary structure. Indeed the technical name for this syndrome is called Holoprosencephaly.
Now all this is very horrible, and actually that's just an initial list of things that can go wrong in infants that have no Sonic hedgehog. But what's really interesting about it is that by looking at infants of this sort you can reverse-engineer and ask what Sonic hedgehog does in the embryo. Instantly it tells you that one of the things that Sonic hedgehog does is to keep our eyes apart because if you don't have the gene the face collapses. It also separates the left and and right sides of our brains. And it's needed for the formation of our arms and legs. In fact, it is one of the most ubiquitous and powerful molecules in the making of our bodies.
And other, more subtle mutations, tell more about it. For example, just as having too little Sonic hedgehog causes the face to collapse in upon itself, having too much causes it to expand. I was recently in San Francisco, in Jill Helms's lab at the University of California, San Francisco, where she's got a jar containing the head of a pig. Or is it two pigs? It's just not clear since the jar contains a pig with two faces, two snouts, two tongues, two throats, and three eyes. It's not a Siamese twin pig; it's just a pig with two faces. Chickens and pigs with two faces crop up periodically, as indeed do humans with two faces, or nearly two faces. There's a syndrome in which you have eyes that are very widely spaced from each other, and in which the nose becomes duplicated. You have two noses side by side in two varying degrees of development.
The gene for this syndrome has recently been cloned, and guess what? It turns out to be the gene that controls Sonic hedgehog, and that, in fact, switches it off. People with the syndrome have too much Sonic hedgehog just as infants with Cyclopia have too little. So by looking at a range of these kinds of syndromes you can put together a very complete picture of how a gene like Sonic hedgehog controls one particular feature of us, the width of our faces. It's a very mundane thing that you'd hardly think about, but that seems to be controlled by this genetic system.
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