Science This article is more than 14 years oldLopsided gene that proves humans are distant cousins of the humble snail
This article is more than 14 years oldPZ MyersA gene shared by birds, fish, reptiles, people – and snails – reveals the fundamental relatedness of all living creaturesIn this, my inaugural column, I face a bit of a problem: I was heavily promoted by the editor when he introduced the Guardian's new online science columnists, and so I feel the need for a particularly dramatic and exciting subject. Fortunately, some recent science news provided me with one, courtesy of a paper by Grande and Patel in Nature:
I know, you're positively floored. Amazing! Enthralling! Say no more; the implications are simply awesome.
Then I bounced the idea off my wife, who is usually more down-to-earth than I am, and she seemed to think I needed to provide a tiny bit more exposition, so I will oblige … but trust me, this is wonderfully interesting news.
Look at yourself in the mirror. You're probably mostly symmetrical: one eye and one ear on each side of your head, features that are at least roughly even, and any lopsidedness is most likely due to postnatal wear and tear.
Deep inside you, though, you are profoundly asymmetrical, and that asymmetry is essential for your well-being. The speech centres of your brain are mostly on the left side, the left side of your heart is larger and more muscular than the right, your stomach coils to the right and the bulk of your liver is on the left, and your large intestine loops just so, with your appendix on the right. With a few medically interesting exceptions, we all have guts consistently skewed in a particular orientation.
This asymmetry is established in the embryo. Beating microscopic hairs called cilia set up counter-clockwise currents that deflect sensors on the left side, which then switch on specific genes (one called nodal, in particular) on just the left side, which in turn activate other genes that bias the formation of organs to one side or the other.
Nodal acts as a little flag thrown up early in development to tell cells whether they are on the left or right side. Nodal also seems to be a universal signal in animals with backbones, the familiar fish, reptiles, birds and mammals, and is used in similar ways in all of those animals.
It was not, until now, found in animals such as molluscs and insects and nematodes, suggesting that perhaps they used a very different mechanism … an idea that we now have to rethink.
Look at a familiar garden snail. Snails are obviously asymmetrical — you don't even need to dissect them to see that. They have a coiled shell on their backs that, in some species, has a left-handed twist, while in other species it makes a right-handed spiral. What genetic mechanisms do these animals use to produce a consistent asymmetry?
This is the surprise: they use the same molecule we do, a copy of nodal. Snail nodal is expressed asymmetrically in the embryo and is crucial for generating adult asymmetries as well.
Doping snail embryos with a chemical that blocks the action of nodal prevents the formation of a coiled shell, yielding strange embryonic snails with perfectly straight, cone-shaped shells.
Obviously, our gene is not exactly the same as theirs — the snail gene has differences in sequence, and is activated on the right side instead of the left, and uses a different trigger than currents from beating hairs.
But it's still an astonishing similarity: a common gene that takes action in some of the earliest stages of development. And it works in animals as far apart in evolution as a snail and a human.
A single gene is a small thing, but it is yet another piece in the growing body of data that reveals the fundamental relatedness of all living creatures.
A snail is a strange-looking beast, at least to us, but right down at the core of its biology it is built with the same toolbox of genes that we use, and we share a common ancestry with it. A very distant ancestry, for sure — our last common ancestor lived over 600,000,000 years ago — but it should at least give you pause as you're exterminating the little pests in your garden this summer.
Explore more on these topicsShareReuse this contentncG1vNJzZmivp6x7tbTEoKyaqpSerq96wqikaKuTnrKvr8RoaWloaWSusb6Oamxon5WjsrW1wqxknqWSp8awv4yapZ1lo6myrnnCnqOlqw%3D%3D