From lefty snails to deadly chemicals, asymmetry in nature is more common than you think.
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Word Count / Video Length:1072 / 15:16 min
Have you ever wondered why your heart is on the left side of your chest? Or why snail shells always seem to coil to their right side? Asymmetry in nature – sometimes known as chirality – is more common than you might have guessed.
From the tiniest molecules to the animals roaming the earth, handedness plays an important part in nature and biology. Let’s explore the science behind these phenomenons.
Asymmetry of molecules
Chirality means that an object or molecule cannot be superimposed on its mirror image, and it turns out our hands are actually a helpful way of visualising this.
Think about it this way: your right hand is a mirror image of your left – both have thumbs, fingers, a palm, and a front and back – but you couldn’t lay one on top of the other and have them match up perfectly, could you?
In chemistry chiral molecules can exist in two forms: as left- or right-handed forms of the same molecule. The pair are known as stereoisomers, meaning that they cannot be superimposed on each other, regardless of which way you rotate or move them. The two molecules have the same chemical properties, except when reacting with other chiral compounds.
Many important substances in biology are chiral compounds, such as carbohydrates, the amino acid building blocks of proteins, and the nucleic acids in DNA. The DNA double helix itself is chiral too, existing with a right-handed turn.
One particularly infamous example of a chiral compound is thalidomide. Sold from 1957 until 1961 as a way to treat morning sickness during pregnancy, it was then discovered to be a teratogen – a drug that causes birth defects.
Well, one of the thalidomide stereoisomers (called an enantiomer) was responsible. As mentioned earlier a lot of biological molecules are chiral, so sometimes two enantiomers can have very different biological effects inside the body.
Unfortunately, thalidomide exists as a racemic mixture in biological conditions – where the enantiomers interconvert and exist in equal amounts. So, separating them before use was ineffective and while one caused the desired sedative effects, the other caused birth defects.
It’s estimated that 10,000 infants were affected by the use of thalidomide during pregnancy; 40% died around the time of birth and others lived with birth defects such as limb, eye, urinary tract, and heart problems.
But it’s not just on Earth that these ‘handed’ molecules exist, they’ve also been detected outside of our solar system. Propylene oxide was detected in a giant cloud of gas and dust called Sagittarius B2 and, although it isn’t used by organisms, it’s presence in space gives astronomers hope that they might one day find other life-based chiral molecules in space.
Asymmetry on the outside of our bodies
Pulling further out to look at our bodies and the bodies of other organisms, symmetry and asymmetry play important roles in both biological development and behaviour.
The left- and right-hand sides of the exterior of the human body are essentially mirror images of each other, and this is also the case for over 99% of animals. One animal that defies this trend is the common garden snail, as almost every individual has a shell that coils to its right.
This asymmetry was explored in depth in the 2019 SCINEMA International Science Film Festival documentary Jeremy the Lefty Sail and Other Asymmetrical Animals. Introducing us to the fascinating story of Jeremy – a one in a million snail whose shell coiled to the left rather than to the right.
The film also touched on other rare animals that also exhibit asymmetry including: the adult flat fish (Bothus mancus) which has both eyes on one side of its body, the top and bottom mandibles of the mature crossbill bird’s (Loxia curvirostra) beak which curve in opposite directions, and the tusks of narwhals (Monodon monoceros) which protrude from their upper left jaw.
But how are animals able to develop symmetrically, or asymmetrically in some cases? The answer lies in the instruction manuals of our bodies – our genetics.
The researchers actually found the gene that determines left-right asymmetry in snails – Ldia2. This gene codes for a protein called formin that interacts with the cell’s cytoskeleton – a complex network of interlinking protein filaments that give the cell structure.
Snails with a mutation in this gene had their production of this protein turned off and developed shells with a coil to the left instead of the right.
Asymmetry on the inside of our bodies
The inside of our bodies is a different matter, however, with many organs – such as the heart or liver – positioned solely on one side of the body while others – such as the lungs and kidneys – grow symmetrically.
There is a rare human condition known as dextrocardia, where the heart is placed on the right instead of the left. This condition is sometimes also accompanied by the condition situs inversus, where the arrangement of all internal organs is a mirror image of normal anatomy.
Usually, this doesn’t cause any problems and an individual can be unaware of it until an unsuspecting doctor examines them and gets a good shock. Though occasionally there are cases where the internal organs aren’t placed consistently one way or the other, and this can cause problems.
Situs inversus runs in families, but unlike snails the underlying genetics of this condition are complex and many different genetic factors or genes may cause it – so it can’t be pinned down to one single gene.
Interestingly, some honeybees (45% to be exact) tend to naturally favour one of their sides, right or left, when flying. They showed a distinct bias when made to choose between flying through two holes, and researchers think that this kind of bias might help the colony as a whole, because it could result in the rapid travel of the group of bees through a cluttered environment.
At ground level, there’s a form of movements known as ‘asymmetric gaits’, where the timing of footfalls is unevenly spread. Galloping is one such example, and it turns out that animals evolved the ability to coordinate their limbs independently around 472 million years ago (mya), long before life emerged on land.
From molecules to physiological processes and anatomy, asymmetry is essential in so many areas of life – see if you can spot it in yours too.
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