Thread by @MedCrisis

You've probably all seen this epic meme. It got me thinking - why *IS* the human body asymmetrical? Why do almost all of us adopt the same asymmetry? [SCIENCE THREAD!]

An object is chiral if it is distinct from its mirror image, derived from Greek for hand as they are the archetypal chiral objects (ie L is not the same as R) 👈🏽👉🏽

Humans are obviously chiral. Hearts left, livers right, colons clockwise etc. We're not absolutely sure why.

Strictly speaking we're not chiral in morphological classification terms - we're pseudobilateral. Snails are chiral - their shells turn in one direction.

Spherical - volvox
Radial - sea anemone
Chiral - snail
Bilateral - planaria
Pseudobilateral - most vertebrates

Pseudobilateral animals have a symmetrical external body plan - which is probably why despite being distinctly asymmetrical internally, our fascination with symmetry as a species is well documented, from physical attraction to geometric patterns.

OK, we have one copy of most organs, they can't all go in the middle, so what's the big deal?

But *why* do we all go in the same direction? True situs inversus (totalis) is extremely rare.

A leading theory focuses on a small pit on the embryo, seen here. Vertebrate embryos start out symmetrical and the origins of L-R asymmetry are likely to start at this pit.

The cells there have tiny hairs - cilia - which are divided into motile and sensory units. The motile cilia beat in a synchronised clockwise manner, creating a right to left flow of fluid (observed by injecting little coloured beads into mouse embryos).

Cilia at the rim detect the vortices created and accordingly cause differential expression of genes. These cause different proteins (which are 3D structures) to be synthesised and within a few hours the L and R sides are different. This eventually leads to different organs.

One interesting and poorly understood phenomenon is the relationship with sex determination: in human hermaphrodites, ovaries develop on the left, testes on the right.

The heart's origins as a midline tube & sensitivity to errors in asymmetry explain why there is a whole field dedicated to congenital heart disease but no other organs. #ACHD

At around 3 weeks, the symmetry is broken as the heart twists. It then develops into a highly asymmetric organ.

A problem with cilia might explain diseases like Kartagener's, which is a ciliary dysmotility disorder characterised by multiple respiratory issues along with dextrocardia (heart on the wrong side)

The asymmetry is thought to come from *molecular* chirality of some component in the cilia. So we might owe our entire anatomy to the shape of an individual molecule.

However - mammals like pigs don't have those cilia and are still laid out in a similar manner to us, so some propose that all cells have a natural chirality.

Again, this is thought to be due to the chiral nature of our most fundamental molecules - proteins, sugars & DNA.

BTW, the chirality of organic molecules was first noticed due to a curious property of light - linear plane-polarised light beams behave in a chiral manner, with L and R 'handed' components. Chiral molecules in the beam act differently with each part (snuck some physics in).

Personally I find the idea that we owe our macroscopic layout - where each organ goes - to the asymmetry present at the microscopic level, which has set off a chain reaction of asymmetry for millions of years of evolution, pretty bloody amazing.

So next time some beauty magazine raves about symmetry being more attractive, remember that in a symmetrical world, we'd all be dead. Then give them two chiral fingers.

BONUS ADDITIONAL TWEETS TO MAKE IT MORE ASYMMETRICAL: Thanks to @Plinz for asking a great question, I have added a few more tweets for a bit more depth 👍🏽 

https://twitter.com/MedCrisis/status/1014412974063316993