We are now 7 episodes into our journey from the dawn of life to humanity, and finally approaching something recognizable, familiar, safe. Here's the journey so far if you missed it - we've all learnt a lot on the way:

Introduction

Archaea, Bacteria & Eukaryota

Bikont or Unikont

Opisthokonta to Holozoa

How animals became animals

Why did we become multicellular?

The question of 'Why' has generally been a very tricky one to approach, and like many parts of this series, the answer is simply 'we don't know'. But the bigger things get and the more general our observations become and the more recent in time we reach out for, the more confident we can be in our assumptions.

Now, we are approaching an organic feature that is shared among every living species from this branch onwards.

Bilateria

As stated at the end of the previous episode, bilateria is a demonstration of bilateral symmetry across the physical body shared among all creatures beyond this point. Think of your face, a dogs face, a ladybird, a butterfly, an elephant.

...You're imagining a butter knife slicing right through them, aren't you? Well if not, you are now.

There are still up to 1.4 million species that fall under this rather large umbrella, but let's take a quick step back to some more grey areas first to see what we're getting rid of before figuring out why:

Eumetazoa

Here we have yet another 'proposed' group that comes after metazoa, which could separate the phylums Ctenophora and Bilateria... So... Basically, Ctenophora are Comb Jellies, a primary contender for the most ancient relative to all animals, and are often mistaken for jellyfish despite being from two separate phyla.

Actual Jellyfish on the other hand fit into Cnidaria which also branches off shortly after in the Planulozoa clade:

That placozoa you see splitting from Bilateria are the simplest known multicellular organisms. Pathetic really, let's ignore them.

So first, Comb Jellies branch off, followed later by Cnidaria - Jellyfish among other things - who move away from Bilateria also, and we've finally filtered out the junk to where we are now.

Um... so, why are we symmetrical?

Bill Gates famously once said:

I will always choose a lazy person to do a difficult job because a lazy person will find an easy way to do it.

I relate to this more than you could possibly imagine, and so does the natural world. It seems the reason for symmetry is simply because nature is lazy.

Thanks to gravity constantly slowing us down and pushing us to the floor, it makes mechanical and aerodynamic sense to create symmetry for maximum stability. With the centre of gravity in the middle, there's little tendency to lean left or right and fall over, which allowed us to take the arrogant step of going upright on two legs.

You can see this natural symmetry in inorganic systems such as snowflakes, cars and planets. It just makes things easier. Stupid gravity.

The thing is, organic life obeys two separate systems to thrive; random genetic mutation, and non-random natural selection. That second one allows us to become variable in our efforts to defy physical laws. Nature can make choices that go against the natural urge to follow the genetic code. Consider the flatfish:

This fish obeys its genetic tendencies at birth, being a symmetrical fish like all its normal buddies, before distsorting and warping its body round to its side, like this hilariously meme-able process image (@trumpman)

Of course, the lifestyle that the flatfish chose created pressure to be a bottom-dwelling species, and this was, somehow, the best idea at the time. I do wonder if they regret the decision, though.

So although symmetry may not be a vital aspect of the lives we live today, it's less biologically expensive to keep it that way (... for most of us) than to completely overhaul our genetic make up and become flat headed freaks. In our human world, most of us could happily add an extra arm on one side and get along just fine, but that's our choice once given the freedom from the womb. From a genetic standpoint, that third arm is a lot of frickin' work.

Genetic symmetry

This symmetry can be observed on the genetic and chemical level too, with DNA simply replicating itself and gluing them together. with a symmetrical stranger. This symmetry that grows into bilateria has a direct effect. Some studies show that facial symmetry is one of the few innate factors of beauty perception in humans and indeed other species.

An acutely symmetrical body implies healthy DNA to pass on to the children; whether or not symmetry actually relates to the health of genetics is still up for debate.

But if it's genetically advantageous to be symmetrical, why are we only talking about it now, 7 episodes in?

For the most part, everything we've talked about so far was either too small to be deeply affected by gravity, underwater away from the effects of gravity, or both. Floating around like @mobbs out of work doesn't particularly require balance one way or the other. That being said, there are different forms of symmetry that came before Bilateral symmetry, such as radial symmetry you may see in Starfish and flowers, and spherical symmetry that, like stars in the sky, provides the most possible volume with the smallest possible surface area.

The specific reasoning for bilateral symmetry is, once again, somewhat unknown but I really should emphasize how important laziness is. From the aforementioned gravitational limitations and mechanical efficiency, it's also just cheap and easy to produce two mirror organs such as the lungs and kidneys than it is to have one big, clunky lung in the way of the heart, or three lungs all over the place.

There are also some interesting quirks and exceptions. The brain, for example, is not very symmetrical, and one study found that autistic mice demonstrated surprising a-symmetry compared to regular mouse counterparts. This does somewhat fall in line with autism in humans. People with ASD have a tendency to process language more on one side than the other. This could possibly be the brain compensating for an imbalance in the brain's structure.

So what can you take away from this?

Well, it seems objectively true now that laziness makes you beautiful and successful, so there's that.

Human technological evolution is in direct accordance with natural evolution, and one could argue that computers and smart phones are completely natural. All our technology is a way to make things more convenient, allowing us to be lazier and lazier as the generations go by - Just the way nature intended it!

So next time your parents or your boss tell you to stop being so lazy, tell them it's hardwired into the Universe and there's no stopping the inevitable. Might as well embrace it.

All Images CC Licensed

References: _{Huffington Post | Facial attractiveness: evolutionary based research | Facial Symmetry | Biological symmetry | Brain’s Asymmetrical Shape Reflects Human Adaptability | Psychology Today}