Why Sex Can’t Exist

It’s impossible.  It’s improbable. And it breaks every rule in the universe.

You are the possessor of the most complex factory ever contrived.  It’s called your genome.  And you have 100 trillion of these jaw-dropping mass-production machines.

What’s your genome?  It’s a tightly integrated collection of chains made from your complete collection of genes. All 30,000 of your genes.  Why do you have almost a hundred trillion copies of these gene machines, these genomes?  Because you have one genome at the center of each of your hundred trillion cells.  

Your genome is not a simple, straight necklace.  It’s not just the long boondoggle of a double-helix you’ve seen in pictures of DNA.  It’s a messy jewelry collection, a bag of beaded key chains.  Keychains with a combined total of 3.2 billion beads, 3.2 billion base pairs.    Yes, 3.2 billion.   

Think of trying to keep a gaggle of keychains with 3.2 billion links in order.  Think of trying to keep them all straight.  But your genome will someday perform a high-wire act in which there will be no room for mistakes.  Get a bead or two out of order and your kids will be monsters.  It’s called teratogenesis—giving birth to monsters.  And in theory—according to statistical probability and the concept of entropy–it should happen nearly every time.  

But it doesn’t.  Why? 

The challenge is even tougher than it seems.  If all the keychains of the genome from just one of your cells were untangled and stretched out end to end, the resulting chain of genes would be over six feet long.  Yes,  that’s the genome  from just one of your cells.   Which means that if all the genetic keychains of every cell in your body were stretched out end to end, the resulting mega-chain would reach to the sun and back.   Not just once, but 22 times.  

But that’s not the way the keychains of your genome are stored.  Believe it or not, the 3.2 billion base pairs in each of your cells spend most of their time knotted and tangled in a tiny ball.  Like all the headphones you have ever owned thrown into a drawer and tangled together.  Times a hundred trillion. In other words, the ball that is your genome looks like  a total mess.  But it’s not.  It is, in fact, very carefully, very methodically, and very precisely packed away.    

Let’s go back to you, the early land plant inventing sex  500,000 years ago. What do you do with your balls of tangled keychains when it’s time to have kids?  The sensible thing would be to follow the path of the spore makers in the sea.  Be narcissistic. Be conservative. Make identical copies of yourself. Then be materialist, consumerist, and indifferent to waste.   Mass produce those identical copies. And finally, send those trillions of duplicates out in the world to do their thing.  In other words, reproduce.  That, in itself, would be a stunning feat.  But you, the land plant inventing sex, are not content to leave well enough alone. 

You are not in search of the simplest path.  You are in search of the most challenging, the  most ornate, the most complex, and the most impossible. 

You use your tangled balls of genetic keychains as a library of blueprints most of the time.  Under ordinary circumstances, you use 40% swatches of your genetic keychains to tell your individual cells how to build themselves and once they are built what role to play.. But you set aside a few of your genomes and use them for nothing until it comes time to reproduce.  Then you begin a process so hard to keep straight that very few of the brightest people on earth can remember all its details.

First you untangle the mess.  Not an easy job.  Remember the last time you tried to untangle that ball of headphone cords in your drawer?  

Then, when you have the beaded keychains isolated and straightened  out,  you end up with 46 of them per cell.   46 keychains.  But these are not just straight strings of genes.  Each of the keychains is really two stubby strands of beads laid across each other and fused in an X.    Those beaded X-like key chains of genes are called chromosomes. 

What’s next?  

A process so complex that it boggles the human brain.  You line up two similar X-shaped keychains next to each other.  You line up two similar chromosomes.  One has a lot of genes from your dad.  The other has mostly genes from your mom.  With those two keychains you perform a trick very much like the shuffles a magician pulls off with cards.  It’s called crossing over. You twist the necklaces together and get a few genes to trade places.   

Trade places?  What does that mean?  You get one or two beads—one or two genes per necklace, one or two genes per chromosome–to switch their allegiance from one necklace to the other.  You get the beads to pull off this switch while staying in the precise location it will take to allow the entire jewelry collection of genes—the entire genome–to succeed in its task in life, making new organisms. 

Crossing over is like asking just a few  pairs of passengers on a train to trade the window seat for the aisle seat.   The girl with the flowery straw hat who was on the aisle in row twenty five will still be in row 25.  But she won’t be staring at the floor of the aisle between  the seats anymore.  Instead she’ll be next to the window taking in the view.  And her date, the guy with the broken nose and tattoos all the way down to his wrist, the one whose face was mashed up against the window trying to see the view ahead of the train, will be on the aisle.  Glowering at the black rubber flooring.  But he, too, will still be in row 25.  

Meanwhile the well-tanned guy in shorts reading a paper on neurobiology on his IPad in row fourteen will have to stop leaning his shoulder against the window and switch to the aisle seat so the elderly lady with shopping bags next to him can finally see the view from the window seat.  And, the elderly lady with the shopping bags and the neurobiology-loving guy in shorts, too, will switch seats without losing their places.  They’ll both still be in row fourteen.

Next comes one of the key tricks that make sex sex.  You separate  the folks in the aisle seats from  the folks in the window seats.  As if you were unzipping the two ribbons of teeth in a zipper.  You set the aisle-seat folks apart on one single strand and you set the window seat people apart on a separate single strand.  Now the muscular guy with tattoos from the aisle seat of row twenty five is in row twenty five of a single strand on the same keychain as the well-tanned guy with the iPad in row fourteen who you forced to take  an aisle seat, too.  But they are no longer seated next to their partners in the window seats.  In fact, their window seat partners have moved off on a separate bead chain far away.  

You pull off this seat switch with roughly 75 of your 30,000 passengers.  Seventy five  out of your 30,000 genes.  Which means that, thanks to the seat switching, each of your new single strands does not replicate your personal gene combinations.  You are not making carbon copies of you.  You are not reproducing.  Far from it. Each new single strand is a one of a kind. Which is why none of your kids look like you or think like you.  But more on that one-of-a-kind-ness in another chapter.

This is barely the beginning of sex’s intricacies..  Now you separate your opposite sides of a zipper. You separate your single-stranded necklaces. You  pull them away from each other and you shove them to opposite ends of the envelope—opposite ends of the cell membrane– surrounding them,  Then you  pinch the membrane in the center, and turn it from one envelope into two.  And you make sure that each new envelope, each daughter envelope, each of your two new cells, has a complete single-stranded jewelry collection.  

Believe it or not, sexual cells manage this mad shuffle with strings of up to 100,000 genes.  Swapping seats in up to 100,000 rows.

Confusing, right?  Hard to understand.   So think how appallingly difficult it must have been for nature to invent.  Much less for nature to pull off .  Trillions of trillions of times.   Think of how tricky and energy-wasting it must be.  Think of all the ways it could go wrong.  Horribly, twistedly, in-the-wrong-row wrong.  And think of how incredibly far it is from obeying the law of least action.  In fact, it is the most-action process in the cosmos.    What’s more, if entropy and entropy’s statistics ruled, crossing over would fall apart every step of the way. 

In this sexual minuet, what have you accomplished so far?  You’ve gotten your cell to divide.    You’ve gone from having one cell to two.  And you’ve done this with hundreds of thousands of cells, not just one or two.  But inside each daughter cell is a collection of single stranded keychains, single-stranded chromosomes, single-stranded x’s of DNA. Single strands slightly different than any single strands that have ever existed before.  And those single strands are hungry.  Hungry for company.  Hungry for a neighbor in the next seat.  Hungry to double-up again. Which is where another of the tricks that make sex sex comes in.

To have kids, to have progeny, you, one of the first land plants, have to find your single strands partners.  You have to make them double strands once again.  Which means that you have to go out and find a mate.   Yes, you have to find another creature of your species.  One that’s willing to entertain your lonely single-stranded key chains and possibly to even embrace them.  And if you think that’s easy, you haven’t tried it.

Yes, the REALLY hard part of sex is the mate hunt.  But more about that in our next exciting episode.