Friday, May 9, 2014

Sex Evolves...




About a billion years ago, two organisms floating within the primordial soup bumped into each other and had sex. VoilĂ ! The revolution that would be sexual reproduction was born and organisms throughout the world have been coupling ever since.
OK... it was a bit more complicated than that, so we'll break it down.
The world was a very different place when sex first arrived on the scene. The ozone was just forming, providing some protection against the sun's raging UV radiation; oxygen was up to ten percent, about half of what it is today; and the continents were clustered in a giant clump known as Rodinia.  The planet was inhabited by single-celled organisms, primarily green and red algae. Needless to say, it was a pretty monotonous place.

But big things were happening. The eukaryotes, those cells sporting a nucleus, had evolved and multicellular critters were about to burst onstage. Fast forward half a billion years to the dawning of the Paleozoic and suddenly, life as we know it (or at least life as the trilobites would know it) was in full swing.
And what role did sex play in all of this? A BIG ONE!!

Scientists are still puzzling out how organisms transitioned from asexual to sexual reproduction, but they take hints from the natural world. Some bacteria are able to exchange genetic information on a rudimentary level, and many critters, such as yeast, are able to switch back and forth between the two methods as conditions warrant. So the basic evolutionary mechanics of sex are slowly being teased apart.
Once it kicked in, sexual reproduction fueled natural selection, as it still does today. Instead of organisms making carbon copies of themselves and relying on the occasional mutation to provide an edge among their competition, sex added valuable variation to the mix. The genetic shuffling and recombination that takes place during sex affords greater variability, which allows greater flexibility in a changing world. Adaptability, pathogen resistance, and the buffering of harmful mutations are all enhanced through sexual reproduction. Instead of relying on a single genetic line, as asexual organisms do, genes provided by two parents offer a broader playing field on which evolution can run rampant.

So how does sex work? Well, the standard penis-in-the-vagina method we humans (and most mammals) employ is but one manner of achieving fertilization. Let's explore.
Sexual reproduction involves the union between the sex cells, or gametes, of two parent organisms. We'll start with the tried and true method of most mammals, intercourse.

For intercourse to work, you need a sender and a receiver. The sender is equipped with a tool for administration (aka, the penis) and the receiver provides a welcoming receptacle (the vagina). Insertion of the tool, followed by a bit of friction, results in a magical deposit (sperm), and if the timing is right, the sperm are greeted by an eager egg (or eggs, in many cases). From this point on, nature takes its course. The egg is fertilized and the zygote is launched on its path to personhood (or elephanthood, porcupinehood… You get the picture).
This is the typical mammalian recipe. However, nature wouldn’t be nature if it didn’t throw in a bit of variety. For those who lack a tool or a receptacle, or simply prefer less fuss and muss, there are other means of achieving fertilization.

For instance, take fish. Imagine a fish penis. Bet you can’t. That’s because most fish spawn, meaning the males and females have adopted a “hands-off” approach to reproduction (I think they’re on to something…). The female typically releases a cluster of eggs and the male then swims over them while discharging a milky little cloud of semen. From there, the eggs and sperm are on their own. But since there are over thirty thousand different species of fish swimming the planet, their crop-dusting method is apparently pretty effective.
So fertilization takes two main forms: internal (like us) or external (as in our fishy friends), with a few variations on these themes. And just as with most aspects of our anatomy and physiology, evolution via natural selection has tinkered, resulting in the most effective method of fertilization for each species.

Not that there aren’t problems, but at least we humans are rewarded with an orgasm (or multiples, on a good day!). But are we the only species that has sex for the sake of sex?
Most animals (as well as certain Republican politicians) engage in sex purely for reproductive purposes. There are just a few exceptions: humans, dolphins, and bonobos (pigmy chimpanzees). Like us, dolphins and bonobos are known to engage in sex merely for the fun of it. Scientists believe sex among dolphins and bonobos might serve some of the same purposes it does among us humans – forging bonds which then promote group cohesion (although I’m convinced the bonobos do it for kicks and giggles). Ironically, bonobos are also one of the few animals (besides us) that practice face-to-face sex. The majority of mammals are better suited for the less personal but oh-so-effective method of mating from the rear.
You’ll be happy to know that scientists are hard at work discovering all the positive effects of sex (don’t question their methods). Regular sex has been shown to improve sleep, reduce stress, increase blood flow to the brain and other organs, and even keep your ticker healthy.

So the next time you get lucky, pause for a moment and reflect on the billions of years of evolution that came before you (no pun intended…).

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