About a billion years ago, two
organisms floating within the primordial soup bumped into each other and had
sex. Voilà! 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.
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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