The start of something big

On some level, every generation of young people thinks it invented sex. (This is SO cool - it MUST be new!) In reality, of course, they know the business of internal fertilization - as opposed to spreading one's eggs on the seafloor for subsequent fertilization by males - has been around a long time. But just how long?

Until very recently, scientists thought our modern form of sexual intercourse was invented by a type of fish that includes early sharks roughly 350 million years ago. Now, however, new fossil study has revealed that copulation was brought into the world by primitive fish called placoderms ("plated skin"), armored creatures with backbones and jaws. They may have been ugly, but they invented the deed at least 25 million years earlier.

This leap back in time apparently has great significance for the study of evolution, but we're interested because placederms are directly on the long line leading to creatures with four limbs, including, eventually, humans.

And if humans are good at anything, it's you know what.

That proficiency took a long time to translate into significant population growth. Warfare, famine and especially disease kept our numbers down. It was one thing to make babies, but qute another to keep them alive to adulthood. Then, a few hundred years ago, all that quickly changed.

And, despite a general slowdown in women's birth rates, late in 2011 the Earth will reach a new milestone: a population of 7 billion souls. According to the UN Population Division, that number is projected to reach 9 billion by 2045.

Those ugly little placoderms had no idea what they'd done.