Friday, July 31, 2009

Glad You Could Make It: The Astounding Improbability of Humanity

"Humans are here today because our particular line never fractured, never once at any of the billion points that could have erased us from history." Stephen Jay Gould

We were almost goners. About 70,000 years ago our fore bearers were on the brink of extinction. According to a study conducted by geneticists at Stanford University, drought and other environmental challenges whittled the number of homo sapiens living in Africa to around 2,000 just prior to the stone age. It is from this small group of brave souls that you and I and six and a half billion others are derived. This fairly shallow gene pool accounts for the lack of genetic diversity among people today.

So how in the world are scientists able to trace our origins back to this small population of hearty hunter-gatherers? As it turns out, our shared history is written in our DNA. Despite the striking physical differences among different people groups and races, at the level of our DNA, we're all pretty much the same. In fact, you have to look pretty hard to find any differences at all.

Enter: "microsatellites" - these are short, repetitive segments of DNA that differ between populations. These little guys have a very high mutation rate as they are passed from generation to generation, so these subtle little mutations begin to stack up when two populations diverge.

Researchers from Stanford University and the Russian Academy of Sciences compared 377 microsatellite markers in DNA collected from 52 regions around the world. Analysis of these various DNA samples show a close genetic kinship with two hunter-gatherer populations in sub-Saharan Africa the Mbuti pygmies of the Congo Basin and the Khosian bushmen of Botswana. This leads scientists to believe that these two groups make up the oldest branch of modern humans. Imagine what life would have been like when the entire population of earth could have fit comfortably in an average sized sports arena with plenty of room left over.

But that close call wasn't our first brush with oblivion. Life on this planet has nearly been extinguished several times. There have been five major extinction events (and several less dramatic ones). The most recent was the Cretaceous-Tertiary extinction, about 65 million years ago. This is well known as the end of the age of the dinosaurs but much of the life on earth also came to an end
about 70% of all species living at that time. All that destruction thanks to a several-mile-wide asteroid that struck the Yucatan Peninsula. It's hard to wrap your mind around an explosion of that ferocity. Some estimates put it at roughly equivalent to 100 million megatons. As a point of comparison, the bombs that were dropped on Hiroshima and Nagasaki combined were 35 kilotons.

Before that around 200 million years ago was the Triassic extinction. About 75% of all living things came to the end of their genetic lines during this period. This housecleaning was more than likely caused by volcanic activity. And we're not talking about a puny little Mount Saint Helens. Think massive floods covering major parts of continents. Now instead of water, think lava. A maelstrom fit for Mephisto himself. The volcanism may have led to deadly global warming with ash blotting out the sun and turning our atmosphere into a convection oven. The death toll: 22 percent of marine families, 52 percent of marine genera. There's no way to know how many vertebrate species bit the dust.

Prior to that, mother nature went on the granddaddy of all killing sprees: the Permian extinction, which happened about 245 million years ago. 95% of species were given their hats and shown the door. Even a third of all insects were swatted. And its hard to get rid of those little buggers.

About 120 million years before the Permian was the Late Devonian extinction. Most everything that was living then was living in the seas. The great majority of them were wiped out.

And, before that, was the Ordovician-Silurian extinction, about 439 million years ago, caused by a drop in sea levels as glaciers formed, then by rising sea levels as glaciers melted. The toll: 25 percent of marine families and 60 percent of marine genera.

In short, it's been a rough ride for creatures great and small. If intentionality could be ascribed to the random acts of nature, one might draw the conclusion the our planet treats complex life as if it were an infection and means to cure it whenever it breaks out. And its antibiotics are climate change, pandemics, volcanism, ice ages and the occasional comet or asteroid. Take two cataclysmic explosions and call me in the morning.

But, somehow, we made it. Bill Bryson writes in A Short History of Nearly Everything, "The one thing we have in common with all other living things is that for nearly four billion years our ancestors have managed to slip through a series of closing doors every time we needed them to."

In light of all this, the idea that humans are the pinnacle and central aim of creation seems a little naive at best and profoundly hubristic at worst. Mankind is the Johnny-come-latey of the biosphere. And it seems clear that the ancient earth was anything but a Garden of Eden. Our privileged planet was a rather inhospitable youngster.

But we are unique in the sense that we are the only intelligent species among the billions who have flourished and perished during our planet's history. In fact, as far as the evidence shows us, higher intelligence has only arisen once in the history of the observable universe. We are the beneficiaries of a billion failed experiments carried out by a blind tinkerer, driven by random mutations, shaped by natural selection and tested by global catastrophes.

So what's the take away? Be happy. You made it! The odds were not in your favor. The genes inside you carry the war wounds of countless devastations. You come from good stock. You're one of the lucky ones a real survivor. And, for the moment, life is good.

1 comment:

  1. I'm glad I stopped by to read this! I enjoyed your perspective on evolutionary history and what's ultimately brought us here. I love the Bill Bryson line you quoted (he's a favorite of mine).

    My background is much more in paleontology (specifically, the rise of tetrapods in the mid-to-late Devonian was my area of study) than archaeology, so I learned quite a bit about the genetic history of humans here. It's pretty spectacular that 6 billion people came from a bottleneck population of about 2 thousand. We truly are survivors.