Upright Origins

Evolutionary studies have conclusively demonstrated that humans and the great apes originated from a common ancestor, some six million years ago. In fact, DNA analyses show that we share some 98% of our DNA with chimpanzees and bonobos. We are close cousins.

So why do Homo sapiens stand upright on two legs, while our ape cousins hunker down closer to the ground, getting about on four legs? While we began running around on the ground on two legs, they maintained the ability to negotiate their way through the trees with strong forearms. Our forearms, on the other hand, morphed into appendages capable of carrying things around, while theirs maintained the ability to swing from tree to tree.

It has become an acceptable idea on the part of anthropologists that at some time in the past—some four million years or so ago—the vast forests of Africa transitioned to vast savannas of grasslands, punctuated by occasional trees. What's a tree-swinging ape to do, when the trees become spaced too far apart to enable it to swing from one tree to another? Well, it comes down to the ground and forages for food there, adapting to the new environment.

The threat on the ground, however, is that now lions and tigers find you at their level—providing them a new tasty treat. Lions love the savanna, which gives them the opportunity of chasing down prey that cannot leap into a tree to escape their charge.

Evolution, ever on the march for one species to be eaten and another to avoid being eaten, transformed our ancestors into critters who could stand upright, so they could better spot a lurking lion, and survive another day. Those savanna grasses could be peered over by ancient upright Homo erectus, giving them a survival advantage. In the meantime, chimps retreated to dense stands of trees along rivers, while our direct forebears came to preside over the open savannas. Not only could we see better in our elevated stature, but our arms were now available to carry food, weapons, and babies.

So what brought about the transition from dense forests to wide-open savannas? Climate change of some sort probably did it. Over the last several million—or even billion—years Earth's climate has spectacularly fluctuated. The causes of change have been diverse. We are often not sure of the origins of climate variation, although we can demonstrate that it did happen and describe the consequences.

Now comes the results of a recent fascinating study at several US universities suggesting that a series of supernovae in our corner of the Milky Way Galaxy showered the Earth with cosmic rays a few million years ago. For quite a long period of time these rays ionized the planet's atmosphere—causing it to be far more electrically conductive. The result could have been an increase of wildfires triggered by lightning in African forests, transforming the terrain from forests into grasslands. With trees much more scarce, grasses took over, and it's an advantage to stand above the grass, to spot those hungry lions.

So, were exploding stars the trigger for Homo species to become dominant? We know that, without supernovae, elements heavier than hydrogen and helium would never have been created in the early universe. Thus, it's true that we are all stardust—from rocks to plants to animals. Maybe we humans even owe our ascendancy to those exploding stars?