Today, let's talk about the extinction of species. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Recently, at Harvard's Natural History Museum, the Stone-Age skeletons caught me by surprise -- strange, oversized animals, as if someone had stirred cats, bears, and mammoths all together.

Now biologist David Burney asks about the extinction of species, and he looks to those Stone-Age beasts for guidance. He finds that widespread extinctions have occurred at different times and in different regions over the past 30 thousand years.

At one time in the Stone Age, European artisans were carving toys from bones of the last woolly mammoths. Saber-toothed cats and giant sloths still roamed North America. And several large Australian marsupial species had only recently died out.

Sure enough, where we find large-scale extinctions, we find humans gaining new technologies at the same time. Take the Americas: How long humans have occupied North and South America mires in debate. But of one thing we're sure. Just 11,000 years ago those natives began making distinctive spear and arrow heads.

Much of North America was a great animal preserve, like the Serengeti Plain. Then those distinctive spear heads appeared, and the extinctions began. Some anthropologists offer the Blitzkrieg Hypothesis. A human population which might've been around for a long time before that finally perfected the technologies of big-game hunting and began killing off species.

That drama played out earlier in Australia -- 15 to 30 thousand years ago. That's when humans, who'd arrived long before, mastered hunting with spears, throwing sticks, and fire-setting.

European cave painters show giant deer and woolly rhinoceroses. It was those same artist cave men who perfected the tools of killing. And as they did, species began to vanish.

Now species are dying at a far faster rate than they ever did in the Stone Age. Burney warns us not to read a wrong lesson from the past -- not to shrug our present losses off. It's the large slow-breeding herbivores that are usually destroyed first. Those are beasts who, like you and me, most influence their environment. When they perish, life changes for the next largest species in turn, and a downward cascade of extinctions follows.

As that arrow points at us, we see another, more sinister, feature of today's extinctions. Armed with guns, chain-saws and bulldozers, we so outrun any hope of normal Darwinian competition among species that we gut the natural selection process. When the only surviving tigers and elephants are ones that live under our care, the stage is set for a new cascade of species extinctions. And they could be more rapid, more dangerous, and closer to home than we'd ever imagined.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Burney, D.A., Recent Animal Extinctions: Recipes for Disaster. American Scientist, Vol. 81, No. 6, Nov-Dec. 1993, pp. 530-541.