Practice Quiz #11

Considering long-term averages, and assuming that we don't deploy space-based defenses against incoming meteorites, a reasonable estimate of the chance of an average U.S. citizen being killed by the effects of a meteorite or comet impact is that this risk is about the same as the chance of being killed by:

Crash of a commercial airplane. Nobody that we know eats Pepsi cans, and while there are still meteorites in the solar system that can hit and kill, there are no dinosaurs left except on "The Flintstones". A reputable study found that a meteorite impact might not occur for millions of years (or might occur next year...) but then might kill billions of people; plane crashes usually kill a few to a few hundred each year. Add up the deaths over a sufficiently long time, and plane crashes and meteorite impacts likely are similarly dangerous. But car crashes, smoking, and being fat and lazy are way more dangerous to us.

The extinction of many types of dinosaurs occurred about:

Humans were trotting around 65,000 years ago, and met dinosaurs only in The Flintstones. 650,000 years is barely enough time for evolution to have changed large animals a bit, and although 6,500,000 years is enough time for noticeable change of large animals—increase in maximum size of members of the horse family, for example—the huge changes since the dinosaurs needed 65,000,000 years. 650,000,000 years goes back before any land creatures, and before all but the simplest of multi-celled organisms.

Examine the two pictures above, labeled I and II. They are from the same sediment core collected in sea-floor muds from beneath the Atlantic Ocean off the coast of South Carolina. (The pictures are scanning electron micrographs by Brian Huber of the Smithsonian Institution, and the scale is the same on both, as shown at the bottom of each.) One picture shows a sample from just below the unique layer marking the extinction that killed the dinosaurs, and the other picture shows a sample from just above that unique layer. Which is which?

I is from below the unique layer, and II is from above the unique layer. Before the impact, biodiversity was high, as shown in I, which includes fossils from below the unique layer and thus deposited before the meteorite hit. After the impact, most of the living types were killed, giving rise to the limited diversity seen in II from above the unique layer after the impact.

Extinction of existing species:

Occurred at a low level throughout geologic history, punctuated by mass extinctions when many types were killed over very short times. Extinction has happened slowly throughout geologic history, but with a few dramatic, catastrophic mass extinctions. We may be causing the latest of those mass extinctions.

There are many large mammals on Earth today. This is because:

Small mammals were not able to outcompete the dinosaurs for big-animal jobs, but after the dinosaurs were killed, some large mammals evolved from small mammals to fill the large-animal jobs. There are "big-animal" jobs—eating tall trees, eating smaller animals, etc. But the total number of big-animal jobs is limited. The dinosaurs filled the big-animal jobs before mammals really got going, and mammals were not able to displace the dinosaurs. Some small mammals survived the meteorite that killed the dinosaurs, and then evolved to give big mammals over millions of years and longer. There were almost no big mammals before the dinosaurs were killed off, volition has nothing to do with evolution, and running away doesn't avoid acid rain.