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roundtable:a modern mass extinction Watch Show 3:
"Extinction!" on PBS
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First, a general question. Are we now in a mass extinction, and if so, why does it matter?
Some Web visitors wrote to say they have heard it said, especially of tropical and marine organisms, that we are losing species before we even know of them. And the question is, if we haven't discovered a species yet, how do we know that we've lost it?
Would Darwin support the Federal Endangered Species Act? Millions of dollars are spent by corporations and government for habitat conservation plans, recovery plans, etc., for species that seem doomed for extinction. Is this a wise use of money? Why or why not?
If extinction and mass extinctions are a natural part of Earth's history, should we really be concerned about our effects on endangered species? Are we trying to fight something that's inevitable anyway and that is much larger than us, as a single species ourselves?
I'm concerned with the loss of genetic diversity because it affects food supply, drug discoveries, and the stability of ecosystems on which we depend. And yet, I'm very skeptical of the theory of evolution as it now stands. How does the theory of evolution -- or we could also say that as evolutionary thought -- explain and predict what's going on with the current cycle of extinction, better than if we had to just rely on what we can observe? In other words, what context and what value does evolutionary thought provide to this discussion that we're having?
Several other questions relate to this: Loss of biodiversity, water and air pollution, and the greenhouse effect are all impacting the biosphere, and if these are indeed combining to produce a mass extinction, do you think our species will survive it? Peter's already said that he thinks yes. How about some other perspectives?
Let's move onto some more specific questions. Why do bigger species go extinct faster than smaller ones?
Do you feel that extinctions of large Ice Age mammals had any human causes?
Another Web user wrote: I know there is a gene databank under construction right now. Could this be used as a modern Noah's Ark for the future?
We'll wrap up with a different kind of question. There was one that came from a younger visitor to the Web site who asked, What can kids do to help save the rain forest and animals?
   

 

Q: Do you feel that extinctions of large Ice Age mammals had any human causes?
Panelist Responses: < back to intro page
Peter Ward
Dan has looked at this a lot more than I have, but certainly [in the case of] mammoths and mastodons, it seems inescapable to me that humans were involved in it. Clearly climate change is involved, as well.

I was privileged to go out onto a [ancient] lake bed in Alberta that was discovered last year. The lake bed was exhumed by a dam, and in this 11,000-year-old paleosurface are the broken up remains of many Ice Age mammals, as well as many Clovis tips. For the first time, the Alberta scientists were able to look at residues on the Clovis tips, and they were able to get protein residues off them, and they found, to their surprise, that the protein on these tips wasn't related to elephants but it was horse blood, dried horse blood, on these 11,000-year-old tips. Which is what a lot of us would have thought, that these Ice Age hunters were getting a lot more than simply mammoths and mastodons, and, perhaps, the disappearance of horses in North America might be related to humans, too.

What has always really fascinated me about the mammoth/mastodon work was that through modeling you can show that you only have to remove a very small percentage of the babies each year, to allow an extinction within, say, a thousand year period. And we always think of these hunters going after these great big, monstrous elephants. I think that's crazy. What you do is you simply get the little ones and herd them off from the big ones and just kill the babies. That's the quickest way. And then you don't have to run big herds off cliffs, you simply take a very slow but steady number of babies and that, accumulated over a thousand years, can cause large mammal extinctions.

So humans certainly weren't the only cause, but I think if humans had not entered North America, then North America today would look very much more like Africa than it does.
Daniel Simberloff
The hypothesis that the questioner is asking about is often called the Pleistocene overkill hypothesis: that the advent of human hunters simply killed many of these large mammals. And for many years there was a lot of controversy about this hypothesis, and the alternative is the very one that Peter was referring to, that climate change was far more important.

And the data that were adduced in support of one or another hypotheses generally had to do with which fossils were found where, and artifacts of hunters, like Clovis tips, were found where. And over the last 10 or so years, I think that the evidence is, increasingly, as more and more remains have been found and dated accurately, the evidence is pretty conclusively tending towards the overkill hypothesis -- that is, that the disappearance of populations of larger mammals is, when we actually have the data, very close in time to the arrival of humans. And there's a similar set of data for islands in the Pacific and the disappearance of birds on the islands, showing with increasing clarity, as more data are available, that the disappearance was very close in time to the arrival of humans. So I think most people believe that overkill is the main factor.
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