The major flashpoint of interest in what has now led to de-extinction technology happened

about 10 years ago when Dolly the sheep was created. In many ways, Dolly was the harbinger

of the kind of technology that will permit us to do even more sophisticated kinds of

creation of species that used to exist, species that still exist and perhaps even species

that never existed.

We can group relevant methods into three large categories. There's back-breeding which humans

have practiced literally for thousands and thousands of years.

This is being done right now in Western Europe, trying to recreate what's known as the aurochs.

The way cows look today is not the way the cattle family looked 10,000 years ago. So,

if your problem is to recreate one of those primitive cows, what do you do? Well you reach

back into the genetic code by selecting for animals that have slightly longer horns or

a bigger bulk or the kind of hide coloration that you think is appropriate. It is a very

viable and low-tech way of de-extinction.

And the investigators trying to do this back-breeding to aurochs have had some success. The animals

look quite a bit like old representations that go right back to cave paintings.

But what is it really? What it is really is a constellation of traits that you've selected

for and you know nothing, really, about how close it is physiologically to an aurochs

because there's no way of examining a living aurochs at present. You also don't know about

behavior. Behavior tends not to fossilize in any realistic manner and the degree to

which any modern cow resembles an aurochs in terms of behavior is a complete unknown.

A second method for de-extinction that has improved greatly in the last 10 years is cloning,

particularly the kind of cloning known as somatic cell nuclear transfer.

You take the genetic material -- the nucleus -- of one cell, you put it into another -- an

egg -- and that egg is then placed in an animal that will act as surrogate mother and produce

an offspring.

There's also been massive improvements in what we can call synthesis. Synthesis involves

making up our own sequence, just like in a recipe.

If you have two species -- one extinct, one living -- you can get the genetic material

of the extinct one and compare that to the living one.

Passenger pigeons died out, finally, almost exactly a hundred years ago. Passenger pigeons

have several very close relatives among pigeons and one is the band-tailed pigeon, which is

very close indeed. In recent years, scientists have been able to isolate the differences

between the genomes of extinct passenger pigeons and living band-tailed pigeons. They've been

able to inject these novel sequences into the germ line of developing band-tailed pigeons

and using surrogates with the sex cells of passenger pigeons. If you cross breed them

one to another, you will in the next generation get something that should look exactly like

a passenger pigeon.

Now comes the basic question here, which is: What are you going to do with passenger pigeons,

assuming that you go ahead and create a whole flock of them?

It's been estimated that there were probably more passenger pigeons than any other kind

of bird ever. I don't know what we would do with a billion passenger pigeons. We don't

like pigeons to begin with in this place.

A very important thing to consider about extinctions in general in the course of life on the planet

is that the waters close very quickly when a species disappears. What tends to happen

is that other species move in. This is how evolution in fact works.

The mammoth behind me perfectly illustrates some of the issues that we're going to face

with de-extinction.

There is no place for several more very large herbivores in a place like the U.S. except

under very controlled conditions. Who's going to take care of them? How are you going to

provision them? Are we thinking that we're going to bring back the Pleistocene by virtue

of having a few extinct species?

What we're really talking about is undertaking a whole lot of unplanned experiments, the

consequences of which are very hard to predict. We don't want a situation in which we drive

out species that are perhaps already endangered thanks to us with something else that we drove

to extinction thousands of years ago.

De-extinction to the degree it will take place should be limited in its scope, limited in

its ambition, limited in the kinds of species that we bring back. But if there was one animal

I could vote for to bring back, it would be one of these giant ground sloths.

If you want people to be interested in not only what the planet still has and how we

might conserve it, but what the planet has recently lost and perhaps most particularly

what it's lost because of human activities, then to see these again, to see mammoths again

in the flesh would be an exciting prospect.

It would raise rather than dampen interest in preserving what we have and what we used

to have.