The movie "Jurassic Park" is probably best remembered for its great animation and special effects. Dinosaurs had never looked so real. Moreover, the movie contained some "science" which at first sight has a powerful plausibility.

Dinosaurs were plagued by insects, much as today's mosquitoes plague us. The insects sucked out dinosaur blood, and then in turn became trapped in sticky tree sap that hardened to amber. The amber dropped to the ground and lay buried until present-day humans came along and collected it. By removing the insects from the amber, and then the blood from the insects, and then the dinosaur DNA from the blood, scientists had all they needed to re-create the original dinosaurs.

How much of this is reasonable? Certainly, we should expect that dinosaurs were preyed upon by small bloodsuckers like mosquitoes and ticks. Plausibly, those insects and arachnids (ticks are not insects, they are closely related to spiders) might become trapped in sap that hardened to form amber. Reasonably, the amber would survive for a long time.

But now let's ask, how much of this is unreasonable? When we say "a long time," we really mean it. The dinosaurs became extinct 65 million years ago, so the amber-trapped insects must have been there for at least that long. Unfortunately, DNA is not a robust form. In a period far less than 65 million years, its structure would degrade. Short sections of the long DNA molecule might survive, but the chances of finding the whole chain are negligible.

So "Jurassic Park" remains as fascinating fiction. But suppose we set less ambitious goals. Suppose that we don't go nearly as far back in time, not a hundred million years but a mere fifteen or twenty thousand. Is there any chance that we might be able to restore animals that became extinct at that time? Not a dinosaur, of course, but maybe we could still have something impressive: say, a woolly mammoth. A mature male stood eleven feet tall and weighed seven or eight tons, as large or larger than the biggest of today's elephants.

As before, we will need a sample of DNA. However, instead of using the elaborate mechanism of bloodsucking insects trapped in amber, we will use a much simpler method: freezing. If we put an animal into a deep freeze, and keep it there, the rate of decay slows to zero. DNA can remain intact.

Fifteen thousand years ago is not very long, even in terms of human existence. Humans have been around for more than a million years. However, although fifteen thousand years ago we had discovered the use of fire, we had not yet invented the refrigerator. Fortunately we can manage without that, because Nature did it for us.

At the time of which we speak, the northern hemisphere was still emerging from the most recent Ice Age. Among the ice age victims were a number of woolly mammoths, which died, froze, and became buried in the permafrost. They have remained there ever since, kept in perfect cold storage in Siberia and the most northern parts of North America, places where the winter temperatures can drop to a hundred below.

The proposed procedure is fairly straightforward, at least in theory. First, locate a frozen woolly mammoth. This is the easiest step. Many intact frozen mammoths have been found in the past, and there was once a flourishing trade in Siberian mammoth ivory dug from the frozen ground. People have even dined on mammoth flesh, which testifies to its fine state of preservation.

Now we delicately defrost the mammoth and extract DNA from cells of its soft tissue. We insert this DNA into the ovum of an African elephant, the closest living relative of the mammoth, having first removed the natural elephant DNA from the ovum. Finally, we return the egg to a female elephant. About twenty months later, perhaps somewhat bewildered by the hairiness of its child, the elephant delivers itself of a living baby mammoth. What are our chances of success? Well, we have to find a cell with intact DNA, which will not be easy because the freezing process itself may have harmed the molecule. However, we will have plenty to choose from, since the whole mammoth body contains more than a thousand trillion cells. We ought to be able to find one good one, although the search process would be extremely tricky.

Then we have to make sure that the implanted egg-containing mammoth DNA is not naturally aborted. This may be somewhere between difficult and impossible. Elephants and mammoths are close relatives, but there are substantial differences. For example, mammoth DNA is contained in fifty-eight chromosomes. The modern elephant has only fifty-six chromosomes. Cross-species implanting is a delicate and formidable task, even with still-living species.

Suppose that we can do all this. There is still a question as to whether we should. Some people argue that the restoration to the world of a woolly mammoth is at best unnecessary, and at worst unethical. We would be bringing an animal back into a world in which it was never designed to live. It had its time and its chances, say those who object to the whole idea. We should leave it at that and let the poor creature remain extinct.

And if it were up to me? I would try to do it, for three reasons. First, we do our best to make sure that endangered species do not vanish from the Earth, and the attempt to bring one back seems fully in the same spirit. Second, there is sheer curiosity. I would love to see a woolly mammoth.

The third reason relates more to guilt than curiosity. As I said, humans were around when the woolly mammoth became extinct. We now suspect that our ancestors had more than a little to do with that extinction. Seven tons of meat. Think how many good dinners that represented to a hungry cave family.

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