Dinosaur Construction 101

kw: book reviews, nonfiction, dinosaurs, DNA, genetic engineering

About twelve years ago, shortly after Jurassic Park hit the big screen, a colleague told me he was briefly famous for the first recovery of proteins from a fossil. In the '70s, when he got his PhD, he discovered a relationship between the normal body temperature of a mammal and the ratios of certain "structural" amino acids in their proteins.

Brief aside: The structure of a protein shifts with temperature. It won't work outside a certain range. About half the 20 amino acids (AAs) are mainly structural, forming the helices and sheets that form the shape of a protein. Biochemistry is mainly geometry. For a protein to work best at a different temperature, a shift in the proportions of certain AAs is required.

When my colleague published his results, a friend asked him if his method would work on the proteins from an extinct animal. He said, "Why not. But how would you get some?" The friend brought him some bones of Smilodon, the best-known sabre-tooth cat, from the Rancho La Brea tar pits in Los Angeles. When the animals died there, they were quickly dried by the tar, and the proteins in bone cavities were often preserved.

They were able to extract sufficient protein to work the method, and published a letter stating their findings. My colleague was at a conference in England when the letter was published. Suddenly, he got many calls from reporters, and a British paper published a cartoon of him, sneaking up on a huge Smilodon, and carrying a spear-sized rectal thermometer!

Now, the tar pits contain bones aged between 40,000 and 10,000 years. Hardly dinosaur-age stuff, which is between 65 million and 200 million years old. But impressive for 1970 or thereabouts.

It is a long way from body temperature to a dinosaur clone. A small measure of the difficulty is presented in Jurassic Park, both the movie and the book by Michael Crichton. A recent book makes it clear how much harder it actually is. Rob DeSalle and David Lindley, a working scientist and a highly expert science writer, in 1997 published The Science of Jurassic Park and the Lost World, subtitled, Or, How to Build a Dinosaur.

Dr. DeSalle isolated the first dinosaur-age bit of DNA in 1992, from an insect in amber. It was insect DNA, though, not dinosaur DNA. Older bits have been found since, as old as 135 million years. So when he outlines how one might (just barely, maybe) retrieve dinosaur DNA and eventually produce a living dinosaur, he has it right.

He agrees that amber is a good place to begin looking, but he prefers amber from New Jersey, which is the right age, to Dominican amber, which is only 30 million years old. But what guarantee do we have, if we find a biting critter with a belly full of blood, that it was a dinosaur's blood?

I have recently read of the recovery of soft tissue from deep inside a Tyrannosaur hip bone. Perhaps we ought to be looking there, instead. Otherwise, you're more likely to find the blood of a proto-possum than a dinosaur, which is quite a bit harder to bite...we do have samples of dinosaur skin, so we know.

The authors go through, step by step, what is needed to do the task. They make clear the uncertainties at every step. For example, the DNA sequencing method called "shotgun sequencing" is probably most amenable to this, but it cannot tell you how many chromosomes there were. We only learn this when we sequence, say, a chicken, because we can look at living chicken cells and sequence them one chromosome at a time. If you have a DNA soup with the entire genome in little bits (say from 200 to 1000 bases per chunk, each broken out of a 2- to 3-billion base sequence), you can't really tell where the chromosomes ended. Telomeres (repeated sequences at the ends) are too variable from one animal to the next to prove anything; one critter's telomere might be another's internal repeat sequence.

Suffice it to say, the undertaking is too expensive for an ordinary billionaire. Given the rate that DNA work's price is dropping, however, I expect it might be possible in another decade or two, making initially one assumption: that we can actually recover large enough bits of 80-million-year-old DNA, in sufficient quantity, in the first place. That may be the biggest hurdle of all.