What Darwin Didn't Know

Zoologists have long been satisfied with the evolutionary theory that holds that humans and monkeys are closely related. But the theorists always welcome additional evidence, and last week they got some striking chemical proof. Drs. B. H. Hoyer, B. J. McCarthy and E. T. Bolton of the Carnegie Institution demonstrated that the DNA (deoxyribonucleic acid) in human genetic material has many sections that are identical with monkey DNA.

The experiments that the three Carnegie men report in the magazine Science were both delicate and difficult.

DNA molecules are made up of two very long strands connected to each other by hundreds of thousands of short submolecules, which are set in place like the rungs of a ladder. The rungs differ chemically, and the order in which they are arranged constitutes the genetic code that controls heredity. When DNA is heated in just the right way, the rungs break and the long strands separate. Though the strands are eager to recombine, they cannot do so unless their broken rungs are matched in perfect order.

The Carnegie men heated a solution of mouse DNA until its strands separated. Then they mixed it with agar that cooled to a stiff jelly, which immobilized the long strands and kept them from recombining. Next, the jelly was reduced to tiny granules, each charged with mouse DNA.

Code Segments. The scientists then prepared a solution of mouse DNA that had been made radioactive with carbon 14. They chopped the molecules into short sections, each carrying a small part of the mouse genetic code, and separated the two strands. When these were mixed with the chunks of agar, their small size permitted them to diffuse into the jelly, and whenever one of them encountered a trapped strand that had a matching sequence of genetic code, the two segments combined firmly.

By measuring the radioactivity of the recombined DNA, the scientists could estimate closely how many of the segments had found matching partners. Under the best conditions, about 25% of the chopped DNA hooked up with that in the agar. This comparatively high figure was only natural, since both samples came from the same species of animal.

Other Species. Next step was to repeat the experiment with different species. Just as predicted by evolutionary theory, mouse DNA combined nicely with that of other rodents, such as rats and hamsters. But it showed much less attraction for the DNA of monkeys and cattle. Human DNA demonstrated only moderate interest in mouse, but it combined with some from a rhesus monkey almost as strongly as if the stuff came from a human. Both mouse and human showed weak interest in DNA from salmon, and almost none in that from bacteria.

What all this means, say the Carnegie scientists, is that bacterial DNA has almost no code sections that are identical with those of higher animals. But fish are vertebrates; thus their genetic code has quite a number of sequences that have survived from the primitive fish that were the ancestors of all vertebrates. When fragments of human DNA combine with salmon DNA, they presumably find matching code sequences that control such common attributes as hemoglobin in the blood or an internal skeleton. Mice are mam mals, so they have more in common with humans (warm blood, hair, a similar reproductive system); and in the world of primates monkeys are so similar to man that long stretches of their genetic codes must be identical. The all-important differences between men and monkeys, say the Carnegie scientists, are not likely to be detected by their DNA-matching method.

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