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model of DNA molecule

Photo by Chuck Eby
and courtesy of the
Telephone History
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model of DNA molecule

arguments for evolution

DNA 'n evolution
Just as nature has "recycled" limb structures and the backbone, a similar parsimony exists on the molecular level. DNA, the "memory chemical" that stores genetic information, comprises the genes of every living organism.

The upshot of the analysis is this, says Goodenough: "The fact that the same kinds of genes are found in all the different kinds of organisms on the planet today, including both bacteria and organisms like ourselves, indicates that these genes developed long ago." To put it slightly differently, the presence of similar or identical genes in two organisms is a genetic fingerprint of a common ancestor at some time in the past.

Bring on the biochemistry
An old crank-style telephone The detailed science of fingering common ancestors and figuring out how their descendants are related is complicated stuff, way beyond The Why Files's powers of comprehension. But the essential technique is obvious to anyone who has played telephone. You remember: You sit in a circle and whisper a phrase into your neighbor's ear, who then passes the phrase along. When the phrase returns to its starting point, "A pox on both your houses," may be transmogrified into, say, "A box of genes in both kinds of louses."

Obviously, the further you are from the origin, the more distorted the phrase will be (although if you are clever in sorting out the phonemes and the rhymes, you can deduce how the two sentences are related and how they were changed while being passed around).

In their shiny labs and clean white coats, this is essentially what the genetic sleuths do. They look for similarities and differences among genes in modern organisms, then try to deduce when they shared a common ancestor.

While taxonomists traditionally analyzed relationships among organisms in terms of shared features, say wings or scales, genetic analysts measure relationships by calculating time to a common ancestor.

In a game of telephone, we might observe that each player averages one more mistaken syllable. In an evolutionary study, we might find that one "letter" of the genetic code changed per million years of evolution. If five "letters" differed between two organisms, we could deduce that they had diverged five million years ago. beer

Yeasty topic
There's another way to show the similarity of genes in various organisms. Remember the lowly yeast, the microbes that raise dough into bread and ferment hops into beer? To demonstrate the similarity between yeast and human genes, experimenters remove genes that make proteins the yeast need to live.

What happens? The yeast croaks, that's what.

But when the deleted genes are replaced with similar human genes, the yeast lives. These results, says Kansas biologist Robert Palazzo, "argue that common molecules exist in systems as distant as yeast and humans."

Much of this analysis is done with RNA in the ribosome, a cellular unit that assembles proteins under the direction of the genes. Because the genes for ribosomes change slowly, this analysis shows the evolutionary links between humans, worms, fruit flies and other organisms that, to the naked eye or microscope alike, look rather different.

And it's just like telephone. As Palazzo points out, variations in non-essential locations on the RNA serve as "yardsticks" for the rate of evolution. When you see long, similar sequences in the "letters" that make up the RNA, you have to assume that the two organisms shared a common ancestor somewhere down the line. Chance just can't explain the similarity.

To Goodenough, these molecular arguments are convincing proof of evolution. "This is pretty overwhelming," she says, because the arguments from the fossil record and the DNA analysis are independent of each other, yet they still "come out the same. For a scientist, that's a real slam dunk. You have to really bend like a pretzel to say those two arguments are related by coincidence."

Sometimes, however, biochemical analysis upsets the applecart rather than confirming existing knowledge...

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