Sat, Nov 27, 2004 - Page 9 News List

Genetic theories have as much to do with culture as science

The overwhelming similarity of human DNA to that of the chimpanzee shows not that we are `mostly' chimp but how little we actually understand about DNA after all

By Jonathan Marks

All cultures impose on their members ideas about who they are, how they came to be, and where they fit in socially. For example, pre-modern Europe believed that a woman who had sex before marriage might carry the imprint of her lover within her, so that her child born in wedlock would resemble the earlier lover, rather than the husband. This served to justify the premium placed on female chastity.

Folk ideas about heredity are a particularly powerful cultural tool, but they are not unique to pre-modern societies. Even contemporary science has its own cultural ideologies about heredity, which are often difficult to disentangle from the complex data and high technology that we believe produces objective, value-free glimpses of nature.

In the field of human origins, it is well-known that the human DNA sequence is nearly 99 percent identical to the corresponding part of a chimpanzee's DNA. From there, it is not uncommon to hear the conclusion that we are "nothing but" chimpanzees, fated to be aggressive or to possess whatever attributes are being imputed to apes these days, or even that apes merit human rights. Once again, social meanings are enmeshed within beliefs about heredity.

Actually, that overwhelming similarity between human and chimpanzee DNA is a product of two cultural facts: our familiarity with the chimpanzee's body, and our unfamiliarity with DNA. After all, when the chimpanzee itself was new and interesting in the 1700s, contemporary scholars commonly judged it to be overwhelmingly similar to the human form -- so much so, that it was commonly classified as a variant of human.

After several centuries studying apes, we are familiar with every subtle difference in form between our species and theirs. But the emergence of molecular comparisons in the 1960s and DNA technologies in the 1980s presented a new way of comparing species. The comparison of linear polymers -- proteins made up of chains of amino acids, or DNA made up of chains of nucleotides -- held out the promise of a simple tabular approach to evolution.

When we compare the genetic material of humans and chimpanzees, we do find that it corresponds at nearly 99 of every 100 comparable sites. But this method omits much of what has been learned about genomic evolution in the last quarter-century. Mutational processes are far more complex than was thought a few decades ago, with strand slippage, duplication, transposition, and illegitimate recombination producing qualitative differences between closely-related genomes.

So, while measuring nucleotide similarity in homologous regions is in the 98 to 99 percent range, this number does not incorporate or acknowledge the measured difference in the size of the human and chimp genomes, or the widespread differences in chromosome or genome structure. Because a DNA sequence is a one-dimensional entity, it lends itself much better to single-number comparisons than a four-dimensional body does. The genes that lead to the production of the human and chimpanzee femur may be 98 to 99 percent identical, but it is impossible to say whether the bones themselves are more or less similar than that.

Perhaps the most seductive aspect of the DNA comparison is its lack of biological context. The value of 98 to 99 percent similarity between human and ape DNA stands outside the rest of life. We can gain some zoological perspective on what that number really means by comparing another species to apes and humans. Compared to the body of an oyster, for example, chimpanzees and humans are at least 99 percent identical ? bone for bone, muscle for muscle, nerve for nerve, organ for organ. If anything, the DNA comparison underestimates the profound similarity of the human and ape bodies in the great tapestry of nature.

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