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 produce objective, value-free glimpses of nature.
In the field of human origins, it is well-known that a human DNA sequence is nearly 99 per cent 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 (and fated to be aggressive, or to possess whatever attributes are being imputed to apes these days), or 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 humans.
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, indeed, find that it corresponds at nearly 99 of every 100 comparable sites. But this method omits much of what has been learnt 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 indeed in the 98 to 99 per cent 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 per cent 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 per cent 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 per cent 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.
Context reveals something at the low end of the scale as well. Two DNA sequences generated at random must be 25 per cent identical, by virtue of the fact that DNA is a sequence of only four bases. Therefore, all multicellular life, having presumably developed from a single common ancestral form, must be over 25 per cent identical in their DNA sequences.
In other words, a human and a carrot have much in common genetically, despite there being little similarity between them physically.
Here the DNA comparison grossly overestimates the actual relationships between species. Genetic comparisons simply do not afford an 'inner view' of the relationships between species, or an encapsulation of those relationships.
In other words, this apparent fact of nature - the overwhelming genetic similarity of human and ape - is a fact constructed from culture. That is not to say it is inaccurate or false; just that its meaning is far less obvious than it may appear superficially.
We make sense of the world, and of our place in it, culturally - and science provides more information for constructing that place. But like any other peoples, our own application of that information to the puzzle of our existence is strongly influenced by our non-scientific ideas, our pervasive folk ideologies of heredity.
The writer is a biological anthropologist at the University of North Carolina at Charlotte and the author of What It Means To Be 98% Chimpanzee: Apes, People, And Their Genes. Copyright: Project Syndicate