THE claim that it is "unnatural" to create genetically modified creatures by moving genes between different species received a knock last week with the discovery that hundreds of our genes probably originated from bacteria.
The remarkable revelation that we are all a bit microbial, that dozens of our genes have been created by parasites called transposable elements, or "transposons", underlines how nature is stranger than we can conceive.
These alien influences on our genetic make-up are reported in the journal Nature by the publicly-funded International Human Genome Sequencing Consortium, including Sir John Sulston of the Sanger Centre near Cambridge, and Dr Eric Lander of the Whitehead Institute, near Boston.
The consortium found 223 genes that are the codes for proteins very similar to those made by bacteria, including one that plays a role in depression. These genes have no similarity to those of the yeast, worm, fly and mustard seed, the other organisms whose genetic secrets are now laid bare.
The team rules out contamination as a possible explanation - despite claims to the contrary by its commercial rivals - and suspects that the genes were transferred to humans from bacteria, given that bacteria often exchange genes in their equivalent of sex.
Bacteria are not the only parasites that seem to have been tampering with our DNA. There are also genetic parasites, "duplicate me" instructions that have been passed down over the generations.
With names such as Lines, Sines, Ltr retrotransposons and DNA transposons, these parasites are a significant fraction of our genetic code, representing 13, 20, eight and three per cent respectively.
The consortium has focused on the most common Sine parasite, called an "Alu", and found that it likes to lurk in gene-rich regions. There is evidence that there has been a 13-fold enrichment in these regions over the past 30 million years and these "selfish" elements "may benefit us in some way".
One scientist has suggested that, under stress, the parasites may go into action to enable a richer repertoire of proteins to be made to help us to cope. In other words, these particular parasites "earn their keep in the genome".
Although transposons have only one aim in life: to copy themselves, the consortium points out that these selfish pieces of DNA have been responsible for important innovations in our genetic recipe.
Before last week's announcements, it was known that at least 20 human genes had been created by transposons. Now the figure is 47. Many more genes have been shaped by the parasites: few hundred genes use genetic instructions donated by LTR transposons.
The papers in Nature and Science also reveal vast stretches of desert-like regions, about one quarter of the genome, where the sequence of genetic letters does not spell out any genes at all. "You find regions of the chromosomes that are very dense with genes," said Dr Craig Venter. "And then regions of the chromosomes that we call deserts that are virtually void of genes."
The "best reads" in the human genome are chromosomes 17, 19 and 22, which are gene rich. In contrast, four, 18, 13 and the sex chromosomes - the X and Y - are comparatively barren. Dr Venter said women will appreciate the discovery that the Y chromosome carried by men "is absolutely pathetic. There is just not much there. It is very sparse in genes".