From the home of Dolly the sheep comes the news that humans are in some ways more like chicken than mice. So say David W. Burt and colleagues from the Roslin Institute near Edinburgh, Scotland, who have been comparing the rough drafts of the 'genomes' or genetic sequences of humans, mice and chickens.

Genomes of a variety of organisms are being catalogued at an increasingly rapid rate. In addition to the genomes of a variety of viruses and bacteria, those of yeast (Saccharomyces cerevisiae), the lab roundworm (Caenorhabditis elegans) and the fruit fly (Drosophila melanogaster) are now known. In less than a year – perhaps by the early summer of 2000 – the human genome will be essentially complete, at least in draft form. Rough 'sketches' of the large-scale features of many more genomes are in place, even though the precise DNA sequences remain to be filled in.

From this flood of information should emerge an entirely new way of looking at evolution. By comparing the major features of genomes all at once – rather than, painstakingly, one gene at a time – researchers will gain powerful insights into the history of human beings and other organisms.

One such insight is the work of Burt's group. As they explain in Nature1, the researchers have been looking at sketches of the mouse, human and chicken genomes in which the locations of large numbers of genes have been mapped, even though the precise details of the genes themselves are still being worked out. (This is like comparing street maps or aerial photographs of cities, rather than walking along each street on foot, noting down the colour of every front door.)

This strategy allows researchers to plot the relative positions of genes on chromosomes in different organisms, and examine how entire banks of genes have been swapped between chromosomes during evolution. It turns out that the overall pattern of gene order in the human genome is similar to that of the chicken, harking back to our reptilian common ancestor more than 300 million years ago. The mouse genome, in contrast, has undergone about twice as much general reorganization as the human genome. This is symptomatic of two things.

One is that bird genomes contain fewer repetitive elements – less 'junk' – than mammalian genomes. The presence of junk makes accidents, such as the inversion or movement of a region of a chromosome, more likely.

Why, then, don't humans – which are mammals – have a more rapidly evolving genome? Why are we outstripped by mice, left gasping in the chicken coop? Because of reason number two: most mutations happen during reproduction. Mice, like all rodents, breed much more quickly and frequently than humans, giving mutation much more scope to act. The researchers hope that studies like these will allow them to look back into the genomic past and, perhaps, reconstruct the 'ancestral' genome of the vertebrates.