By Richard Black BBC science correspondent

They propose moving common chimps and another very closely related ape, bonobos, into the genus, Homo, the taxonomic grouping researchers use to classify people in the animal kingdom.

Humans, or Homo sapiens to give the species its scientific name, are the only living organism in the genus at the moment - although some extinct creatures such as Neanderthals (Homo Neanderthalis) also occupy the same grouping.

Six species

"Since people have been studying primate evolution, there's been this dichotomy between humans and the apes," said Dr Derek Wildman, who has published the findings of the genetic study with colleagues in the Proceedings of the National Academy of Sciences (PNAS).

"And so what we've shown is that humans and chimpanzees are actually more similar to each other than either is to any of the other apes," he told BBC News Online.

Modern genetic science offers researchers another way to establish the relationships between different species, by measuring the similarity of their DNA code.

It is a far cry from the traditional way of categorising organisms on the basis of what they look like, either live or in fossil form.

The Detroit team compared 97 important genes from six different species: humans, chimpanzees, gorillas, orang-utans, Old World monkeys, and mice.

From this, the scientists constructed an evolutionary tree that measured the degree of relatedness among the organisms.

Horses and donkeys

According to this analysis, chimpanzees and humans occupy sister branches on a family tree, with 99.4% genetic similarity. Next on the tree are gorillas, then orang-utans, followed by Old World monkeys.

None of the primates were closely related to mice, which were used as a control.

Dr Wildman said: "You could say that humans and chimps are as similar to one another as say horses and donkeys.

"And there really isn't much evidence for them to be divergent at the family level, which would be something like the divergence between apes and monkeys.

"There has been this notion since Aristotle's time of this great chain of being with humans at the top and then less complex life at the bottom. But while that might seem intuitive to some people, it doesn't appear to be borne out by the data.

"There's been as much change on the lineage on the line leading to chimpanzees as there has been on the lineage to humans since they last shared a common ancestor around six million years ago."

Chimp troubles

The Detroit team says its work supports the idea that all living apes should occupy the higher taxonomic grouping Hominidae, and that three species be established under the Homo genus.

One would be Homo (Homo) sapiens, or humans; the second would be Homo (Pan) troglodytes, or common chimpanzees, and the third would be Homo (Pan) paniscus, or bonobos.

Not all scientists will accept the new classification.

Whereas Dr Wildman's team find that chimps and humans are 99.4% similar, other researchers last year put the similarity at around 95%; the figure you get depends on precisely which genetic differences you look at.

As to whether this will improve the lot of chimpanzees themselves, a spokeswoman for the conservation group the Jane Goodall Foundation was sceptical.

The problems of habitat loss and commercial bushmeat hunting would continue whatever genus we put them in, she said.