I've just been reading a fascinating article in The January 21, 2009 issue of the New Scientist, Why Darwin was wrong about the tree of life, which discusses the emerging significance of horizontal gene transfers. For all those who've read Gwyneth Jones's Life, this will certainly ring a glorious, wonder-inducing bell.
Some researchers are also convinced that hybridisation has been a major driving force in animal evolution (see "Natural born chimeras", and "Two into one"), and that the process is ongoing. "It is really common," says James Mallet, an evolutionary biologist at University College London. "Ten per cent of all animals regularly hybridise with other species." This is especially true in rapidly evolving lineages with lots of recently diverged species - including our own. There is evidence that early modern humans hybridised with our extinct relatives, such as Homo erectus and the Neanderthals (Philosophical Transactions of the Royal Society B, vol 363, p 2813).
Hybridisation isn't the only force undermining the multicellular tree: it is becoming increasingly apparent that HGT plays an unexpectedly big role in animals too. As ever more multicellular genomes are sequenced, ever more incongruous bits of DNA are turning up. Last year, for example, a team at the University of Texas at Arlington found a peculiar chunk of DNA in the genomes of eight animals - the mouse, rat, bushbaby, little brown bat, tenrec, opossum, anole lizard and African clawed frog - but not in 25 others, including humans, elephants, chickens and fish. This patchy distribution suggests that the sequence must have entered each genome independently by horizontal transfer (Proceedings of the National Academy of Sciences, vol 105, p 17023).
Other cases of HGT in multicellular organisms are coming in thick and fast. HGT has been documented in insects, fish and plants, and a few years ago a piece of snake DNA was found in cows. The most likely agents of this genetic shuffling are viruses, which constantly cut and paste DNA from one genome into another, often across great taxonomic distances. In fact, by some reckonings, 40 to 50 per cent of the human genome consists of DNA imported horizontally by viruses, some of which has taken on vital biological functions (New Scientist, 27 August 2008, p 38). The same is probably true of the genomes of other big animals. "The number of horizontal transfers in animals is not as high as in microbes, but it can be evolutionarily significant," says Bapteste.
Whatever happened to the Great Chain of Being? Gone the way of the Dodos, I'd say.
(Link thanks to Gwyneth Jones.)