Affairs to Remember
Affairs to RememberBy OLIVIA JUDSON - New York Times, May 28, 2006 - London, originalEVER since scientists realized that the fossilized bones of ichthyosaurs and mastodons were relics of organisms past, debates have raged about what fossils mean for our understanding of the history of life on Earth, and especially of evolution. No longer. Fossils have become unnecessary to the argument: since we've learned to sequence whole genomes, we've had far more powerful ways to examine the past.
This point is the most significant aspect of a recent study of primate genes published in the online edition of Nature. The researchers matched big chunks of the human genome to the genomes of several other primates, including gorillas and our close cousins the chimpanzees. By dint of evolutionary forensics, complex statistical analyses of DNA sequences, the researchers revealed that the ancestors of humans and chimpanzees exchanged genes — they had sex — a million or so years more recently than we had thought.
Prurience aside, our ancestors' antics are of no particular note. The claim that humans and chimpanzees started to become different species but then some individuals got back together for a final fling — well, such shenanigans go on in nature all the time. For example, Europe's white-headed ducks are supposed to be a different species from North American ruddy ducks. Yet some years ago, a few of the ruddy ducks arrived in Europe and to the consternation of conservationists, who tend to disapprove of such miscegenation, white-headed ducks have been getting it on with the ruddy ducks. As a result, "pure" white-headed ducks are disappearing
No, the remarkable fact is not what our ancestors did five million years ago, but that we can find out about it by scrutinizing our genes.
Bit by bit, our ability to penetrate the mists of time is improving. At first, using only single genes, we began to reconstruct patterns of human migration. Then it became possible to examine small stretches of DNA, like the Y chromosome (passed from father to son). The immense sexual success of Genghis Khan is witnessed by the large number of men today who have his Y chromosome. Now that we can sequence whole genomes, we are undertaking investigations of far greater scope to reconstruct our ancient history and that of other organisms.
But that's the least of it. A less salacious but more salubrious use for this data is to observe, in rich detail, how organisms are made and how they evolve. This has led to some surprising discoveries. For instance, the number of genes has turned out to be lower than expected. We once grandly estimated ourselves to have around 100,000 genes. Now we know the number is more like 23,000 — only a few more than chickens have.
Another fascinating find: genomes are complex patchworks of genes from a startling variety of sources. Moreover, we can use the data to trace exactly which genetic changes have led to changes in an organism's appearance, or in what it does. In other words, we can examine evolution with a sophistication and subtlety that, even 10 years ago, we could only dream of.
This is not to say that the study of fossils is unimportant. Without fossils we would know nothing of life's also-rans — trilobites and pterodactyls, giant carnivorous sheep and vegetarian crocodiles, humongous dragonflies and the myriad other marchers in the parade of the extinct. Fossils also provide a measure of the passage of time. And excitingly, some fossils — mammoths, cave bears and Neanderthals, for instance — are yielding DNA, allowing us to examine the genomes of organisms that lived tens of thousands of years ago. But when it comes to the traditional use of fossils — the demonstration that evolution has happened — that's antediluvian.
Olivia Judson, a research fellow in biology at Imperial College London, is the author of "Dr. Tatiana's Sex Advice to All Creation." In June, her column "The Wild Side" will appear on the TimesSelect Web site.