The secret ingredient appears to have been sex. Asexual reproduction, in addition to being rather boring, doesn't introduce anywhere near the possibility for diversification of a genome like sex does. So somewhere around 700 to 1000 million years ago, life discovered this new way to do things, and that seems to have been the real turning point. Others have suggested additional reasons for early stability and later explosion 1) Oxygen content of the atmosphere. Complex life could not arrise until there was sufficient oxygen to make complex life feasible. 2) There may have been complex life before the Cambrian but it lacked hard body parts that could fossilize. Hard parts evolved when life got nasty (predation) 3) There needed to a major disruption of the status quo before new forms could gain a foot hold Enter snow ball earth about 600 million years ago. By the way sex does not introduce diversification it facilitates it. Without sex, life would be limited to about 6000 genes. Sex is an error correction tool. Think of the game of telephone where a message passed from individual to individual is degraded to the point where it no longer is intelligable. (In modern lingo - its information is lost). Same holds true for organisms. After repeated copying the genetic message is lost and organism cannot reproduce. Basic rule is that an individual must produce at least one good copy of itself if its lineage is to survive. But as you increase the number of genes the number of errors increase. So the upper limit of genetic complexity is determined by how good your error correction tools are. DNA surplanted RNA because it was better at correcting errors. Other tools like reverse transcriptase allow for correction of DNA errors. Sex achieves its error correction by randomly dispersing genes to multiple descendents. The number of mutations does not decrease but some offspring get stuck with lots of bad genes (and die) and others get only good copies. By the way this stuff comes from Mark Ridley's book "The Cooperative Gene" and Matt Ridley's (no relationship) " The Red Queen". When you understand that the number of genes is limited in this matter it does not come as a shock that humans have only 30,000 genes. To have more we would have had to invented a whole new way of correcting errors. Mark Ridley published The Cooperative Gene (US title - it was called something else in England) before the genome was decoded and he was very skeptical that we would have the 100,000 + genes that had been generally predicted for humans. Just another example of human hubris.
But life's apparent diversity today is still not the whole picture; proximally 95% of all species that ever existed are now extinct. Some of that is due to mass die-offs, but a lot of it is also do, simply, to evolution. Eventually, in two populations isolated by geography (for instance), one group of organisms simply cannot interbreed with the other, and so you get a new species. Ring species are one rather interesting example of this. If you go one way you have populations that can interbreed; but if you go the other way, you have a disconnection: <http://www.pbs.org/wgbh/evolution/library/05/2/l_052_05.html> And if ? when ? one of the populations fails to survive, perhaps a harsh winter or similar local catastrophe, you can argue that the species is extinct, but there's still continuity there in the surviving and altered branch. Hence finding that the majority of species which used to be here are gone isn't really that surprising, though some interpret it as a warning rather than recognizing it as simple change. After all, Pangaea is no longer on the maps either, yet no one mourns its passing. >> Homo sapiens as we know it today is doomed, like 90%+ of all >> other terrestrial species, to extinction. > > hmmmm, perhaps... Or maybe HomoSapients-Universalis will be able to > > interbreed with all species..... I've allways wondered about that > Polyploid > Honey from "The Fifth Element"... What do you think she's > got hiding in > those genes? Everything. IIRC the doctor in that movie claimed that *all* her genes were active, not 97% or so deactivated like the junk DNA humans have. That's a cute but impossible idea, since you really wouldn't want most of that stuff switched on. It might code for scales and gills, for instance, fins instead of fingers, etc. Junk DNA is why the "loss of information" argument from ID-iots is specious. The information to, say, grow eyes ? or segmented abdomens ? is not lost. It's still there; it's just switched off, but it can be switched on again any time. It seems that once DNA "learns" how to do something it doesn't forget; it simply stops doing it in favor of some other variation that, for whatever reason, is more optimized to fit a given environment. Finally, it's easy to overlook the quiet mutations that don't have any apparent effect at all on an organism's ability to succeed ? *until* some major change to the local scene takes place, and suddenly an unimportant variation could well be the single difference between survival to breed or vanishment. Genetic change isn't always about producing something that's clearly immediately superior (or inferior) to other members of the population. It can often be about these kinds of minor and seemingly unimportant variations which end up being crucial because of entirely stochastic environmental effects. One example would be the famous case of peppered moths living in England, which appeared to have taken on a sooty color in response to large levels of coal soot in the 1900s. IIRC it turned out that the moths didn't change their color; the black shading was one variation, which had been present long before the industrial revolution, that ended up being more successful because of pollution than their more lightly-colored counterparts: <http://www.millerandlevine.com/km/evol/Moths/moths.html> That is, the color differences had been there long before pollution; they only became more successful *after* soot helped them hide more effectively. -- Warren Ockrassa, Publisher/Editor, nightwares Books <http://books.nightwares.com/> Current work in progress "The Seven-Year Mirror" <http://books.nightwares.com/ockrassa/Flat_Out.pdf> <http://books.nightwares.com/ockrassa/Storms_on_a_Flat_Placid_Sea.pdf> _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
