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NS 18 may 2002
Creepy-crawly On the trail of the early worm
A MYSTERY is afoot. Something appears to have been crawling over the seabed more than a billion years ago. Yet according to the textbooks, animals didn't exist until hundreds of millions of years later.
The puzzle originates from some fossilised trails found in Australian rock. The ridges are a millimetre wide and up to 2 millimetres apart, and look like slime trails left by some worm-like creature inching across fine sea-floor mud.
When geologists found the trails 10 years ago, they assumed that the rocks must date from the Ediacara epoch 550 to 600 million years ago-the time from which the oldest generally accepted animal fossils come. But when Birger Rasmussen of the University of Western Australia dated the fossils using a new technique that was developed for non-volcanic rocks, he was surprised to find that they formed between 1.2 and 2 billion years ago.
Studies of the genetic differences between animals suggest that they began diverging between 700 and 1500 million years ago. But palaeontologists have always been sceptical about that because no animal fossils that old have been found, and because genetic clocks can underestimate the effects of rapid evolutionary spurts. And an earlier claim that rocks more than a billion years old found in India contain fossil trails has never been widely accepted (New Scientist, 10 October 1998, p 6).
Rasmussen asked palaeontologist Stefan Bengtson of the Swedish Museum of Natural History in Stockholm to take a closer look at the trails. He interpreted them as slime trails left by a worm-shaped organism that glided alopg by stretching its body forwards and then contracting to pull up the rear. Modern ribbon worms move in the same way.
A worm-like animal would be the logical suspect if the traces dated from the Ediacara period, but because they existed at least 600 million years earlier Bengtson says "we have been quite careful not to call these things animals". He says the mystery organism may not be the ancestor of any of today's creatures, but could have been an evolutionary "dead-end experiment" that went extinct.
"It's a very interesting structure," says Bruce Runnegar of the University of California at Los Angeles. He thinks the dating is accurate, but isn't yet convinced the fossils are trails rather than some other biological structure. He plans to go to Australia to look at the rocks in the field.
Rasmussen and Bengtson have found more trails since writing up their findings. "They don't change the story, but they make it considerably richer," Bengtson told Jeff Hecht
New Scientist. More at: Science (vol 296, p 11121
GM So Far How Good?
SO FAR THE PROPHETS OF DOOM HAVE DRAWN A BLANK. BUT WHERE ARE THE SPECTACULAR BENEFITS OF GENETIC MODIFICATION WE WERE PROMISED? NEW SC/ENT/STWATCHES THE DUST SETTLE ON THE GM CROP CONTROVERSY
WHEN genetically modified crops first hit the market in 1996, opponents warned that they could seriously damage the environment and human health. Proponents countered that the new technology was safe and would "feed the world", as Norman Borlaug, the Nobel prizewinning agronomist, later wrote in an editorial in The Wall Street foumal Since th the debate has shed more heat than light. But GM crops have built up enough of a track record for us to begin assessing what they really mean for farmers and the environment. And, peering up the biotech companies' pipelines, we can get a sense of what's coming next.
As things stand, neither proponents nor critics of GM crops can yet say "I told you so". Despite a few alarms, there's no real evidence that GM crops have hurt human health or the environment in the past five years during which their use has risen steadily but neither have they made the world a much better place. GM crops have delivered real, though not stunning environmental benefits and they have nudged yields upwards for mechanized farmers in industrialized countries very farmers who already produce so much food they have a hard time selling it at a profit. The near future promises more of the same however the biotech crops that might really help feed the world's hundry remain but a hazy future promise. Meanwhile, bold advances in conventional breeding mean that transgenic plants offer fewer advantages than we once thought. In short, the debate over GM crops has less riding on its outcome than either side admits. Most obviously, GM crops haven't produced the apocalypse that critics warned of. In 1998, Scottish-based plant chemist Arpad Pusztai provoked a furore when he claimed
that GM potatoes caused abnormalities in rats that ate them. But most scientists agreed Pusztai's experiments were seriously flawed, and there is still no convincing evidence that his claims are correct. Many also feared the worst the following year, when researchers in the US reported that in the lab, monarch butterfly caterpillars died after eating milkweed leaves dusted with pollen from GM corn. The corn had been engineered to contain a gene for a form of Bt, a bacterial toxin that acts as an insecticide. Suddenly, it seemed possible that all those waving fields of com could be killing off one of the best-loved species of butterfly in North America. But two years of follow-up studies showed that the pollen of most varieties of Bt corn wasn't very toxic and that, in the field, caterpillars didn't seem to eat enough of it to harm them. That seemed to settle the main question and helped convince the US Enviection Agency to re-approve Bt corn for another five years. The case isn't closed, though. For instance, how often do butterfly larvae accidentally eat corn anthers ( the pollen-producing structures), which contain high levels of toxin, when these fall onto the leaves of their food plant? and even if the pollen doesn't kill the catterpillar are there harmful long-term effects? No one knows, even as farmers plant Bt corn on millions of hectares all across the US.
Shortly after the butterfly controversy broke out, another human health scare surfaced. Once again, GM com was responsibleand once again, there's no clear evidence that it caused any real problem. This time, the culprit was a corn variety called StarLink that expressed another Bt toxin called Cry9C. The structure of Cry9C made it possible that some people might develop an allergy to it, so the EPA approved the corn only as animal feed.
But in September 2000, traces of StarLink corn turned up in some taco shells, and soon other corn was found to have been contaminated. US corn exports suffered, and StarLink has since been taken off the market. No one knows yet whether anyone actually suffered an allergic reaction to the com, although it's unlikely that serious or widespread harm occurred. Still, the incident reinforced the idea in the public mind that genetic engineering could make food unsafe. Another fear expressed ftom the beginning has been that genes wilf lump from GM crops into related weeds, resulting in "superweeds" that are harder to control. That hasn't happened yet, although a milder version of the same danger has turned up in Canada. There, researchers have found canola plants-a variety of oilseed rape-that are resistant to three different herbicides, even though no commercial seed carries more than a single resistance gene. This stacking of resistance genes is proof that they can spread, apparently through cross-pollination of different herbicide-resistant varieties. These superresistant plants, which can appear as weeds in ditches and fields planted with other crops, are close to being the superweeds that environmentalists feared from the startalthough farmers can still wipe them out with other weedkillers. But if GM crops have produced no big environmental disasters so far, they've also taken only a few halting steps towards making the world a better place. Herbicide-tolerant crops-which make up 7 7 per cent of GM crops planted today-make it easier for farmers to make the environmentally friendly move of abandoning their ploughs (see p 38, main feature). They don't reduce the total amount of herbicide sprayed, which the USDA says is still about a kilogram per hectare of US
soybeans, for example-the same as in 1995. But they do allow farmers to use less toxic choices, such as glyphosate, best known as Roundup, the chemical that most GM crops are engineered to tolerate. The other main class of GM crops, those that contain Bt toxins, have also produced some benefits. Bt cotton, for example, allows farmers to spray fewer insecticides. In China, where one,cotton plant in five is now armed with the Bt gene, growers have cut their use of toxic pesticides by 80 per cent. Only S per cent of farmers growing Bt cotton reported pesticide-associated illness, compared with 22 per cent among growers of conventional cotton (Science, vol 29S, p 674). The other main Bt crop, corn, hasn't led to reduced pesticide use, mostly because farmers rarely spray corn against its major pest, the European corn borer, which hides in the soil out of reach of pesticides. But by reducing losses, some farmers using Bt corn in the US reaped yields 9 per cent higher during heavy corn-borer infestations, according to a report last year by the National Center for Food and Agriculture Policy in Washington DC. Still, it's too early to tell whether this makes up for the higher cost of GM seed.
What GM crops haven't done yet is put more food into the bowls of hungry people. That may change, at least in China, where the government is aggressively pursuing the new technology with about 2SO GM varieties now approved or being tested. Some 90 per cent of field trials in China are aimed at reducing losses to pests or diseases, especially viruses. Elsewhere, government-sponsored centres are testing a few similar crops-virusresistant sweet potato in Kenya, for example. In the US, by comparison, only 20 per cent of trials are of pest or disease-resistant crops, with most of the rest being herbicide-resistant crops. The difference shows the huge bias in industry towards developing plants that enable more agrochemicals to be sold as part of their overall GM package.
Even with pest and disease-resistant crops, though, there are big worries that bugs can and will overcome the single genes, such as Bt, that defend the crops. Then you're back to square one. But Andy Maule of thejohn Innes Centre in Norwich predicts that crop scientists will change their strategy from toxins that kill pests outright to multi-gene traits that discourage but don't kill them. Unlike kilter toxins, which.leave only the rare, resistant insects alive to reproduce, "tolerance' genes spare even vulnerable insects, thus slowing the development of resistance in the pests. Genetic engineering could do many other things to build better crop plants. Maurice Ku of Washington State University in Pullman is working to improve the photosynthetic system of rice by inserting genes ftom the much more efficient system found in maize (New Scientist, 1 April 2000, p 19). And still others are altering staple crops so that they produce vitamins such as folic acid, which helps prevent birth defects if consumed by pregnant women, and vitamin A, as in the well-publicised "golden rice' announced in 1999. Further in the future, genetic engineers may learn how to force crop plants to reproduce exact copies of themselves by setting seed asexually, so that poor farmers can get the benefit of elite hybrid varieties without having to pay for seed every year. The trait would also make it easier to maintain varieties tailored to local conditions, and it would prevent leakage of genes into wild relatives, says Briah Johnson, biotechnology adviser to English Nature. But farmers will not be exploiting these traits any time soon. Private enterprise, the biggest source of funding for GM crops, is understandably Teluctant to invest in products that would be mostly useful to poor farmers who lack the cash to buy them. "Biotechnology companies are not philanthropists," says Val Giddings, vice-president of food and agriculture at the US Biotechnology Industry Organization. With costs of between $5 million and $30 million to get regulatory clearance for each GM crop, it's easy to see why the companies concentrate on the big potential earners.
Some knowledge should transfer easily from major crops, which will help bridge the gap. "What works in soya also works in chickpeas," says Roger Beachy of the Donald Danforth Plant Science Center in St Louis, Missouri. But without extra money from governments, and in the face of environmental protests, progress is likely to stall.
And anti-GM protesters may also be holding back improvements that could benefit developing countries. Maule says there are many crops in Africa which could be improved through GM if the technology were accepted. But he believes government officials have been "spooked" by anti-GM propaganda. "They are nervous that if they get into international markets, they'll have difficulty selling stuff in places which are antiGM, particularly Europe,' he says.
Ironically, crop researchers may not need GM to accomplish many of these things. Old-fashioned plant breeding, newly supercharged by plant genomics, promises to deliver many of the same benefits without the political strife (see 'There's no substitute for good breeding"). If so, that may prove the most expedient solution for struggling farmers in developing countries who have yet to see the benefits trickle down to their fields.