The Big Question: Why did dinosaurs die out, and why should it matter 65 million years later?

Why are we asking this now?

There are almost as many theories about the demise of the dinosaurs as there are numbers of scientists working on the problem. The latest comes from George Poinar, a "courtesy" professor of zoology at Oregon State University in Corvallis in the United States, who believes that the dinosaurs may have died out with the help of infections transmitted by biting insects.

Professor Poinar and his wife Roberta, who have just written a book called What Bugged the Dinosaurs? Insects, Disease and Death in the Cretaceous, argue that the deadly diseases carried by mosquitoes and other biting insects would have caused a heavy toll among the leviathans of the prehistoric terrestrial world.

What evidence has the professor come up with?

Professor Poinar contends that during the late Cretaceous period, the associations between insects, microbes and disease transmission were just emerging. "We found in the gut of one biting insect, preserved in amber from the era, the pathogen that causes leishmania a serious disease still today, one that can infect both reptiles and humans.

"In another biting insect, we discovered organisms that cause malaria, a type that infects birds and lizards today. In dinosaur faeces, we found nematodes, trematodes and even protozoa that could have caused dysentery and other abdominal disturbances. The infective stages of these intestinal parasites are carried by filth-visiting insects."

Does the theory of insect bites spreading infection have any merit?

It has less merit than many other more plausible extinction theories. What one has to remember is that the dinosaurs were just one of many different groups of animals and plants to go extinct about 65 million years ago.

In fact, the largest mass extinction of animals at that time took place in the oceans, where biting insects that transmit disease are not an issue. And then there is the problem of trying to explain how a blood-sucking mosquito or tick could cause a terrestrial plant species to go extinct, which they did in their thousands.

Professor Poinar is not saying that biting insects were the sole cause of the dinosaurs' demise, just an extra burden that would have made their lives more difficult. As he said: "We can't say for certain that insects are the smoking gun, but we believe they were an extremely significant force. Our research with amber shows that there were evolving, disease-carrying vectors in the Cretaceous, and that at least some of the pathogens they carried infected reptiles. This clearly fills in some gaps regarding dinosaur extinctions."

Haven't we heard about this link before?

It was the central theme in Jurassic Park, the novel by Michael Crichton that was made into the block-buster movie. Biting insects feeding on the blood of dinosaurs were supposedly trapped in amber, the fossilised resin of tree sap. The fictional scientists in the book extracted the dinosaur blood from the insect's gut, extracted the DNA and used it to bring the dinosaurs back to life.

All the evidence suggests, however, that this scenario is impossible. Dinosaur DNA does not survive the fossilising process, certainly not to the extent of it being used to recreate an entire organism.

So why did dinosaurs die out?

No one knows for sure but there are several highly plausible theories, which may have interacted with one another. But the really important point is that the dinosaurs disappeared at about the same time as many other species of animals and plants, at a point in geological history known as the K-T boundary. This was the time about 65 million years ago when the Cretaceous period (K) came to an end and the Tertiary period (T) began. Whatever killed off the dinosaurs almost certainly had a bearing on the rest of this mass extinction, which was global in scale and affected the flora and fauna in both the terrestrial and the marine environments. So whatever triggered the ultimate death of the dinosaurs, it must have been a pretty epic phenomenon affecting the entire globe for a very long period of time.

What is the most persuasive theory?

The most well known is certainly the idea that the Earth was hit by a giant asteroid, causing widespread firestorms and kicking up so much dust that it cut out sunlight, stopped life-giving photosynthesis and caused a prolonged winter period.

Who came up with this theory?

The idea was first postulated in 1980 by father and son Luis and Walter Alvarez of the University of California at Berkeley. At sites around the world, they discovered deposits of iridium a mineral found in meteorites at the K-T boundary layer, which suggested a massive global impact which vaporised an object from space about 65 million years ago.

About a decade later, scientists found a massive underground impact crater at Chicxulub on the Yucatan peninsula of Mexico, which matched the size and age for just such an asteroid crash site. Although the evidence seemed overwhelming, not everyone agreed with the asteroid-impact theory.

Other scientists suggested that a set of supervolcanic eruptions at a site called the Deccan traps in India could have caused an equally cataclysmic change to the global environment and it too would have dispersed iridium around the world. So the global catastrophists are split between those who think it was volcanic in origin and those who believed it came from space.

Any alternative arguments?

We are used to the idea of dinosaurs suddenly becoming extinct, and in geological terms measured over tens or hundreds of millions of years, their death was indeed quick. But there is abundant evidence from the fossil record that dinosaurs were already in slow and perhaps terminal decline long before the K-T boundary "event". It may be that they had just stopped evolving and diversifying, which made them more vulnerable to competitors.

Remember that dinosaurs first arose about 230 million years ago, so they had lived for about 165 million years during which time they came to dominate the land before they became extinct. As the largest land animals they would have been more vulnerable to changes in the environment especially if they could not control their body temperature in the same way as the warm-blooded mammals.

Should we care why the dinosaurs died out?

Yes...

* They were the most unusual land creatures, and their demise is inherently fascinating

* We may learn important lessons about how such a dominant group of animals lost its footing in the world

* Studying the evidence for dinosaur extinction teaches children about life, death and evolution

No...

* What happened 65 million years ago bears little or no relevance to today

* We can only speculate about what caused them to disappear, and such speculation is meaningless without hard evidence

* Scientists are wasting too much time and effort on a question that can never really be answered satisfactorily