What Really Killed the Dinosaurs?

The idea that mass extinctions are caused by impacts from outer space has been the most popular explanation. It is certainly one of the best marketed pieces of popular science. Not only is it accepted by most scientists, it has even inspired Hollywood movies. We dealt with this question in Reason in Revolt nine years ago, where we challenged the prevailing theory that this mass extinction was caused by the impact of a massive meteor. We wrote the following:A recent Horizon programme on Channel Four asked the question “What Really Killed the Dinosaurs?” For a hundred and forty million years, the dinosaurs were dominant. Then, all of a sudden they all disappeared. Something must have killed them off. What was it?

“There is empirical evidence to suggest that some kind of explosion took place, which may have been caused by a meteorite. The theory has gained ground in recent years with the discovery of a thin layer of clay amongst fossil remains, which would be consistent with the effect of dust produced by such a large impact. The idea has, for example, seemingly been accepted by Stephen J. Gould. Nevertheless, there are questions which have still to be answered. First of all, the dinosaurs did not disappear overnight, or even in a few years. In fact, the extinction occurred over several million years – a very short time in geological terms, but sufficiently long to cast some doubt on the idea of a meteoric catastrophe.

“While the meteorite hypothesis cannot be ruled out, it has one major disadvantage. As we have pointed out, there have been many mass extinctions along the evolutionary road. How is this to be explained? Do we really have to resort to an external phenomenon such as a sudden meteor impact to do so? Or does the rise and fall of species have something to do with tendencies that are inherent within the process of evolution itself? Even at the present time, we can observe the phenomenon of the rise and fall of animal populations. Only recently have we come close to understanding the laws which govern this complex process. By looking for explanations that lie outside the given phenomenon, we run the risk of abandoning the search for a real understanding. Moreover, a solution which seems attractive because it removes all difficulties at a stroke can create even greater difficulties than the ones it was alleged to have solved.” (Reason in Revolt, p. 261)

We favoured an alternative explanation – one that did not require cosmic intervention of any kind:

“The period under consideration was characterised by widespread volcanic activity. This, and not a meteorite impact, could well have caused a change in the climate which the dinosaurs were unable to cope with. It has also been suggested that the disappearance of the dinosaurs was connected with competition from the mammals. There is a parallel here with the disappearance of most of the original marsupial population of South America under pressure from the mammals from the North. Indeed, it is possible that the extinction of these creatures was the result of a combination of these circumstances – volcanic activity, destruction of the existing environment, excessive specialisation, and competition for reduced food resources by a species better-equipped to cope with the changed conditions. It is unlikely that this particular controversy will be resolved in the near future. What is not in dispute is that, at the end of the Mesozoic some fundamental change ended the domination of the dinosaurs. The main thing is that it is not necessary to introduce external factors to explain this phenomenon.” (ibid.)

At the time even some of our supporters thought we were wrong to reject the impact theory. They argued that the discovery of a huge crater in the Yucatan Peninsular in Mexico, together with additional proof, had confirmed it beyond reasonable doubt. However, more recent research has thrown the whole question back into the melting pot.

The dialectical method

In essence, this was a disagreement about method. As Marxists we approach all questions from the standpoint of dialectical materialism. Dialectics attempts to derive the laws of motion of nature, society and human thought from a careful analysis of objective phenomena and processes. Formalism, on the contrary, seeks to impose a preconceived schema on reality. It is characteristic of formalistic thinking to invent a hypothesis and then try to get the facts to fit into it. Dialectics, on the contrary, tries to work out the laws that flow from the thing itself.

In general, it is an unsatisfactory method that seeks to explain natural phenomena in terms of external elements and accidents. This does not mean that such elements play no role. They can and do play a role. But, as Hegel explained long ago, necessity expresses itself through accidents. An accidental event (such as an impact from a passing meteor) is an event that might take place or might not take place. In that sense it has no necessity. To attempt to explain such important phenomena as mass extinctions purely in terms of accidents is to avoid a real explanation altogether.

Let us pose the question concretely: what would have happened if the meteor had missed the earth? Would the dinosaurs have escaped extinction? Or to pose the question differently: “The problem becomes clear the moment we pose the question in the following way: very well, let’s accept that the extinction of the dinosaurs was caused by an accident in the form of a sudden meteorite impact. How do we explain all the other mass extinctions? Were they all caused by meteorites?” (Reason in Revolt, p. 262.)

This question is not so pointless as it may seem, because some scientists have attempted to do just that. This has led them into a blind alley, as we shall show. But first let us set out the essentials of the impact theory that is still generally accepted.

What the theory says

What does the existing theory say? It states that sixty five million years ago a huge meteor came from outer space. The impact caused a catastrophe in which the whole world burned. Dust blocked out the sun and the earth was plunged into deep freeze for months or years. This was what was supposed to have driven the dinosaurs to extinction.

How the question should not be posed

The generally accepted theory points to a layer of rock called the KT boundary, which was formed sixty five million years ago. It is seen in mines and rock outcrops around the world. Because it is such a thin, sharp line we know something dramatic must have happened here, and this something has been linked to the disappearance of the dinosaur. Below this layer there are lots of dinosaur fossils, above it there are none.

In 1979 a clue was discovered in the KT boundary. There was a high concentration of an element called Iridium. Such quantities are extremely rare on earth and usually come from outer space. The conclusion was drawn from this that somewhere on earth a very big impact must have happened by an asteroid or a comet of great size – possibly some ten kilometres in diameter. If the theory were right, the impact would have created a fireball, equivalent to ten billion Hiroshima bombs. The shock wave alone would have destroyed all life for hundreds of miles around.

This seems superficially attractive, but the defects in this theory are self-evident. They were well expressed on the programme by Norman Macleod of the Natural History Museum, London: “The impact theory says in effect that a rock fell out of the sky and killed everything, except for the things that it didn’t kill. I don’t think that’s much of an explanation.”

In order to prove the theory they first had to find the evidence. Jan Smit of Amsterdam University first discovered traces of a huge impact. Hidden in the KT boundary layers he found these tiny balls of rock called spherules. These are made of round rock globules condensed from a vapour cloud, originating at very high temperatures that may be associated with an impact.

The spherules were used as evidence to suggest that the fireball caused by the impact had vaporised billions of tonnes of rock. In outer space the vapour condensed into tiny droplets, which fell back all over the earth as white-hot spherules. Then scientists discovered another clue in the KT boundary, high levels of soot, which seemed to be evidence of massive burning at the time of the impact.

It was assumed that the world’s forests had spontaneously ignited as the rain of spherules heated the atmosphere by up to a thousand degrees centigrade. Such temperatures would instantly have ignited all the plant matter across the world and the entire planet would go up in flames. The impact was also thought to have created a deluge of deadly acid rain, as strong as battery acid. This would literally burn everything on the land, not just the dinosaurs.

Lastly, there was a high concentration of fern spores, known as a fern spike. Ferns flourish whenever all other plants have been killed off by some environmental devastation. So the predominance of fern spores suggested something had wiped out every plant on the planet. It was taken as evidence that there was something like global darkness caused by an impact.

It was suggested that vast amounts of dust created by the impact must have blocked out the sun. This could have plunged the world into freezing darkness for months or years. Any dinosaurs which had escaped burning must either have frozen or starved to death. But the mammals were small and could burrow to escape the heat and the cold. When conditions recovered they would emerge to inherit the earth.

It was an elegant theory, but there was one thing missing – the impact should have left a crater, two hundred kilometres across. To prove that the theory was right they had to find it. Alan Hildebrand of the University Calgary in Canada found a likely candidate in Mexico, near a village called Chicxulub: a horseshoe shape feature a hundred and eighty kilometres across, on the Yucatan Peninsula. Since this is within the region where the ejecta thickness says the crater should lie, he suggested that this might be the crater that killed the dinosaurs.

Jan Smit searched for rock outcrops the same age as Chicxulub. He hoped they would reveal the remaining secrets of the impact. Finally he came to explore an area one thousand kilometres from the crater itself. It was a massive rock outcrop. It seemed to be exactly the same age as Chicxulub.

Anomalies

At last the whole history of the impact could be read in a rock. There were spherules from the crater and Iridium from the asteroid. But in between the spherule layer and the layers rich in Iridium there are huge masses of sandstone. How did it get there? Sand is normally found only on the coast, but sixty five million years ago this whole region was deep under the ocean. The sandstone should not have been here.

But Smit was not worried by such anomalies. Smit thought it could only be evidence of another disaster caused by the asteroid. There must have been a huge Tsunami wave, a tidal wave like no other. It would have travelled at hundreds of miles an hour. It could have been a thousand feet high and it could have churned up billions of tonnes of sand from the coast and dumped it on the ocean bed. This became the sandstone and when the sea level fell, the rock was exposed. If Smit was right the entire outcrop was created in a couple of days.

With Smit’s Tsunami, the impact theory seemed complete. The idea became accepted science, and it captured the popular imagination. It was even reflected in Hollywood disaster movies. Everything seemed to corroborate the meteor theory. It appeared to be unassailable. But in science it often happens that a beautiful theory is overturned by a single awkward fact. And in this case there are lots of awkward facts.

Gerta Keller, a geology professor from Princeton, and her colleague, Wolfgang Stinnesbeck, investigated fifty rock outcrops in the same region of Mexico that Smit had explored. The same sandstone feature Smit described occurred everywhere they looked, but Smit’s Tsunami analysis didn’t seem to fit the facts.

The first awkward fact was evidence of ancient life, fossilised in the sandstone. At a lower level they found feeding structures, feeding tracks of a probably worm-like animal that ate the sediment and we have another layer here with dwelling structures of a crab. This suggested worms had colonised the lower layer of sandstone over months or even years. Only afterwards was the upper layer deposited and colonised by the crabs. It seemed to them that there were two layers created at different times and not within a couple of days as Smit had suggested.

The next awkward fact was the existence of layers of muddy sedimentary rock, interspersed in the sandstone. These are made of fine grains, which tend to build up slowly on the sea floor and would have taken a long time to form. That meant that these sandstones would be separated by some time. They could not have been formed in a couple of days. Near the bottom of the outcrop they discovered their third awkward fact – a layer of limestone, which can also take a very long time to form.

Then they discovered another layer of spherules in places where, according to Smit’s hypothesis, they were not supposed to be. Chemical analysis suggested that these remains of tiny meteorites, like the layer higher up, could only have come from the Chicxulub crater. According to Keller and Stinnesbeck, this changed everything.

The Chicxulub crater

These spherules were eight metres below where any had been found before. In geology, lower normally means further back in time, so for them it suggested an entirely different explanation for the whole rock formation and the extinction of the dinosaurs. After the Chicxulub impact, the spherules settled on the sea floor within minutes. These were the spherules just discovered by Doctor Keller. On top of the spherules there seemed to have been a period of normal sedimentation. Eight metres of mud had built up. Keller and Stinnesbeck estimate this would have taken two hundred and seventy thousand years.

Next came the limestone layer, which also could have taken thousands of years to form. Only then came the sand, washed in to form the sandstone. Then those fine-grained layers built up by sedimentation that could have taken centuries. Then worms apparently had months, maybe years in which to colonise some of the layers. Only then, at the very top came the Iridium layer, which coincided with the extinction of the dinosaurs. This would mean that three hundred thousand years had passed since the Chicxulub impact. In other words, Chicxulub was far too old to have caused the extinction of the dinosaurs.

If this is correct, then the Iridium must have come from an asteroid but it must have been a second impact, three hundred thousand years later. But nobody knows where the crater is and it is by no means certain that impact “probably finished off the dinosaurs”, as Professor Keller believes.

Bitter controversy

This has sparked one of the bitterest scientific controversies of recent years. Gerta Keller’s work provoked a major scientific clash. Defenders of the old impact theory attacked her ideas. Jan Smit was furious because Keller has questioned the accepted theory about what wiped out the dinosaurs. In fact, he was desperate to rescue his impact Tsunami hypnosis. It is always the way! What began life as a hypothesis has acquired the status of an accepted dogma, which must be defended at all costs. The argument soon turned vicious.

The heated nature of the controversy should be no surprise. There is an Establishment in the world of science just as there is in society at large. What are at stake are not just scientific truth but prestige, careers and salaries. So there are powerful interests that are compelled to defend the official dogma. Heresies are not welcome. But all new theories always begin as heresies and without heretics no progress would ever be possible and we would be permanently sunk in stagnation.

As a matter of fact, the controversy over Chicxulub, while it casts serious doubt over the dominant theory, misses the main point, which is that you cannot explain mass extinction events by accidents such as meteorite impacts. Meteorites have hit the earth on many occasions, most of which do not coincide with mass extinction events. On the other hand, there are well-documented mass extinction events that do not coincide with such massive impacts.

Norman Macleod comments: “For the very large craters that we think might have produced global effects, in some cases they don’t line up with any extinction events whatsoever and in other cases extinction events, very well dated extinction events have occurred that are not associated with any impacts.”

The “universal conflagration” theory

The KT boundary

Because of this, some scientists (and ourselves) have long doubted that any impact could have wiped out the dinosaurs. Now there is new evidence, which adds weight to these views. This relates to the forest fires that were said to have raged all over the world as a result of the impact. This is supposed to be proven by the soot found in the KT boundary layers. The problem with the wildfire theory is that it is based on computer modelling and theoretical arguments. And there is a problem with soot, because it can be blown thousands of miles from where the burning actually happened.

Claire Belcher, a researcher at Royal Holloway, University of London, decided to investigate a more reliable kind of evidence. She explains that large pieces of charcoal cannot be transported great distances. They cannot be picked up and lifted in smoke plumes or carried around the world, as soot can. Therefore if charcoal can be found at each KT boundary location we can be sure that burning occurred at each site.

In 2003, Claire Belcher scoured North America looking for charcoal. She collected samples from eight KT boundary sites, from New Mexico in the south right up into western Canada. After examining the samples in a laboratory, she was amazed at what she discovered there was almost none.

Claire Belcher’s results were consistent from Canada, all the way down to the southern United States, just two thousand kilometres from the crater. Even here there was no evidence of burning. What she did find was a lot of non-charred plant material. To have this amount of non-charred plant material in the KT boundary rocks means that there cannot possibly have been a globally extensive wild fire. It simply wouldn’t have left so much non-charred material on the rock record.

“It’s significant because most people model the KT boundary in terms of the thermal energy released,” Belcher told BBC News Online. “It’s often said that temperatures on the ground reached 1,000 [Celsius]. But 40% of species survived the impact. How could a small mammal survive temperatures of 1,000 Celsius?”

Claire Belcher’s conclusion is emphatic: “If there was no burning across north America then how on earth could there be burning across the rest of the world because north America is so close to the impact site so basically it seemed there were no fires.”

Acid rain?

So yet another prop has been knocked from under the meteorite theory. A major predicted effect of the impact simply did not square with the facts: the dinosaurs had not roasted. But that immediately raises a whole series of other questions. What about the idea of acid rain as strong as battery acid that should have destroyed the plants that many dinosaurs lived on and wiped out many sensitive animal species?

Biologist Dave Archibald has made a statistical study of the kinds of animals that became extinct and those that survived. He found that many that should have died out because of acid rain did not. He cites the case of the tree frog, which, like all amphibians, is very susceptible to acid rain. They have problems with their metabolism and finally they cannot breed in the water.

If acid rain had occurred at the end of the Cretaceous, all amphibians should have disappeared. They did not: almost all of them survived, as did all the turtles, the crocodiles, their cousins the alligators and most fishes. This proves that there cannot have been battery acid-strength acid rain at the end of the Cretaceous. The same objection can be made to the so-called impact winter. This states that dust blocked out the sun, plunging the whole world into deep freeze for months or years. But yet again those little frogs got in the way.

David Archibald explains: “Amphibians like all cold blooded creatures have problems dealing with sudden drops in temperature, especially if it lasts for many months or years. If this had happened at the end of the Cretaceous, then many cold-blooded organisms should have become extinct. They did not. This tells us that there could not have been a long term drop in temperature.”

The impact theory is full of holes. The dinosaurs did not freeze to death, nor did they roast and nor did they die from the effects of acid rain. Moreover, none of this is necessary to explain the extinction of the dinosaurs or any other species. Such events can be explained by the classical evolutionary ideas without recourse to exotic external events.

It is quite possible that the dinosaurs died out gradually and for completely different reasons. About ten million years before the KT boundary, the dinosaurs’ environment began to get markedly worse as sea levels fell dramatically. The lush coastal planes on which they depended began to turn arid, destroying their natural habitat. This is quite sufficient to explain the demise of the dinosaurs without having recourse to accidents or external factors.

A few months ago Jason Phipps Morgan and his colleagues at Kiel University in Germany suggested that flood basalts might be accompanied by huge explosions, which they dubbed ‘Verneshots’ because, like the launching gun in Jules Verne’s novel ‘From the Earth to the Moon’, they would be capable of launching things into space.

According to Dr Phipps Morgan a Verneshot would produce almost all of the evidence thought to indicate an asteroid strike. Iridium, which, according to the impact theory, could only come from outer space, would in fact be brought up from deep within the Earth, shocked quartz, buckyballs, spherules, and big impact craters caused by the return to Earth of huge gobbets of material the explosion ejected. Meteorite fragments are more difficult to explain. But since meteorites hit the earth all the time, they could well be “contamination” from smaller impacts.

At the moment, this idea still has the character of a hypothesis. But Dr Phipps Morgan points to a strange, circular gravitational anomaly buried under the lava of the Deccan Traps. This, he speculates, could be the muzzle from which a Verneshot was fired. It would be ironic if the newly discovered Australian crater, which is now being put forward as evidence for the extraterrestrial origins of mass extinctions, turns out to be yet another nail in the impact theory’s coffin.

For a dialectical theory!

The dialectical method seeks to explain natural phenomena as the transformation of quantity into quality: a long period of slow, gradual change is interrupted at a critical point by a sudden change of state, a quantum leap, a phase transition or, to use the language of dialectics, a qualitative leap. This method of analysis was first developed by Hegel two hundred years ago and then placed on a scientific basis by Marx and Engels. But it is only in recent years, thanks to the development of chaos theory and its derivatives that it has begun to be taken seriously by scientists.

However, this wonderfully profound idea was for a long time resisted by scientists. Long ago Newton insisted: “nature does not make leaps”. But this idea, so fundamental to classical physics and mechanics, has been completely overthrown by the advances of science in the course of the last hundred years. Even Darwin, that great innovative genius, was incapable of seeing the existence of sudden leaps and revolutions in nature. He saw evolution as a long, unbroken line of gradual change.

This undialectical standpoint meant that he could not explain phenomena as transcendental as the Cambrian Explosion. He admitted to friends that he could not sleep at night thinking about this problem. To his dying day he insisted that it could not be a sudden change and that new discoveries in the fossil record would show that it was part of a process of gradual evolution.

We now know that there have been many “leaps” along the line of evolution. The extinction of the dinosaurs at the end of the Cretaceous period, 65m years ago, accompanied by that of many other creatures at the same time, is the best known but not the biggest mass extinction event in world history. That distinction belongs to the end of the Permian period, some 250m years ago. The end of the Permian was marked by the extinction of 90 percent of the Earth’s species.

Such events cannot be explained purely in terms of accidents such as meteor impacts. On closer examination they will be seen to be the result of a combination of circumstances that signify that the adaptations of a given species clash with a change in the natural environment that exposes their limitations and makes it increasingly difficult for them to continue.

The existing life forms then enter into a prolonged period of decline. The extinction of species, in that sense, is no more a sudden event than dying in the case of individuals. We begin to die the moment we are born. Trillions of cells die all the time, only to be replaced by new cells, until the organism reaches a point where healthy new cells cease to be reproduced in sufficient numbers to prevent decay and eventually death.

The final event that brings on death may be sudden or even spectacular (a heart attack, a stroke etc.), but it is rooted in the inevitable process that has been prepared over a long period. We see something similar in socio-economic systems. History is rich in examples of socio-economic systems that arise, flourish and expand, only to reach a culminating point where it enters into a terminal decline.

This decline may be prolonged for centuries – like the decline of the Roman Empire – and the decline may be accompanied by periods of apparent recovery. The downward tendency is arrested and even reversed, only to reassert itself with redoubled force. And in the last analysis, whether a given socio-economic system is viable or not, whether it enters into an ascending or descending phase, depends on its ability to develop the productive forces, the real mainspring of all human progress, culture and civilization.

“Everything that exists deserves to perish.” For a hundred and forty million years, the dinosaurs dominated the planet. They were well adapted to the existing conditions and filled all the important niches of the environment. But when the conditions changed, they went into a long decline. Finally, they were so weakened that any external shock could push them over the brink to extinction.

The final shock (which in all probability was caused by volcanic activity rather than a meteor impact) acted as a catalyst. At this point quantity became transformed into quality. Necessity revealed itself through an accident.

The crisis of capitalism

One could certainly draw an interesting analogy between phenomena like the extinction of the dinosaurs and the present situation of capitalism on a world scale. This assertion is not as arbitrary as you might imagine. Recent developments in a derivative of chaos theory known as ubiquity have shown that the same basic laws of motion underlie phenomena of the most diverse kinds.

We can observe the kind of qualitative leaps predicted by dialectics in history (wars, revolutions), in economics (stock exchange crises), in health (heart attacks, nervous breakdowns) and also in evolution, where they are usually characterised by the disappearance of certain species (mass extinctions) and the rise of new species better adapted to live in the changed objective conditions. Although there are some important differences between these phenomena, the underlying laws remain the same. They can even be expressed mathematically.

Therefore, with the necessary reservations, it is possible to establish a broad analogy between the rise and fall of socio-economic systems and the rise and fall of animal species. For a long period after the end of the Second World War, capitalism entered into a phase of expansion characterised by a greater integration of the world economy and a phenomenal development of the productive forces. There was a powerful development of industry, agriculture, science and technology and a colossal upswing of world trade that impelled the whole thing forward.

However, that stage came to an end with the first world recession of 1973-4. The accident that triggered that crisis was a sharp increase in the price of oil, which was itself a result of turmoil and war in the Middle East. Ever since then, capitalism has been struggling – unsuccessfully – to recover the kind of position it enjoyed in the upswing of 1945-73.

Although there was a temporary recovery in the second half of the 1990s, which many economists predicted would be permanent, this has proven to be illusory. Growth has slowed to a trickle in most countries. The growth in the USA has a largely artificial and unstable character; that of China is even more unstable. The world economy is balancing tenuously on a knife edge. Any shock – a sharp rise in US interest rates, a big increase in the price of oil, a worsening of the situation in Iraq – can push it into recession.

After the fall of the Soviet Union, the strategists of Capital were euphoric. They imagined that all their problems were solved and that capitalism would last forever. In the space of just a decade all these dreams have been reduced to ashes. All the contradictions that have been accumulating for decades are coming to the surface: everywhere there is instability, wars, terrorism and crises. These are merely the outward symptoms of a system that is sick unto death.

The capitalist system finds itself in a blind alley. In reality it has exhausted its progressive historical potential and is rotten ripe for overthrow. The constant shocks that are shaking it to the foundations are merely the reflection of this fact. They are rooted in the objective situation. So degenerate has the system become that any serious shock can have far-reaching consequences and threaten its very existence.

There is no future for the human race under the rotten, decaying system of capitalism. Beneath the surface new forces are maturing, which are destined to inherit the earth. But the forces of the old society will not accept their demise with passive resignation. Like any other animal, the capitalists will fight against extinction. But it is a battle that is lost in advance.

Does this mean that capitalism will collapse of its own accord, that we just have to wait with arms folded until that happens? The comparison with the kind of crises we see periodically in evolution is very real, but of course it is only approximate. There are big differences, and the main one is that social evolution, as opposed to natural evolution, demands the conscious intervention of men and women. The subjective factor plays a decisive role.

Men and women make their own history. But they do not make it as entirely free agents. We reflect the prevailing social and economic conditions of the world in which we live. These conditions are increasingly favourable for socialism. The crisis of senile capitalism is daily providing more arguments for its overthrow. More and more people are drawing the conclusion that “things cannot continue like this” and that “we need a fundamental change.”

What is needed in every country is to unite the most advanced and revolutionary elements and carry out systematic work to win over the masses. In that sense the revolutionary tendency performs a role in society analogous to that of a catalyst in chemistry. The tide of history is now flowing strongly in our direction. We must redouble our efforts to prepare for the decisive break in the situation that is inevitable.

Explosive events are being prepared. They will be the equivalent in social and political terms to the tremendous volcanic eruptions at the end of the Crestacious. A fundamental change is being prepared in the consciousness of the masses. Once that takes place, the capitalist system will follow the dinosaurs to where it belongs – the museum of extinct fossils.

18th October 2004.

Postscript:

I have just learned of a new theory, which puts forward a different explanation for the extinction of the dinosaurs. The theory put forward by David Miller and Jonathen Summers of Leeds University focuses on the effects of climate change on the dinosaurs’ reproductive system.

It is known that variations in temperature determine the sex of the offspring of certain species. In humans and other mammals, sex determination is genetically controlled to ensure an approximate 50/50 male to female ratio. But some reptiles and fish use temperature-based methods to determine sex of offspring. They produce roughly 50/50 ratios under ideal conditions but a variation of temperature produces a bias towards males or females, with a skew towards males leading to population decline.

The rising temperature during the Cretaceous period, according to this theory, affected the incubation of dinosaur eggs, favouring the hatching of males over females. This fatal imbalance would be more than sufficient to explain the extinction of the species over a period.

Crocodiles, which are essentially similar to dinosaurs, use temperature sex determination, and Dr Miller argues that this could mean the extinct giant reptiles used the same mechanism. Dr Miller said, “We think that there was an abrupt and prolonged change in the environment and that the dinosaurs could not adapt to it in time to save themselves. A temperature change made them produce too many males and they died out as a result.”

He thinks that temperature control of sex determination may have been the original mechanism for animals. He says: “Birds, which probably descended from dinosaurs over 170 million years ago, may have been early adopters of genetically determined sex and hence immune from the environmental vicissitudes that affected their dinosaur cousins. Nevertheless, temperature based sex determination continues to exist because it may offer an effective means of rapid niche exploitation favouring one sex over the other. But it works best in reasonably stable conditions and that was one luxury that the dinosaurs didn’t have 65 million years ago when they vanished from the fossil record.”

Dr Miller apparently hit upon the idea during research into the evolution of the Y chromosome in humans, which he is carrying out in collaboration with Dr Sherman Silber of St Louis Infertility Centre in the US. Dr Summers brought in mathematical techniques to model the effects of variations in male/female ratio on populations. More complex modelling is planned to explore the effect of temperature change on different sized dinosaurs and eggs.

Dr Miller and his colleagues cannot prove that the suggestion is correct, but they have gone a long way towards showing that it is at least feasible. It is too early to say whether this latest explanation is the correct one, but it sounds quite plausible, and at least it does not depend on any extraterrestrial or accidental causes to explain evolution on earth.

[For more information see: Miller, Summers and Silber, 2004. ‘Environmental versus genetic sex determination: a possible factor in dinosaur extinction?’ Fertility and Sterility, 81, pp 954-964.]