The conventional and widely-accepted explanation for the extinction of dinosaurs 65 million years ago is that this traumatic development resulted from the collision of a large asteroid with Earth.
There is certainly much evidence in support of the asteroid theory, but there is other evidence that strongly indicates that the asteroid is only part of the explanation. This latter evidence commands much less attention, both in the public domain and within professional circles, than does the hugely popular asteroid theory.
Advocates of the asteroid theory fiercely defend their position against all sceptical argument. However, the facts of what happened 65 million years ago will not be swayed one way or the other by current scientific squabbles.
The asteroid theory was first published in 1980 by Louis and Walter Alvarez, Frank Asaro and Helen V. Mitchell. They had discovered that rocks deposited 65 million years ago in several parts of the world contained high concentrations of the element iridium. Iridium is naturally rare in the earth but is relatively more abundant in meteorites.
Alvarez and colleagues proposed that an asteroid collided with the Earth, exploded and scattered its iridium worldwide, which then settled into the sediments formed at the time.
Palaeontologists (who study fossils in rocks) at first dismissed the asteroid theory, pointing out in particular that no crater had been found on Earth of the size predicted by the asteroid theory.
However, overwhelming evidence soon came to light that a large asteroid had in fact collided with Earth at that time. At more than a hundred sites around the world, a thin layer of clay deposited 65 million years ago was found to contain high concentrations of iridium. This clay layer also contained other geological evidence (for example, glassy spheres) indicating that its deposition coincided with a hugely violent impact with the Earth.
A clinching piece of evidence was announced in 1991 with the discovery of an immense crater (the Chicxulub crater) under the Yucatan peninsula in Mexico. This crater was formed 65 million years ago and is of just the size predicted by the asteroid theory.
The asteroid theory of extinction goes something as follows. An asteroid about the size of the is land of Manhattan plunged through 300 feet of seawater and smashed into the sea-floor at what is now Yucatan. The enormous impact ejected a cone of material thousands of miles upwards through the atmosphere.
Much of this was vaporised, but it also contained bigger blocks of rock. Huge tidal waves from the impact swamped the North American coast. Some of the tower of vaporised material cooled quickly and fell back to earth as glassy spheres. Other, finer material remained aloft in the atmosphere, blotting out the light of the sun.
Horrendous wild fires raged across the globe. High levels of carbon dioxide, sulphur dioxide and soot polluted the air for years, killing plants and thereby the food chains that depend on them and eventually destroying most higher life forms on Earth.
The asteroid theory exerts a powerful effect on the imagination, but there are several important questions it does not answer.
For example, the material released into the atmosphere by the asteroid collision (carbon dioxide, sulphur dioxide, dust and soot) would have caused high levels of acid rain and an initial drop in temperature followed by an extended period of global warming.
The acid rain and the initial cold period should have first killed amphibians, fishes, and cold-blooded vertebrates. The dinosaurs were adapted to climates ranging from the tropical to the arctic and should have survived. Instead, the dinosaurs disappeared but amphibians, crocodiles and turtles survived.
The story in the worldwide fossil record is not fully explained by the simple asteroid theory. While the fossil record for North America, next door to the asteroid impact, shows the rapid disappearance of dinosaurs 65 million years ago, a somewhat different picture emerges from the fossil record elsewhere in the world.
The worldwide fossil record shows that the rate of extinction increased several million years before the event recorded in the iridium clay layer, and, in fact, the rate of extinction accelerated in the 300,000-year period before the iridium layer was created. The further away from North America the fossil evidence is examined ,the more gradual and extended the extinction rate appears.
AN alternative theory to explain the mass extinction of life 65 million years ago was proposed in 1981 by the palaeontologist Dewey M. McLean.
This theory proposes that the extinctions resulted from worldwide volcanic activity that choked the air with carbon dioxide, sulphur dioxide and volcanic ash, causing darkness, global warming and acid rain. There is, of course, evidence of intense volcanic activity about 65 million years ago.
Rather than depending on the asteroid theory to explain everything, what may have happened is something along the following lines. As the Cretaceous period (lasting from 135 to 65 million years ago) ended, world sea levels dropped significantly, damaging many environments and killing off many species, including some dinosaurs. This was followed by intense volcanic activity that stressed the world environment for half a million years and killed off many more species.
When the Chicxulub asteroid struck, it killed off much of the life that remained. Nevertheless, some prominent Cretaceous species survived for thousands for years but were eventually replaced by new species filling in ecological niches emptied by the dropping seas, volcanic activity and the Chicxulub asteroid. Among these new successful species were the mammals, our ancestors.
The asteroid theory is very entertainingly described by Walter Alvarez in T.rex and the Crater of Doom (Penguin, 1998). The alternative broader theory is outlined by Vincent Courtillot in Evolutionary Catastrophes: The Science of Mass Extinction (Cambridge University Press, 1999). The subject is also reviewed by Robert Zimmerman in the July/August (1999) issue of The Sciences.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC.