The impact hypothesis - III

The Alvarezes wrote up the results from their tests and sent them, along with their proposed explanation, to Science. Their paper, “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction,” was published in June 1980. An asteroid six miles wide collided with the earth sixty-five million years ago. (The date was later revised to 66 mya.) Exploding on contact, it released energy of more than a million of the most powerful H-bombs ever tested. Debris, including iridium from the pulverized asteroid, spread around the globe. 

Sunlight disappeared and temperatures plunged and a mass extinction ensued. The Alvarezes proposed that the main cause of the K-T mass extinction was not the impact itself or even the immediate aftermath. The truly catastrophic effect of the asteroid was the dust which spread around the globe and shut out the sunlight and blocked photosynthesis in plants. In the intervening decades, this account has been subjected to numerous refinements. 

It generated lots of excitement, much of it beyond the bounds of paleontology. In the context of “hard-core uniformitarianism,” the impact hypothesis was worse than wrong — it couldn’t have happened. A few years later, an informal survey was conducted among paleontologists. A majority thought some sort of cosmic collision might have taken place. But only one in twenty thought it had anything to do with the extinction of the dinosaurs. Among professional paleontologists, the Alvarezes’ idea and in many cases the Alvarezes themselves were reviled.

But evidence for the hypothesis continued to accumulate. First was tiny grains of rock known as “shocked quartz.” Under high magnification, shocked quartz exhibits what look like scratch marks caused by high pressure that deform the crystal structure. Shocked quartz was first noted at nuclear test sites and later found near impact craters. In 1984, grains of shocked quartz were discovered in a layer of clay from the K-T boundary in eastern Montana. 

It occurred to Walter Alvarez that if there had been a giant, impact-induced tsunami, it would have left behind a distinctive "fingerprint" in the sedimentary record. He scanned the records of thousands of sediment cores that had been drilled in the oceans, and found such a "fingerprint" in cores from the Gulf of Mexico (oops! Gulf of America). Finally, a hundred-mile-wide crater was discovered beneath the Yucatán Peninsula buried under half a mile of newer sediment. 

This crater had shown up in gravity surveys taken in the nineteen-fifties by Mexico’s state-run oil company. When Walter located the cores in 1991 and examined them, they were found to contain a layer of glass—rock that had melted, then rapidly cooled at the K-T boundary. To the Alvarez camp, this

was the conclusive proof that they required about there having been an asteroid impact.  It was enough to move many uncommitted scientists to support the impact hypothesis. By this time, Luis Alvarez had died of complications from esophageal cancer. The crater became more widely known, after the nearest town, as the Chicxulub crater.

When the Alvarezes had published their hypothesis, they knew of only three sites where the iridium layer was exposed. In the decades since, dozens more have been located. The confirmation of the impact hypothesis was a challenge to a uniformitarian viewpoint that basically every geologist and paleontologist had been trained in. 

On land, every animal larger than a cat seems to have died out. The event’s most famous victims, the dinosaurs — or, to be more precise, the non-avian dinosaurs — suffered a hundred percent losses. Around two-thirds of the mammalian families living at the end of the Cretaceous disappear at the boundary. Everything (and everyone) alive today is descended from an organism that somehow survived the impact. 

Change one detail, and we can imagine a completely different world. If the asteroid had hit a moment earlier or later, it would have hit deep ocean instead of shallow seas, releasing far less toxic gas, and killing many fewer species. If the asteroid had been delayed by just one minute, it might have missed Earth entirely. An astrophysicist has proposed that tiny oscillations of the sun's orbit flung the asteroid from the distant Oort cloud toward our planet. But for one small vibration in an unfathomably distant reach of deep space, dinosaurs might have survived — and humans might never have existed. 

Natural Selection is often presented as a relentless improvement from worse to better. Richard Dawkins once said that “Nature is a miserly accountant, grudging the pennies, watching the clock, punishing the smallest extravagance." But evolution at times proceeds in an unpredictable fashion. This is obvious when you consider that the evolution of mammals only happened because of an asteroid strike. But we mostly hear about survival of the fittest not survival of the luckiest. 

The impact hypothesis - II

Enter Walter Alvarez. He came from a long line of distinguished scientists. His great-grandfather and grandfather were both noted physicians, and his father, Luis, was a physicist at the University of California-Berkeley. Walter attended graduate school at Princeton and took up geology. In the early 70s, he got a research post at the Lamont-Doherty Earth Observatory. Alvarez decided to try to figure out, on the basis of plate tectonics, how the Italian peninsula had formed.

In this quest, he found himself working in a hill town of Gubbio, about a hundred miles north of Rome, with an Italian geologist who was an expert on foraminifera or “forams” for short. They are the tiny marine creatures that create little calcite shells which drift down to the ocean floor once the animal inside has died. They can only be seen with microscopes. The geologist drew Alvarez's attention to a curious sequence. 

In one centimeter of clay separating two limestone layers, there were no fossils at all. In the older layer that lay below the clay, the forams were much larger than in the younger layer above the clay. The same of distribution of forams above and below the clay layer was present everywhere he looked. What had caused such a change in the forams? How fast did it happen? These were the questions that puzzled Walter. The pursuit of these questions led him to one of the biggest discoveries about one of the most important days in the history of life.

First a brief description of Deep Time. The history of life is divided into three chapters called "eras". The first is called the Paleozoic (“ancient life”), the second the Mesozoic (“middle life”), and the third the Cenozoic (“new life”). Each era comprises several “periods”; the Mesozoic, for example, spans the Triassic, the Jurassic, and the Cretaceous. The next period is the Tertiary (now renamed the Paleogene).

               Geological Time Scale 

The boundary between the Cretaceous and Tertiary layers, where the clay layer is found is called the K-T boundary. K is used as the abbreviation for Cretaceous because C was already taken by an earlier geological period known as the Carboniferous; today, the border is formally known as the Cretaceous-Paleogene, or K-Pg, boundary. It is a line that definitively marks the end-Cretaceous mass extinction everywhere in the world where the right aged rocks are preserved. It happened 66 mya (million years ago). 

                                                             KT (or KPg) boundary 

 Alvarez had been used to believing in uniformitarianism. He had learned that the disappearance of any group of organisms had to be a gradual process, with one species slowly dying out, then another, then a third, and so on. But the sequence in the Gubbio limestone gave him a different picture. The many species of forams in the lower layer seemed to disappear suddenly and all more or less at the same time. He also realized another thing. These forams appeared to vanish right around the point the last of the dinosaurs were known to have disappeared.

In 1977, Alvarez got a job at Berkeley, where his father, Luis, was working. He brought with him to California his samples from Gubbio. While Walter had been studying plate tectonics, Luis was busy winning the Nobel Prize in Physics in 1968. He’d also developed the first linear proton accelerator, invented a new kind of bubble chamber, designed several innovative radar systems, and codiscovered tritium.  In 2007 the American Journal of Physics commented, "Luis Alvarez was one of the most brilliant and productive experimental physicists of the twentieth century."

Luis Alvarez was interested in all sorts of riddles. An example was whether there were treasure-filled chambers inside Egypt’s second-largest pyramid. He would often come up with innovative ideas to approach the problem. When Walter told his father about the puzzling fossil distribution in Gubbio, Luis was fascinated. He came up with the wild idea of clocking the clay using the element iridium which is extremely rare on the surface of the earth. But Luis knew that it was much more common in meteorites. 

On earth, the tiny amounts of iridium come from bits of meteorites that are constantly raining down on the planet. Luis reasoned that the longer it had taken the clay layer to accumulate, the more cosmic dust would have fallen; thus the more iridium it would contain. By this technique he would be able to find out what length of time the clay layer represented. Walter gave him some limestone from above the clay layer, some from below it, and some of the clay itself. 

When the results came from the lab, it was puzzling. The amount of iridium that was present in the layers above and below the clay layer was what was normally present on earth. But the amount of iridium in the clay layer in the middle was 30 times higher. No one knew what to make of this. Was it a weird anomaly, or something more significant? Something very unusual, and very bad, had happened at the K-T boundary. The forams, the clay, the iridium, the dinosaurs, were all signs — but of what?

Two other sites having sediments dated to 66 mya when the Dinosaurs disappeared were found - one in  Denmark and another in New Zealand. They had the same pattern as the ones at Gubbio - a thin clay layer between earlier and later layers. They too showed an iridium “spike” in the clay layer. The Alvarezes knew they were onto something and started thinking up theories that would fit the available data. Finally, after almost a year’s worth of dead ends, they arrived at the impact hypothesis. 

The impact hypothesis - I

More than 99 percent all species that have ever lived on Earth have become extinct. For many people, when they think about extinction, they think about dinosaurs. The dinosaurs were huge terrestrial animals that lived during the period about 240 to 66 million years ago (called the Mesozoic Era). They had rich varieties in body size, shape and way of life. They ruled the Earth for more than 100 million years, till somehow they suddenly vanished on the Earth more than 65 million years ago. 

The mystery of the extinction of the Dinosaurs has been the focus of research and debate for long. Many different theories have been put forth as explanations. Some of the well-known ones include invoking climate change to which the dinosaurs could not adapt; continental drift causing climate change; flipping of earth’s magnetic poles leading to the dramatic changes of natural environment; acid rain leaching away important micronutrients; rodents eating dinosaur eggs as food; etc. 

The most commonly accepted explanation is that a meteor struck the earth 66 million years ago leading to a nuclear winter that led to the extinction of the non-avian dinosaurs (birds are accepted as having evolved from a branch of the dinosaurs). The rock slammed into the Yucatán Peninsula moving at something like forty-five thousand miles per hour. The asteroid blasted into the air more than fifty times its own mass in pulverized rock.

The resulting huge cloud of very hot vapor and debris raced over the North American continent incinerating anything in its path. Owing to the composition of the Yucatán Peninsula, the dust thrown up was rich in sulfur and particularly effective at blocking sunlight. After the initial heat pulse, the world experienced a multi-season “impact winter.” Forests were decimated. Marine ecosystems collapsed. And the non-avian dinosaurs died out. 

The interesting question is: how did scientists find out about a meteor-strike that happened all that long ago? Before that story, one needs to know about a clash between two schools of thought in evolutionary biology: uniformitarianism and catastrophism. 

In the opinion of uniformitarians like Darwin, the emergence and disappearance of species are the outcomes of natural evolution. When there is change in natural environment, the species is no longer able to adapt to the new environment and if there are no other proper places for migration, the population of the species will diminish till it becomes extinct. They believe that the emergence and disappearance of species is the effect of slow natural selection. The uniformitarian view denied sudden or sweeping change of any kind.

On the other hand, catastrophists believe that sudden, short-lived, and violent events lead to the extinction of many organisms. The leading scientific proponent of catastrophism in the early nineteenth century was the French anatomist and paleontologist Georges Cuvier. He believed that the history of life on earth indicated that there had been several of these revolutions, like earthquakes and floods, which he viewed as recurring natural events, amid long intervals of stability. 

The more that was learned about the fossil record, the more difficult it was to explain the sudden disappearance and appearance of large numbers of species, which, according to Uniformitarians, should take millions of years. The Uniformitarians said that maybe the losses shown in the fossil record did constitute a “mass extinction.” But mass extinctions were not to be confused with “catastrophes.” They maintained that the fossil record was incomplete and the missing spans of time would eventually be found. 

In the war of words between the two groups, Uniformitarians held the upper hand for many decades. The feeling between the two groups was so bitter that Uniformitarians described catastrophism as 'evolution by jerks'. In retaliation, Catastrophists described Uniformitarianism as 'evolution by creeps'. Who said academics don’t have a sense of humour?