The Context Of Large Early Collisions

It seems likely that life emerged well before 3.5 billion years ago, during the most collisionally tumultuous time in Earth’s history. A related question is whether life emerged as a localized phenomenon, and how long it took to spread out to the four corners of the planet. Or, perhaps, life originated concomitantly at multiple locations. This bears important consequences for subsequent evolution. In a rapidly evolving young planet, punctuated by massive collisions, a small oasis of life would have been very fragile. This could imply that life had multiple consecutive starts. Alternatively, life could have emerged and spread rapidly all around the globe, in which case localized catastrophes would have constituted only minor threats. Gene sequencing of living organisms has provided a means to find genetic patterns that can be traced back in time to a common ancestor to all living beings. These molecules can be harvested by microorganisms to gain energy to perform vital functions. Life developed for several billion years before the emergence of the first multicellular organisms, capable of exploiting the chemical resources of the surrounding world through a complex progression of metabolic advances. While the details of timing for the move to life on land are uncertain, it is commonly accepted that life originated in watery environments.

We Can Work It Out

So researchers need to consider the environmental consequences of these events, and for this, we could gain some insights by studying more recent impacts. This number is destined to increase as more older rocks are being scrutinized for the subtle signs of ancient collisions. What were the environmental consequences of these catastrophes? The environmental consequences of early collisions are not easily decoded, and not just because of the incompleteness of the geological record. But some insights can be gained by looking at relatively recent events. The largest collision for which we have a direct eyewitness account is the Tunguska event. On June 30, 1908 at about 7:14 a.m. The meteoroid traveled at a speed of about 25 km/s, from north to south. Modern computer models estimate that the meteoroid was about 50 m in diameter. The explosion generated a rapidly expanding shockwave that was strong enough to flatten about 2000 km2 of forest. It is estimated that approximately 1000 humans saw the event from various distances, of which about 50 humans were inside a radius of 120 km from the epicenter. What they described was an apocalyptic experience. The meteoroid produced a bright trail and when it exploded became brighter than the Sun, unbearable to view.

Don't You Want To Be There?

The explosion occurred with a peal of thunder. Within 50 km from the epicenter, several people were knocked down, the covers of their tents blown away. The region close to the epicenter was pasture for reindeer, and hundreds of the animals disappeared, the rest scattered around. The wave of heat from the explosion was intense, and some witnessed the Taiga set ablaze. It is estimated that at least three people died as a result of the event. We can only imagine with horror what would have happened if the meteoroid had struck a major city. The Tunguska event tells us that even small cosmic collisions could produce significant local damage, but it fades into insignificance when compared to the more powerful collisions that the Earth has borne. Perhaps the best studied collision of all is the Chicxulub event, which has been associated with a mass extinction that killed off 75 percent of all living species about 66 million years ago. But what, precisely, was the series of events leading to the mass extinction? Lacking eyewitness accounts, we must rely on circumstantial evidence and our ability as scientific detectives to reconstruct events that took place a long time ago. The day of doom started with an asteroid some 5–10 km in diameter taking aim at the Earth. The asteroid suffered only minor erosion while penetrating the atmosphere and hit the surface hard, delivering an energy of 100–200 times that of Tunguska. Within minutes, a staggering 100,000 km3 of rocks were excavated and scattered, leaving behind a crater about 150 km wide.

Running to Stand Still

Traces of this event have been identified in about 350 localities worldwide. It is conceivable that these processes were the main triggers of the decimation of fragile organisms, such as marine phytoplankton. On land, low light and disruption of large forests would have suppressed photosynthesis, the most important biochemical process for life on Earth. This, in turn, could have introduced starvation higher up in the food chain, both in the ocean and on land. At the top of the food chain, dinosaurs did not stand a chance. Scientists debate the actual chain of events leading to the mass extinction, and some researchers do not agree that Chicxulub was the trigger of the K–Pg mass extinction. The bulk of the magma came out in multiple eruption phases, over a period of about one million years. Precise determination of the ages of various eruptions has shown them to be close in time to the K–Pg extinction, leading to the idea that degassing of carbon and sulfur species from solidifying lava could have triggered a global climatic change, in ways similar to the vapors produced by the Chicxulub impact. Of five large mass extinctions in the past 500 million years, during which most evolution of complex life took place, only this most recent example has been connected with a cosmic catastrophe.10 There is no evidence of other collisions comparable in magnitude with Chicxulub in this time frame. Certainly, smaller impacts occurred, but they were not enough to induce the scale of global environmental perturbations required to trigger a large mass extinction. At least two other mass extinctions have been ascribed to episodes of massive volcanism, including the end Permian event, the biggest of all. More generally, a dozen minor episodes of extinction have been found, whose triggers are uncertain.11 Whatever the precise concatenation of events leading to the K–Pg extinction, it is undeniable that the evolution of the biosphere in the past 66 million years, what geologists call the Cenozoic Era, has been largely shaped by the Chicxulub event. It is sobering to realize that it is entirely possible humans would not have emerged if mammals had been kept in check by fierce and massive dinosaurs, which had ruled for more than 150 million years prior to their sudden demise.