Events in space may have changed course of evolution
We think of outer space as distant and unreachable, but in fact events out in the cosmos may have helped and hindered the evolution of life on Earth
Philip Ball
We are here by the skin of our teeth. Evolution could well have turned out differently, and the fact that it did not may well be down to freak, chance events. Life on Earth has faced a string of accidents, weird situations and outright catastrophes, from sudden ice ages to collisions with asteroids and it is how life responded to these contingencies that ultimately led to us.
If that is so, we can only understand the story of life by taking the broadest possible view. Organisms are shaped by their environments, and those environments are shaped in turn by huge geological forces like volcanoes and ice sheets, and by the shifting climate. But we should cast the net even wider. What if these great forces were influenced by even greater forces from the wider Universe? Might cosmic events in our Solar System and even our galaxy have also played roles? Do we literally have to thank our stars that we are here?
The most well-known example of an evolutionary shift caused by astronomical events is the hypothesis that the dinosaurs were driven to extinction by a gigantic meteorite impact 66 million years ago. This was proposed in 1980 by physicist Luis Alvarez, his geologist son Walter, and their coworkers.
The researchers discovered that sedimentary rocks, laid down all over the world at the time of the extinction, contain large amounts of a rare element called iridium. The team suggested that the iridium might have come from the dusty debris of a meteorite that smashed into the Earth. Iridium is more abundant in asteroids, the most likely source of such a meteorite, than it is on Earth. Quite how such an impact might have killed off the dinosaurs remains a matter of debate, but there are many possibilities. The energy released could have triggered global wildfires.
The researchers estimated that, to deliver the required amount of iridium, the meteorite would have to be about 6 miles (10km) across. The impact of such a monster would have released millions of times more energy than a hydrogen bomb. Whats more, the dust and debris thrown up into the air by the blast could have blocked sunlight and sent temperatures plunging for several years after. In 1991 the impact hypothesis got a boost when scientists found an impact crater more than 100 miles wide at Chicxulub on Mexicos Yucatan peninsula. Its geological age coincided precisely with the extinction.
How much the impact drove the dinosaurs demise is not clear; there is evidence that they were already on the wane. Still, there is good reason to expect that such a dramatic event would leave some mark on evolutionary history. The discovery helped to prompt concerns about potentially devastating meteorite impacts today.
Besides, a meteorite impact is not the only explanation for the extinctions 66 million years ago. Tokuhiro Nimura is a researcher at the Japan Spaceguard Association, which was formed to monitor near-Earth objects that might strike the planet. In March 2016, Nimura and his coworkers suggested that the extinctions, global cooling and iridium layer might have been caused by the Solar System passing through a molecular cloud: one of the great clouds of gas and dust in space from which stars form. As dust accumulated in the atmosphere, it would have formed a haze that reflected sunlight and cooled the planet.
The basic idea goes back to a 1975 suggestion by British astronomer William McCrea. He thought that if Earth passed through an interstellar dust lane it could cause an ice age. At the time, astronomers Mitchell Begelman and Martin Rees pointed out that such dust might instead affect the way particles streaming from the Sun impinge on the Earths atmosphere and expose the planet to high doses of radiation, causing extinctions as well as climate changes.
Nimura has now resurrected McCreas idea, arguing that the Chicxulub impact does not seem to have been catastrophic enough to account for the extinctions at the end of the Cretaceous.
However, for now this is mostly speculation. The idea strikes me as highly interesting and plausible, but as yet undeveloped and without clear supporting evidence, says astronomer Martin Beech of Campion College at the University of Regina in Saskatchewan, Canada.
The event 66 million years ago is just one of several known mass extinctions, in which many species worldwide seem to have died out suddenly. The biggest was at the end of the Permian period 252 million years ago, when no less than 96% of all life on Earth seems to have died out. All life today is descended from the surviving 4%, so it is easy to see that evolutionary history would have been very different if this extinction had not happened. When species die off, those that survive get opportunities to expand and diversify that they would not otherwise have had.
Palaeontologists have long debated what causes these mass extinctions. It is possible that, just like smaller-scale population crashes, they could be an inherent part of the way ecosystems work. Because all life is interdependent, a small shift in one population might occasionally create a domino effect that sends shockwaves through the entire system. But it is more likely that at least some mass extinctions are caused by influences outside the living world.
One such mass extinction occurred at the end of the Triassic period. About half of all species on Earth disappeared. The event might have been triggered by increases in volcanic activity, perhaps producing changes in climate, but it might also have been caused by a meteorite impact.
Such catastrophic collisions might not be the result of sheer chance, of stray asteroids or comets blundering into Earth. Instead, cosmic circumstances could systematically cause such objects to come close to our world. The most well-known of these ideas is that our Sun has a dim companion star, which is so far away that it has never been observed directly. This star, dubbed Nemesis or the Death Star, might periodically pull lumps of icy rock from the fringes of the Solar System and send them careening through our neighbourhood. This idea was proposed in 1984 by two teams of astronomers: Daniel Whitmire and Albert Jackson, and Marc Davis, Richard Muller and Piet Hut. They were all prompted by the apparent discovery earlier that year that mass extinctions have happened at regular intervals, about 26 million years apart, over the past 500 million years.
The idea is that the gravitational pull of Nemesis, circling the Sun in an orbit about 1.5 light years away, would disturb the Oort cloud: a host of icy objects that lie beyond the orbit of Pluto at distances of about 0.8 to 3 light years, and which are only loosely bound by the Suns gravity. The Oort cloud is the source of long-period comets, which return to the inner Solar System every few hundred years or more. Nemesis would be a tiny star: perhaps a red dwarf, or even a brown dwarf not much bigger than a giant planet like Jupiter. So it is not altogether surprising that it has never been spotted. It would be hard to see at such a distance, even with the most powerful telescopes. But that is not the only problem with the Nemesis theory.
In a study published in 2010, astrophysicist Adrian Melott of the University of Kansas and palaeontologist Richard Bambach of the Smithsonian Institution in Washington DC re-examined the fossil record using the latest data. They confirmed that mass extinctions recur every 27 million years. However, they say this pattern is actually too regular to fit with the Nemesis idea. Such a distant dwarf star would inevitably be disturbed by other nearby stars, producing a more irregular influx of comets.
Instead, maybe the waves of mass extinction are caused, not by a companion star, but by another planet.
In 1985, Whitmire and his colleague John Matese suggested that there might be a relatively small rocky planet, around five times the mass of the Earth, orbiting in the Solar System far beyond Neptune. This planet might pull comets, not from the Oort cloud, but from the much nearer Kuiper belt. This is another disk of icy rocks on the edge of the solar system, of which Pluto and its moon Charon are now recognized as members. Whitmire and Matese called their hypothetical object Planet X. The Washington Post
