the galactic grim reaper theory

Mass extinctions take a terrible toll on the planet’s biodiversity, wiping out more than half of all life in a breathtakingly short time span. Each one is a puzzle that leaves the scientists studying them guessing what could’ve triggered such devastation and if an extinction that severe could happen again. A recent and popular theory could provide some answers to those questions and unfortunately for us, they’re not very comforting. Earth, it seems, is on a 64 million year cycle during which mass extinctions are much more likely.

Virtually everything in space is in constant motion. Asteroids, moons, planets, stars, and galaxies are all following gravitational tugs. While Earth orbits the Sun, our entire solar system is orbiting around the galactic center and the entire Milky Way is making its way towards the Virgo Cluster, generating a huge shockwave of heated gas and cosmic rays on the side facing the giant swarm of galaxies. And that shockwave may have an effect on our planet. As our solar system goes through its 250 million year orbit, it undulates through a thin layer of stars. Every 64 million years, it rises just above the galactic plane on the side of the Milky Way producing a bow shock and gets hit with cosmic rays streaming from the shockwave.

According to researchers at the University of California at Berkeley, that corresponds with what seems to be a 62 million year cycle for marine diversity and matches up with two of the biggest extinctions of all time; the Ordovician and the Permian. Could a bombardments of cosmic rays play an important role in triggering them when all the other elements align in their favor? If so, how would we prove this effect? Would it be enough to note that three mass extinctions match the predicted cycle if you give or take a million years? Just like with any other theory, the devil is in the details. When we consider how a mass extinction is defined, other possible and far more potent triggers for them and the general history of life on Earth, we find that the theory isn’t as firm as it may seem at first glance.

First, there’s the question of what constitutes a mass extinction. What percentage of life has to die before we can assign the label? During the Permian event, about 90% of all species vanished over several million years. The now famous end of the dinosaurs’ reign thanks to the impact of a massive meteor, unfolded on a scale of a human lifetime and took some 75% of all species on our planet with it. And just to add an interesting twist to it, many biologists believe that we’re in a mass extinction right now. It even has a name; the Holocene Event. Don’t feel like we’re living through a massive dying? Just look at the ever expanding extinction lists and the statistics that the last time Earth had the same rate of extinctions was the end of the Permian. When the event finally plays itself out, possibly millions of years into the future, it will look dramatic according to the fossil record but to us, it will look like business as usual.

And that brings us to our second point. Mass extinctions are caused by confluences of events, not just a single impact or volcanic eruption. In the Permian, the supercontinent of Pangea was under a great deal of stress from forces within the Earth. Fissures could open up, spewing hot, noxious gases and molten rock for millions of years, heating up the planet until the balance of life is ultimately broken and food chains collapse. When the meteor that ended the Cretaceous slammed into what is modern day Mexico, it arrived on a world in which dinosaurs were having a lot of trouble and their lineages were declining. It’s very possible that the Deccan traps, which formed a vast lava bed in India, triggered climate change with which dinosaur evolution simply couldn’t keep up. And today, human pollution plays a very large role in destroying habitats and creating the kind of food chain collapses only volcanoes were thought to be capable of.

But there is evidence of a cosmic killer in one extinction; the Ordovician. It seems that gamma ray bursts destroyed the oceanic food chain by irradiating the topmost layers of water where a countless array of photosynthetic creatures formed the base of the planet’s food chain. While the Ordovician event does match up with the 64 million year cycle, we’re looking at a potential killer a few thousand light years away taking a direct shot at our planet, not a long exposure to high levels of cosmic rays that slowly poison life on Earth. And what about the Devonian event that took its toll on approximately half of all life? Or the Triassic-Jurassic extinction which was probably caused by the final breakup of Pangea and the volcanic turmoil that followed? They’re not matching up with the galactic cycle by tens of millions of years. Three out of five seems to be more like a random coin toss than a statistical match.

We can’t rule out the effect space has on our world. Earth is an open system and it’s very likely that highly sensitive and very fragile plankton floating at the surface of the ocean are dying off when hit with just a little bit too much radiation coming form the Milky Way’s bow shock. There would be less food around and natural selection would trim down oceanic biodiversity as food chains are put under stress and survival by attrition sets in. Deep marine life is very tough and will probably survive just about anything to help repopulate the oceans after we’re back in the galactic plane. However, the increase in radiation set against such powerful forces as pollution and volcanism driven by plate tectonics as well as gamma ray bursts from hypernova just isn’t a persuasive grim reaper capable of nudging life on edge into another mass extinction.