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The Whole Galaxy Shaped the History of Life on Earth

There would not have been catastrophic impacts, and evolution might have been much simpler. And less interesting.


Would humanity survive if the Sun somehow escaped the orbit of the Milky Way and broke free? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

If we (gently) left the Milky Way today, the main difference would be the absence of starry nights. But if it had happened a billion years ago, the evolution of life would have been dramatically different.

Without the Milky Way, we would have had few cometary impacts on the Earth. Comets have stable elliptical orbits, and in the first few hundred million years of the solar system, any comet with an orbit that intersected the inner solar system would crash (or be kicked out by Jupiter) and be wiped out. This would happen before life began.

Remarkably, most cometary impacts occur because of the local tidal force of the Milky Way galaxy. (That is a theory first published by Donald Morris and myself, and now generally accepted.) The next most common cause is gravitational perturbation from passing stars. If we were out of the Milky Way, neither would happen. Without either effect, there would not have been catastrophic impacts, and evolution might have been much simpler. And less interesting.

We now think that cometary impacts played a huge role in evolution. Take the simple case of the dinosaurs. They were powerful and intelligent, and far better suited to survival than the tiny mammals that lived along side. But then came the cometary impact, blocking sunlight, killing the plants that were the foundation of life.

The large animals could not survive. Indeed, neither could most of the smaller ones. Probably 99.99% of all individuals were killed. But the little animals were more abundant, and some of them made it through. The only dinosaurs which survived were indeed the mobile seed-eaters, those that didn’t depend on fresh plants. We now call those survivors the birds. And in a few thousand years, the little creatures spread exponentially (like the famous rabbits in Australia) and repopulated the Earth.

Remarkably, being big helps you in competition with other species. But being small helps you to endure catastrophe.

Evolution is driven by “survival of the fittest”. But what makes for fitness? We once thought it mean competition with other creatures, both peers, and microscopic organisms that are trying to eat us. But the discovery of the role of impacts added something new. To survive for millions of years, species must be fit to survive catastrophe. The small and numerous ones have an advantage. But so too do the intelligent ones. (Good news for humans.) Without the occasional catastrophic interruption of the ecology that took place during the evolution of life, there might not have been sufficient advantage to have a calorie consuming big brain.

The world would have had a different history of life. It is fun to speculate on how it would have been different. Would dinosaurs still be at the top of the food chain? Or maybe it would be something simpler, like worms. Or trilobites.

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