Wednesday, 5 October 2016

A world without mass extinction?

How would life evolve on a planet with few, or no mass extinctions? This is a question posed recently during my lab group's discussion on the role of mass extinctions on the evolution of life on Earth. While the group’s topic moved on, I found myself chewing over this idea. It’s a delicious thought experiment.
We’ve all heard of the term Goldilocks planet: referring to a planet that is not too close nor too far from the sun, but “just right” for life to evolve and thrive. With increasing computation and more powerful equipment, astronomers are searching the skies for signs of other inhabitable worlds. Such worlds are announced by excitable journalists every few months or so; for example the European Southern Observatory found the nearest one in August. NASA has a series of Kepler worlds, illustrated with beautiful concept art on their website.

NASA is searching
What I didn’t know is that there are not only Goldilocks zones around individual stars, but within each star cluster, and within the galaxy itself. Astrobiologists don’t look too near the centre of the galaxy, nor at the edge. They look for planets neither too big nor too small, somewhere not too hot nor too cold. Life develops within a narrow, balanced band on habitability.

On our planet earth, there have been five major mass extinctions, and multiple smaller ones. The causes range from the infamous striking asteroids of doom, to massive gut-spillings of lava from the earth’s interior (which belch monumental volumes of greenhouse gas), sea level changes and mountain building - both of which can so radically alter the atmospheric composition and ocean currents that huge swathes of the planet are changed. Each of these events can lead to extinctions on a massive scale, often with knock-on effects that cause ecosystem collapse or re-arrangement on timescales from many thousands to millions of years.

This image was just so over the top, I couldn't resist it. (Source)
Most extinctions are not caused by external forces (asteroid strikes), but are the result of earth movements. And yet without plate-tectonics, life wouldn’t get very far. First off, the movement of earth’s molten core generates our planetary magnetic field, deflecting dastardly space particles and solar winds, preventing them messing up life’s precious DNA. Without the recycling of nutrients that results from plate-tectonic processes, could there be the complex webs we see on our own interconnected world? There would be a huge dearth of ecological niches. New forms usually emerge to exploit new niches, or those left vacant by others.

So let’s say we have a Goldilocks planet in a Goldilocks zone, in a Goldilocks galaxy, but this one – unlike earth – is never hit by asteroids, nor does it have plate-tectonics. The main causes of mass extinction are absent. What path would life take?

So this brings us to mass extinction again. What role does it play in evolution? Undoubtedly extinction opens up space that is eventually refilled, either by similar organisms, or those that evolve novel ways to exploit that niche. A good example are the marine reptiles of the Mesozoic, which rapidly moved into the oceanic space left vacant after the disastrous end-Permian mass extinction. And when the marine reptiles bit the dust after a the K-Pg extinction… well along came the whales and dolphins. Life has a tendency to converge. If a niche was worth its salt, some critter is going to come along and exploit it.

So the question remains: would life be so diverse if there were no mass extinctions?

Sepkoski’s famous diversity curve seems to show that life has consistently been slammed by extinction, only to diversify to levels still even greater than before in each aftermath. While the tricks played by a patchy rock record mustn’t be overlooked, it appears to be the case that life is more diverse now than ever before. Did extinctions cause this, or would it have happened anyway?
Sepkoski curve, showing the big 5 extinctions, and the elusive Ediacaran/Cambrian changeover (marked as 0).
One of the earliest turnovers in life – and increases in complexity – is seen where the Ediacaran biota gives way to the Cambrian. There has long been a search for a mass extinction to explain this. Did something wipe out most of the soft-bodied microbial mat dwellers, paving the way for Cambrian settlers? Or was the Cambrian “revolution” a takeover by organisms that had hit on more “winning designs*”? The jury is out. But if there was no great calamity responsible for the emergence of Cambrian life, it could support the idea that organisms would inevitably become more complex, rather than it being the result of plate-tectonic – or cosmic – intervention.
The Ediacaran: the age of doormats. (Source)
 Like many people, I think basic life probably springs up all over the galaxy, where the conditions are right. As for it evolving into more complex forms… This is a trickier question, one I cannot answer.
The role of plate-tectonics is, on the other hand, almost indisputable. A planet with no mass extinctions is probably oxymoronic: life is unlikely to exist without plate-tectonics, and most mass extinctions are caused by earth processes that result from plate-tectonics, therefore a planet with life, is naturally a planet with mass extinctions.

Perhaps the mythical planet without extinction would consist of little more than a billion year microbial soup (I think I ate that once while travelling). But then again, life may just... find a way.

*I use design in the colloquial sense. Of course nothing is designed in the natural world, it is selected for, based on advantage.


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