Is Earth Special?

Reconsidering the uniqueness of life on our planet

By | March 1, 2014

BASIC BOOKS, APRIL 2014Earth is almost the perfect place for life as we know it to begin, prosper, and diversify. That makes it a very odd world. Most planets are too hot or too cold; too wet or too dry; too small or too big; or just plain wrong for life in any one of a hundred other ways.

One life-friendly property of Earth particularly intrigues me and compelled me to write my latest book, Lucky Planet—it has had a remarkably stable climatic history. Global average temperature is controlled by just three things: the brightness of its sun; the fraction of sunlight a planet reflects rather than absorbs; and the concentration of greenhouse gases in its atmosphere. This has been known for more than a century, but we have only recently realized that astronomical, geological, and biological processes have massively altered all three factors over the 4 billion years that life has existed on Earth. In that time our Sun has warmed 40 percent as it has aged; the Earth’s reflectivity has altered as clouds, ice caps, and continents have changed. And thanks largely to biology, our atmosphere’s composition is now utterly different from that of 2 billion years ago. These changes could have produced surface temperatures varying by hundreds of degrees Centigrade—making complex life impossible—but, somehow, these multifarious influences counteracted each other to produce a climate that has generally only fluctuated by tens of degrees Centigrade.

There are three scientifically respectable explanations for our 4 billion years of life-friendly weather. Firstly, it could be a fundamental principle that biogeochemical processes on inhabited worlds tend to stabilize climates. Thus, we shouldn’t be surprised by Earth’s suitability for life because the physical, chemical, and biological laws of the Universe guarantee the existence of many such worlds. Alternatively, it may be that life is extraordinarily adaptable and will thrive under a wide range of conditions. Again, we shouldn’t be surprised that Earth fits life because, in fact, life has adapted to fit Earth. Finally, perhaps planets suitable for complex organisms occur only rarely and purely by chance. But even then we shouldn’t be surprised that we inhabit one of the few lucky worlds. Intelligent observers can only arise on planets where conditions allow complex life, even if such worlds are so unusual that we’d need to search a billion galaxies to find another. The tautology that we must inhabit a habitable world, even if such planets are extraordinarily peculiar, is an extreme example of the scientifically common problem of observational bias. This anthropic selection effect, as it is known, is a central theme in Lucky Planet.

In making the case that our climate is anthropically selected, Lucky Planet takes the reader on a journey through the 4.5-billion-year history of our planet, global warming, life on Mars, and the surprisingly biology-friendly nature of our Universe. In addition, it shows how my own research on the climatic influences that drive ice ages has led me to view good fortune as an important component of Earth’s biological story. The book also discusses how recent discoveries of planets orbiting other stars will allow my ideas to be tested in the near future. At the end of this tour of cosmology, geology, climatology, and biology, the book comes to a simple conclusion: Earth is a rare, beautiful, and very special place—one of the luckiest planets in the Universe.

David Waltham is an astrobiologist, geophysicist, and former head of the department of Earth Sciences at Royal Holloway College, University of London. You can read an excerpt of Lucky Planet.

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Avatar of: Michael Tigges

Michael Tigges

Posts: 8

March 14, 2014

I'm a engineer.   I don't "believe in" luck or chance.  I want hard facts and plausible hypothesis.  I don't accept a theory on the anthropormorphic 'luckiness' of Planet Earth.   Show me the Math, show me the Physics, show me the Biological pathways.  If all that fails, if all that approach can do is point to "luck" or "chance", then I will open my Bible.

Avatar of: Paul Stein

Paul Stein

Posts: 237

March 14, 2014

So, there is something to that Gaia Hypothesis.

Avatar of: Roy Niles

Roy Niles

Posts: 112

Replied to a comment from Michael Tigges made on March 14, 2014

March 14, 2014

There are no plausible hypotheses that don't consider luck and chance.  None of your biological pathways would have found a reason to evolve without chance and the luck to take advantage of it.

Avatar of: Stuart21


Posts: 14

March 25, 2014


March 25, 2014

We've been lucky but maybe only up till the present day.

Peter Ward's Medea Hypothesis:

... the  Medea Hypothesis in which complex life, instead of being in symbiotic harmony with the environment, is actually a horrible nuisance. In this hypothesis, the planet and microbial life have worked together multiple times to trigger mass extinction events that have almost succeeded in returning the Earth to its microbe--dominant state. In other words, Mother Earth might be Microbe Earth and she might be trying to kill her kids.

In addition the gradual sequestering of CO2 [by natural processes] may diminish and finally end the production of foodstuffs by photosynthesis.

Avatar of: EvMedDr


Posts: 14

March 25, 2014

Professor Waltham is sending his audience a mixed message. On the one hand he's suggesting that we humans lucked out by inhabiting the Earth. On the other he tangentially alludes to the fact that we have adapted to Earth's environment- so is it luck or evolutionary adaptation? When biology is seen as a loose association of parts it would seem like luck. But when you drill down to the cellular origins of complex physiology, you see how and why form and function have evolved from unicellular to multicellular life. Once we understand the 'history' of life as a series of cellular-molecular interactive steps, we'll be better equipped to predict where in the Universe we can survive and thrive- it doesn't have to be a an exact replica of Mother Earth; but how similar is close enough? We have evolved to adapt to change, but just how plastic are we as a species? The answer to this question will put us in better stead both here on Earth and elsewhere in the Universe.

Avatar of: mightythor


Posts: 83

March 25, 2014

"Always remember that you're unique -- just like everybody else." -Hallmark

Avatar of: Tony S

Tony S

Posts: 5

March 25, 2014

I've always thought life tends to create for itself an executive species, some creature that will look out for the interests of the biosphere. We humans are on probation. In my imaginings I think it likely the biosphere would be willing to start over again, from scratch, if the climate starts to become inhospitable due to our mismanagement.

Out of your three explanations for the earth's historic climate stability, I intuitively favor your first, but with likely assists from the other two. 


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