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Special life-creating things about the Earth

by Roger Bourke White Jr., copyright Aug 2002


Why do we seem to be alone? Why does Earth seem so special for life?

There should be millions of worlds in our Milky Way Galaxy capable of supporting hydrocarbon-protein-lipid-based life such as ours, and billions of worlds that could support other life systems that we are not yet aware of.

There should be, but if there are not, why not? If we are one of the first of very thinly scattered life forms, are there good scientific reasons why this could be?

Here are some factors that may be important to the spontaneous creation of complex life forms, and if they are, they would dramatically reduce the number of worlds that would spontaneously produce complex multicellular life.

The role of the sun

The Earth is roughly 4.5 billion years old. Assuming Mother Nature hasn't dallied in her creation and development of Homo Sapiens, most of those years were necessary, and it was necessary that for all those billions of years, there be a constant energy flow that complex molecules could take advantage of. On Earth, that energy flow is sunshine. The role of the sun in creating complex life on Earth is that of providing a constant, unchanging energy source to the Earth's surface for billions of years.

This is important because solar energy is not a typical energy source in the universe. Most energy sources decay exponentially in intensity with time. Radioactivity and cooling rock, two of the most common energy sources in the galaxy, are typical energy sources. These produce a big energy flow early on, but the flow is constantly declining with time -- the quantity and quality of the energy changes with time. Constantly declining sources are hard to create life around because the climate they produce is constantly changing. Complex molecules evolving and adapting to one energy level will be mismatched to the environment as the energy flow changes, and the life creating process will have to start again. For example, life molecules that thrive in very hot rocks will stagnate as the rock cools; those that thrive in medium-hot rocks will stagnate as the rock continues to cool, and so on.

Solar energy is also special because it can't be consumed by its primary users. Plants do not eat the sun's hydrogen, instead they eat the energy that radiates from the hydrogen's fusion process. Plants eat sunshine, they don't turn hydrogen into helium. In contrast, a bacteria that oxidizes hydrogen sulfide for energy consumes the hydrogen sulfide. This bacteria will try to grow exponentially until all the hydrogen sulfide in the place it lives in is consumed. It doesn't matter much what the size of the energy-giving reservoir is, an exponentially growing life form will consume it all in an eyeblink on the geologic or evolutionary time scale. Hydrogen sulfide eaters that survive to this day do so by eating the trickle of new hydrogen sulfide produced by geologic and biologic processes active today. Their biomass is tiny because they are forced to live on just a trickle of new energy.

So, having a long-lived sun shine on a planet may be a vital characteristic for creating life. Planets that are warmed only by their own internal heat won’t give life the same kind of opportunity. This opportunity is created by a monotonous energy source, something I cover in this next section.

Monotonous Energy

Solar energy is an example of something I call monotonous energy -- it's the same day after day. The most common monotonous energy sources in the universe are those associated with a phase change. When water boils in an open container at sea level, the steam produced is always 100 degrees C in temperature.

Phase change is a way of transforming a constantly declining energy source into a constant energy source. A block of uranium boiling water could produce 100 degrees C temperature for billions of years. At first there would be huge amounts of 100 degree steam, in the end just a few bubbles, but all the steam for billions of years would be the same temperature -- it would be a monotonous energy flow.

A star is a phase change in progress. Matter is changing from tenuous hydrogen gas into a condensed form -- white dwarf, neutron star or black hole matter. As this happens, a monotonous stream of solar radiation is produced.

If life of Homo Sapiens complexity requires billions of years of evolution, how many other kinds of phase changes are there in the galaxy that can be sustained for billions of years?

The role of the Earth's core

The Earth has a core that is liquid iron on the outside and solid iron on the inside. The core is freezing slowly. Guess what? It's another phase change! This means that the surface of the inner core will be held a constant temperature. This could be why Earth has plate tectonics that have lasted for billions of years and few other solar system bodies have them at all. Venus, Earth's twin planet in size and location, does not seem to have experienced plate tectonics. It could be that Venus's core is either completely molten or completely solid. And as a result, it can't sustain a steady energy flow from the core, and that steady flow is necessary to sustain plate tectonics.

Why are plate tectonics important? Two reasons come to mind:

o First, plate tectonics may be why the Earth’s atmospheric CO2 (carbon dioxide) level is so low compared to Venus. The cycling of calcium-rich rock to and from the surface may have allowed a whole lot of gaseous CO2 to be transformed into calcium carbonate rock.

o Second, the moving continents have produced slow, random changes in the climate. Plate tectonics has added variety to the climate, and the variety has been within the life sustaining range. This variety has accelerated the evolutionary engine. Climate changes bring extinctions, and extinctions lead to adaptive radiations, and the new species that radiate are more efficient and advanced than the old.

Plate tectonics has also brought about rock recycling, through subduction and volcanism. This recycling means that elements important to life, such as carbon and oxygen, are constantly being refreshed, and that some important trace elements, such as heavy metals, are getting concentrated in veins, which are now important to human civilization.

The Earth's core, by driving billions of years of plate tectonics, may have thinned and stabilized the atmosphere and accelerated evolution. If the evolution we have witnessed in 4.5 billion years on Earth takes 10 billion years on a world with an all solid or all liquid core and a rigid crust, then plate tectonics have played a decisive role in creating complex life on Earth and making us an early evolver in the galaxy.

The role of the moon

How many planets have a large satellite orbiting them? In the solar system, Earth does. Some asteroids and some dwarf planets, such as Pluto, do as well.

Does having an orbiting near-twin make a difference for life? This has been a hard one, which I have wrestled with for years. My hunch has always been that it could, but the most obvious phenomenon of a twin -- large tides in oceans -- can't be the cause of the difference. With no moon, Earth would still experience solar tides, which are roughly a quarter of the moon's in strength. Tides are not a difference for life between twin and solo planets.

Then I read an article that did point out a significant difference: The moon stabilizes Earth's tilt perturbations – how much Earth tilts changes with time. Mars, which has no big moon, can randomly change in tilt over millions of years between roughly 0 and 80 degrees. Tilt has a big effect on climate. When a planet is nearly upright -- zero tilt -- it is cooler and has almost no seasonal variation. Mars's tilt wanders randomly over millions of years as the tilt trades energy with Mars's other orbital features. The Earth's tilt also wanders, but not the way Mars's does. Earth's tilt changes in 20,000 year cycles, and only from roughly 20 to 25 degrees. The moon is responsible for this difference. Twins that stabilize tilt perturbations can be significant to life.

Tilt produces changes in climate that are more extreme than those of shifting continents. Solo world climates may vary too much for complex life to establish.

The Mitochondria accident

As life became established on Earth, it changed the atmosphere. Two crises had to be solved for life to thrive on Earth on a large scale, and large amounts of life help life evolve rapidly.

The first of these was the “carbon crisis”. Photosynthesis was allowing plants to consume carbon dioxide at an exponentially growing rate. If something wasn't done, the world's air would be leached of carbon dioxide, and like today's sulfur bacteria, photosynthetic life would continue only around rare places where geologic processes, such as hot springs, were releasing carbon dioxide to the sunlit surface. If the carbon crisis were not solved, the Earth's surface would be covered with cellulose-like compounds that didn't rot, and the air would be mostly oxygen. On land, fire would do the work animals do today, and life would thrive where fire brought fresh carbon dioxide and rid the surface of carbohydrate. In the sea, carbohydrate would produce proto-coal beds and "sink" carbon out of the biosphere.

The carbon crisis was fairly easily solved: Some early organisms became animals and fungus -- both reverse the plant's carbon capturing process by eating plants and then respiring to release CO2 to the air. This problem was easily solved, but solving it was very important. It allowed life to become as common as water, instead of as common as, say, gold.

The second crisis was the “oxygen crisis”. This one was not so easy to solve. Oxygen gas is a poison to most of life's chemical processes. Some early organisms that survived where oxygen was getting thicker learned how to build barriers to oxygen, and some survived by avoiding oxygen -- they lived in places where there is little free oxygen floating around. Their successors are with us today in the form of anaerobic bacteria and other "extremophiles" who stay away from air. Those that stayed in the oxygen evolved better cell walls and metabolic pathways to detoxify the oxygen that slipped through the walls. A straight forward way to detoxify oxygen is to combine it with a carbohydrate of some sort and create carbon dioxide and water.

Solving the oxygen crisis was a necessary step in creating the carbon cycle that allowed carbon, hydrogen, and oxygen to cycle through Earth's metabolic systems in massive quantities.

A few of those early organisms learned a bonus trick: They learned how to make adenosine triphosphate (ATP) as they reacted the oxygen with carbohydrates. ATP is by far a modern cell's most useful energy source, so this was a valuable trick indeed. One kind of organism that learned this trick early on was the ancestor of today's cell structure called the mitochondria.

Today the mitochondria structures in cells are the powerhouses of modern complex cells. And they are essential, not something optional: Nerve, muscle, and organ cells are not possible without mitochondria -- alternative metabolic processes can't provide the concentrated energy needed to run these specialized cells fast enough for them to be useful.

This trick of creating ATP while detoxifying oxygen was so useful that it essentially split early life into those who learned it and those who didn't. Those forms of life that didn't learn the trick remained very small and developed very little internal cell structure. Today these kinds of cells are called prokaryotes and they are still with us living in places that eukaryotes find too hostile. (bacteria are prokaryotes and human cells are eukaryotes) Prokaryotes are both very primitive and very diverse in the metabolic pathways that they have discovered how to use. Most of the extremophiles are prokaryotes. But what all these alternate pathways have in common is that they are feeble power sources compared with oxygen respiration of carbohydrate, and they are feeble carbohydrate builders compared to photosynthesis. In a word, they are slow.

Those forms of life which did learn the ATP-from-oxygen trick developed into eukaryotes. Eukaryotes are much bigger cells, and their insides are much more formally organized. One of the cellular structures eukaryotes have that prokaryotes don't is mitochondria (another is a nucleus).

Mitochondria lived billions of years ago as separate organisms from the proto-eukaryote cells, but at some point in that early era, some mitochondria became symbiotes with some of the proto-eukaryote cells. The combination thrived, and the symbiosis became so close that the mitochondria are no longer separate entities, they are now an integral and vital part of every eukaryote cell.

Evolving oxygen detoxifiers may be a fairly easy evolutionary step; evolving a detoxifier that can produce ATP may be tougher; getting an ATP synthesizer inside a cell using DNA and a nucleus may be next to impossible.

In other words, the leap from prokaryote-style cells to eukaryote-style cells may be very difficult to evolve.

If it's difficult to evolve a nucleus for DNA and detoxifiers that create ATP in the same organism, then the “mitochondria accident” -- the symbiosis of mitochondria and a nucleus in the same cell -- is something that would distinguish evolution on Earth from average evolution in the galaxy. Without the mitochondria accident, life will evolve on a world, and it can become prolific, but it will remain much like prokaryote cells on Earth (primitive single-cell algae and bacteria) because it can't generate enough surplus energy to sustain being multicellular. Such a world will be covered with all sorts of bacteria-like slimes, but nothing more.

The uncommon symbiosis of mitochondria and cell nucleus may have put Earth millions or billions of years ahead of the pack in developing complex cells.

Complex life versus simple life

We are complex, multicellular life and we are on the verge of reaching the stars. If as we continue to explore, we continue to find that we are "first", are there some scientific reasons why we might be so lucky? It could be that worlds truly suitable for evolving complex life are rarer than we now speculate.

It could be that the galaxy is filled with simple life. It could be that the big evolutionary barriers are not between barren pre-life and life, but between simple life and complex life. As we explore the planets around other stars, we could find hundreds or thousands of worlds that evolve cyanobacteria (primitive algae) and bacteria equivalents, but only one or two that go further.

Thousands of worlds covered with simple life would be great news for colonizers and humanity spreaders. These worlds would have everything complex life needs; they just haven't had time to evolve it. These worlds should be easily colonizable, with oxygen-rich atmospheres and lots of organic gunk lying around. If there have been no plate tectonics on these worlds, soils will be leached and minerals may be hard to find, but those will be trivial problems compared to creating an oxygen atmosphere and a climate suitable for carbohydrate-protein-lipid life in the first place.

Life's Magnitude

The life that we have on Earth may be distinctive in the sheer magnitude of the material and energy cycling through the biosphere.

Consider what life on Earth would be like without a steady supply of sunshine. Recent theories of life origin propose that life began around the thermal vents that dot the mid-ocean ridges at the bottom of our seas. If that is so, then life can begin on dark planets that have thermal springs, but are otherwise hostile to life as we know it.

What will life be like on planets that don’t develop a massive biosphere?

It will remain a tiny phenomenon, associated with rare, exotic environments. Finding life on such a world would be like finding gold or diamonds on ours: A rare event that happens after much prospecting. So little biomass would be created on a dark planet that life forms would evolve slowly and remain primitive for a long time.

Widespread life that covers a world thickly may not be life's average condition on the life-sustaining worlds of our galaxy. On many worlds, life may be an exotic, and finding it may be more akin to finding gold on Earth than simply finding the nearest tropical tidal pool bathed in sunshine.

Star-filled skies

(2015 update) Humanity has always been fascinated with the star-filled night sky. This may be yet another distinctiveness of the human situation. Consider what it would be like to become a civilized species under Venus's skies -- look up, and you would always see clouds.

For a curious Venusian, the answer to the question, "What is above the clouds?" is "More clouds... What are you getting at?"

In such a situation wondering about stars and space is not going to be part of instinctive thinking. For the beings populating such a planet wondering about "What's up there?" is something that will come only after the beings become civilized enough to invent high-flying aircraft of some sort. For these people, wondering about space and stars is going to be like humans wondering about the Higgs Boson -- it is going to come late in their science evolution and not be terribly exciting when it is finally discovered.

This means that for such a world spending on space travel will come late and be minimal.


Life should be widespread throughout the galaxy, but we don’t know yet whether it is, or not.  We haven’t searched long enough or well enough to have any idea how common it is.

The more uncommon we find life to be, the more special the circumstances of life on Earth becomes. Some of the things that might make Earth uncommonly suited to life are:

• Monotonous energy from the sun

• A partly-molten core producing a long-lived plate tectonic era in the crust

• A large twin satellite which stabilizes the tilt

• A “mitochondria accident” which allows multicellular life to thrive

• So much life on the surface that evolution progresses rapidly

In the end, we may discover that Earth is a pretty special place for life.

This concept of Earth being a special place for life shows up in the novel “The Honeycomb Comet” and the short story “The Colonists” in “Tips for Tailoring Spacetime Fabric – Vol. 1

Update: This 18 Oct 14 Economist article, Bolts from the blue The history of life in the universe may have been governed by the frequency of giant stellar explosions, talks about this topic. It discusses the correlation between gamma ray bursts, the mass extinction they may cause, and why the Sun being far from the center of the Milky Way has likely reduced its exposure to nearby bursts.

From the article, "But there is another possibility. In a paper published on arXiv, an online repository, two astronomers, Tsvi Piran of the Hebrew University of Jerusalem and Raul Jimenez of the University of Barcelona, argue that some regions of the galaxy are less friendly to life than others. Moreover, the friendly areas may have been smaller in the past than they are now. If that is true, then it may be the case that complex life on Earth is just about as ancient as it is possible for complex life to be. And, since complexity necessarily precedes intelligence, that might mean human beings really are the first intelligent life forms to evolve in the Milky Way."

-- The End --

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