What has allowed the earth to maintain stable conditions that are favorable for life? To answer this question we need to look at our planet’s history, and ask why the earth has been habitable for almost 4 billion years. This is an extremely long time, and probably the time needed for intelligent life to evolve. Scientists have second-hand evidence to go by, like entering a crime scene after the fact. But there is plenty of evidence to reconstruct the major historical events of our planet; this comes from a wide range of scientific fields.
The earth is the home of all life that we know about, possibly the only home humans will ever have. However, given enough time, it is possible that much like ancient sailors settled new lands, spaceships will cross space to new worlds. Up until now we should consider ourselves very fortunate that our planet has maintained the conditions necessary for life. For sure, the earth has gone through dramatic changes in its lifespan, but not significant enough to snuff out life. Let us examine a number of plausible reasons why a life-friendly earth has endured for so long:
The Goldilocks Zone
The earth’s location in relation to the sun has been called the ‘Goldilocks Zone’ or ‘Habitable Zone,’ because it is just the right distance from the sun to support life. Specifically, the temperature on earth is within a range that allows for water to flow (life as we know it needs liquid water). The right location is the starting point for a living world.
It is possible that life could exist with other chemicals that are liquids at other temperatures. For example, it has been suggested the liquid methane at extremely cold temperature could support life, such as the lakes of Titan (Saturn’s largest moon). But this is speculative, and that form of life would be unfamiliar to us. Nevertheless, finding evidence for liquid water on other worlds is challenging, as Goldilocks Zones are hard to come by. Although over 2 thousands exoplanets (planets outside our solar system) have been discovered, planets in habitable zones are rare.
The Solar System
The earth is in constant motion and in relation with other celestial bodies in the solar system. Somewhat like a mobile hanging above a baby’s crib, all the bodies have influence on the system. In addition to a habitable zone, a long-term stable system is necessary. At least, the overall effects of the celestial bodies must stabilize the movement and climate of one body (like the earth). Here are 4 earth-friendly characteristics of our solar system:
- Earth’s Tilt: The earth is tilted at an angle of 23.5 degrees away from the plane of the elliptical orbit. The tilt gives us our seasons, which allows a greater surface to attract heat from the sun. Without seasons, only the region around the equator would be habitable. This would have drastically changed life on the planet?
- The Sun: The sun is 4.5 billion years old and will live for another 5 billion years. Some stars only live for a few million years. For complex life to evolve it takes several billion years, therefore a long-lived sun is needed.
- The Moon: The earth-moon system seems to have attained a stable relationship. The moon is just the right size to help prevent a chaotic wobble of the earth’s axis. The moon also aids in creating larger tides, which is thought to have played a role in transitioning life from water to land. And the speed of the earth’s spin has slowed over time due to the moons presence, thus moderating climate extremes.
- The Gas Giants: Jupiter and Saturn are the largest of the outer planets. Their orbits outside the earth’s orbit have protected the earth from large impacts. In the early development of the solar system, there were many large moving objects. The gas giants are believed to have ejected some of the large debris out of the system, and aided the inner planets to form sooner. And who knows how many potential collisions with the earth were absorbed by the gas giants.
It is remarkable that the earth has maintained a stable climate for billions of years. I mean stable in the sense that the climate has not varied enough to wipe out life. Since life has appeared the earth has gone through a number of ice ages and periods of intense warming. Average temperatures may have varied by as much as 100 degrees C. But for reasons only partially understood, the climate has always returned to moderate levels.
Factors controlling the temperature have fluctuated throughout planetary history, such as: the heat generated by the sun, the earth’s heat absorption rate, and the amount of greenhouse gases that trap heat in. Could there be a regulating effect or cancelling-out effect that has prevented a runaway process? The earth has avoided irreversible climate change, unlike our two cosmic neighbors (Venus is to hot and Mars is to cold). Currently the average global temperature is about 15 degrees C.
Moderate and Gradual Change
Changes to the climate and environment are essential for the evolution of life, provided that the changes are moderate and gradual. Evolution is a multi-generational process, in which individuals that are better suited to their environments survive longer and reproduce. Beneficial genes are passed on to future generations; however, what constitutes beneficial genes is unstable, because the earth is constantly changing.
As a result of moderate and gradual changes species evolve into other species. If the planet was unchanging, the earliest life forms would not have evolved into more complex forms. On the other hand, if the changes were too drastic life could not have adapted successfully. Earth’s history shows that environmental changes have caused some species to go extinct, while others have evolved and branched out into new species.
The Gaia Hypothesis
James Lovelock, a NASA chemist in the sixties, proposed the Gaia Hypothesis when he was searching for life on other planets. While comparing the atmospheres of Earth, Mars and Venus he noticed that the earth was chemically in a state of flux. Conversely, Mars and Venus were chemically unchanging and predominately composed of carbon dioxide. The fact that the earth’s atmosphere was an active mixture of gases and still retained its overall composition, suggested some form of planetary regulation. His conclusion was that life regulated the atmosphere by its many processes.
Lovelock expanded the Gaia Hypothesis (also called Gaia Theory) to include the whole biosphere (climate, rocks, oceans, biology, etc.) and described the earth as a self-regulating system. In other words, the earth acted as one organism. Gaia was controversial as a scientific hypothesis when first proposed. The main objection was evolutionary theory, as organisms are not believed to act in concert with their environment (sometimes supportive and sometimes destructive). The argument against Gaia Theory was that organisms would somehow have to communicate with each other, and act altruistically towards the planet. This was impossible.
Lovelock’s counter-argument was that Gaia was not intentionally achieved, yet that natural selection was critical in shaping the regulatory patterns of the planet. Gaia did not need a controlling center; it was a consequence of natural selection. Nevertheless, loosely applied it points to life processes as being critical in creating and maintaining living conditions. Over time Lovelock’s idea gained more popularity as evidence grew for an ever more interconnected and interdependent biosphere.
It could be that natural selection allows life to adapt to whatever conditions arise, giving the impression of Gaia. Or possibly, that long-term climate and atmospheric stability is in large part due to the existence of life.
It could be that the earth is a rare and unique planet, which has benefited from an extraordinary amount of good luck. Evidence for planets outside our solar system is mounting. There are a number of earth-like candidates, but the odds are stacked against finding a place just like earth. This does not mean that other earths don’t exist, just that they would be extremely far away. Paradoxically, the unfathomable size of the universe could mean that life is both rare and plentiful.
Anthropic reasoning would suggest that the earth has endured through a long succession of fortunate events. Intelligent observes are the result of anthropic selection, of which other lifeless worlds have no one to observe them. If events had not worked out just right for us, we wouldn’t be here. Still, it is difficult to comprehend the many unlikely phases of earth’s evolution. For example:
- The emergence of life.
- Multi-cellular life.
- Atmospheric transformation from carbon dioxide rich to available oxygen.
- Life moving from water to land.
- The rise of consciousness and intelligence.
These examples are major thresholds that were crossed, yet countless other variables could have changed the course of history. Life could have taken a completely different direction, even to the point of total extinction. Obviously, this has not happened, either from cosmic events or global catastrophes. When life began there was no guarantee that it would survive for nearly 4 billion years. And the specific circumstances that led to human beings were even more tenuous. We should consider ourselves very lucky to be here, on such a special planet.
References: David Waltham, Lucky Planet (New York: Basic Books, 2014).
Beautiful Minds – James Lovelock – The Gaia Hypothesis / Gaia Theory, Published on Sep. 12, 2013.
Life on Earth Can Thank Its Lucky Stars for Jupiter and Saturn, By Sarah Lewin, Staff Writer | January 12, 2016 07:30 am ET, http://www.space.com/31577-earth-life-jupiter-saturn-giant-impacts.html
What Makes Earth So Perfect for Life? Dec 13, 2012 03:00 AM ET, http://news.discovery.com/human/life/life-on-earth-121019.htm.