The Evolution of Complex Life
Microorganisms, or single celled life forms, are hypothesized to be ubiquitous throughout the galaxy. Our own solar system has perhaps six planets and moons which may harbor living organisms. Life leaps into existence even in the most hostile environments and under the most difficult situations. But complex life is a much different matter. There is a wonderful book entitled, Rare Earth: Why Complex Life is Uncommon in the Universe, by Peter D. Ward and Donald Brownlee. This book discusses the long and arduous journey of life from microorganisms to complex life forms. The book defines complex life as meaning animal life.
While microorganisms may be ubiquitous, complex life is exceedingly rare. Our only example of a world supporting complex life is the Earth. From our sample of one body it appears that it takes some four billion years for the evolution of life from microorganisms to complex life forms. And, for the evolutionary process to work, those four billion years must be remarkably stable to allow life to flourish and develop into complexity. The reason that complex life is so rare is that there are so few venues with stable conditions over billions of years.
In the 1961 Frank Drake wrote his famous Drake Equation, which attempts to quantify the number of advanced civilizations in the galaxy. Advanced civilization was defined as a civilization with radio telescopes making interstellar communication possible. In 1984 SETI began scanning the heavens, looking for those advanced civilizations. People such as Carl Sagan used Drake’s basic model and came up with estimates of perhaps 10,000 advanced civilizations.
Now it is believed that the old estimates were wildly optimistic. Now the hypothesis is that because complex life is so difficult to create, Earth may be the only venue for complex life in the galaxy. This hypothesis, the theme of Rare Earth, has been supported by the likes of Stephen Hawking and Neil deGrasse Tyson. The assumption is that the chance of microbial life developing into complex life is so miniscule that the Earth may be the only place where this has happened. If this is true then SETI searches in vain.
Here are some of the requirements for the multi-billion year stability required for life to flourish into complex forms:
- Complex life cannot exist near the center of the galaxy. A combination of excessive radiation and gravitational perturbations would be too severe for complex live to evolve.
- Complex life cannot exist anywhere radiation hazards, such as pulsars, which can sterilize planets thousands of light years away.
- Complex life would be virtually impossible in any binary star system. The gravitational perturbations of a binary star would cause orbital instability, and perhaps cause the planet to crash into another object or to be ejected into the cold darkness of interstellar space.
- The development of complex life requires a stable solar system with nearly circular planetary orbits. Many of the extra-solar planets that we have discovered have been “hot Jupiters,” formed in the outer reaches of the system and then spiraling inward, smashing lesser planets in the process.
- The development of complex life requires a planet in the “Goldilocks zone”, at the right distance from its star so as to be neither too hot nor too cold. Moons require similar circumstances that allow for the presence of liquid water.
- Stars and planets have life cycles. Overtime the Goldilocks zone can move. Originally Venus and Earth were twins, both in the Goldilocks zone, and both most likely had life forms. Over time as the Sun’s radiation increased (10% every billion years), Venus became a hellish world of extreme temperatures and pressures. If there are any life forms now on Venus they would be microorganisms in the upper atmosphere.
- It is not only the distance from a star that is important, but also the planet’s or moon’s climate. Climate is a complex mixture of atmospheric temperature and pressure, atmospheric composition, transparency, the presence of greenhouse gases, the presence of oceans, carbon cycles which regulate the amount of Co2 in the air, and many more factors as well. As the comparison of Venus and Earth demonstrate, climate is a critical factor in the development of complex life. And this climate must remain essentially stable over billions of years.
- The development of complex life requires magnetic shielding. Earth is surrounded by a strong magnetic field, caused by a hot, rotating iron core that creates a “force field” surrounding our planet. Mars failed in the development of complex life because it lacked this force field. Mars is smaller than Earth and it cooled more quickly. As the core was solidified the magnetic field collapsed. This allowed the solar winds to strip Mars of most of its atmosphere and water. Also, with the magnetic field removed, there was no protection from intense solar radiation. Earth is protected from radiation not only by our magnetic field bus also by our ozone layer in the atmosphere.
- Complex life depends upon its home world surviving various extinction events. The Earth has undergone as least five mass extinctions, each of which destroyed at least 75% of life on the planet. Most scientists would say that the Earth is now in the early stages of its sixth extinction event. This time it is caused largely caused by human activity such as agriculture, which destroys eco-systems to create single species dominance, pollution, habitat destruction, and the burning of fossil fuels.
Given all of the impediments to the development of complex life forms, the chance of microorganism evolving into complex life forms is a very small number, and perhaps one in a trillion.
The Rare Earth book stops at complex or animal life. It does not deal at all with the emergence of technological civilizations. There are many intelligent species on Earth such as octopus, pigs, dogs, crows, and dolphins. But none of these species are apt to create civilizations or to build radio telescopes. The leap from complex life to advanced civilization may be as difficult as the leap from microorganisms to complex life.
Life in the Universe
Part 1: The Solar System
It is a virtual certainty that we will discover extraterrestrial life within the next 20 years. There are many places in our solar system that would seem to make good cradles for life. Some of the best prospects include Mars, Europa, and Enceladus.
Mars has been tantalizing us for decades with its prospects for life. We have recovered a meteor blasted off of the surface of Mars that seems to contain fossilized life forms, although not all scientist agree with this premise. There have been experiments conducted by Mars landers that gave ambiguous results on the presence of life. But perhaps the most tantalizing sign comes from the seasonal methane cycles on Mars. Methane can be produced by either biological or geological processes. But there is no geological explanation of seasonal methane cycles. The prospects are good that these seasonal methane cycles are produced by organisms on Mars undergoing seasonal transformations. Methane, that gas from decaying garbage dumps and bovine flatulence is a necessary byproduct of organic life.
Europa has a vast ocean underneath a planetary ice cap. Ice fissures allow the subsurface ocean to leak through to the surface. When we look at the surface of Europa we see a fractured ice cap covered by reddish-brown crud along the fissures. It may be that this surface crud is some sort of life form resembling an algae bloom.
The extraterrestrial life be find in our solar system will be simple, single celled organism.
Life on Earth is a thousand times more diverse than I was taught in high school biology class. In school we were taught that there are plants and animals. The Sun was the source of all biological energy. Plants converted sunlight into sugars through photosynthesis. Herbivores ate the plants and carnivores ate the herbivores. All life was beholden to the Sun.
But now we know much more. There is life everywhere on Earth, and much of it totally cut off from the Sun. There are tube worms and shrimp in the deepest ocean trenches, using chemosynthesis to convert sulfur into energy in extreme high temperatures and pressures. There are organisms living in the boiling, caustic paint pots of Yellowstone Park. There are organism living deep underground, drawing their energy from the rocks. There are organism living in frozen glaciers. When glaciers calve, these life forms create a rich biomass in the ocean, a biomass that is the bottom of the food chain for all life in the polar oceans. There is even life forms living in the cooling ponds of nuclear reactors.
Along with plants and animals, there are fungi such as mushrooms, slime molds, algae, protozoa, and these are just our closest relatives. It has been suggested that there is more biomass below ground than there is above ground.
With life on Earth this diverse and abundant, we can expect that life will be prolific throughout vast sections of the Universe. Our own solar system is full of water and organic compounds, the two essential ingredients for life as we know it. Comets and asteroids have both in abundance. Spectroscopy of the Universe suggest that the same chemistry exists everywhere. Water and organic compounds exist throughout the Universe. On Titan there are lakes of liquid methane. On Europa there is more water than exists on Earth.