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Rare Earth Debate Part 3: Complex Life

Space.com, July 2002

original source |  fair use notice

Summary: This five-part debate will cover a variety of topics prompted by the hypothesis of "Rare Earth," a book by Peter Ward and Donald Brownlee that suggests complex life may be unique to Earth. Today the participants examine complex life and the possibility of its occurrence in the universe. Complex life is generally considered any living thing with multiple cells -- as opposed to single celled, microbial life -- and, on Earth anyway, includes everything from the simplest slime molds to human beings.

This five-part debate will cover a variety of topics prompted by the hypothesis of "Rare Earth," a book by Peter Ward and Donald Brownlee that suggests complex life may be unique to Earth.

In Part 2, the participants discussed how far (or near) alien life might be. Today they examine complex life and the possibility of its occurrence in the universe. Complex life is generally considered any living thing with multiple cells -- as opposed to single celled, microbial life -- and, on Earth anyway, includes everything from the simplest slime molds to human beings. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.


Michael Meyer: I presume that we are in agreement that microbial life, at least, may be common in our stellar neighborhood and even may be present on other planets in our solar system. That being the premise, the probability of complex life elsewhere is then dependent on the probability of the transition from slime to civilization. It happened here, so why not elsewhere? Do you think that complex life should develop on a sizeable fraction of worlds around other stars?

Christopher McKay: As David Grinspoon pointed out earlier, the Earth is our only example of planetary life. This makes it difficult to unravel what is universal and what is accidental about the nature and history of life. Still, one data point is better than none, and when we look at the question of complex life, our one data point seems to say that complex life arose as a result of the rise of free oxygen. If we take this as being generally true, then we can ask the geophysical question: On what types of planets will free oxygen arise and how long will it take to reach high enough levels?

On Earth it took billions of years for oxygen to rise to present levels. Partly this is because the Earth is efficient at recycling by plate tectonics. This recycling keeps the Earth habitable by cycling the essential elements, but it also would have been a barrier to the buildup of oxygen. Earth probably is not the best possible planet for complex life development, since less plate tectonics would allow a faster rate of oxygen build up.

Mars took this to the extreme. With no plate tectonics, a shallow ocean, and only 38 percent of the Earth’s gravity, Mars might have built up oxygen much faster than the Earth. But the lack of plate tectonics doomed Mars to lose its atmosphere through mineralization. We might find that complex life arose on Mars only to be extinguished later. Perhaps the optimal planet for complex life would be an intermediate between Earth and Mars.

There may be a range of planet types on which oxygen could arise -- and therefore complex life. I would hazard a guess that most -- maybe two-thirds -- of terrestrial planets with life go on to develop complex life at some stage of their history. An optimist’s view.

Simon Conway Morris: The problem in my view is, why did complex life take so long to evolve on Earth? Evidence from oxygen data is frankly equivocal. Maybe the redox state of the Earth's mantle was peculiar in comparison with other similar planets. Alternatively, ocean chemistry may have put the lid on things.

There could be other dimensions that could explain why there was such a brake on the evolution of complex life -- why there were no Meso-Proterozoic dry martinis, but on the other hand, once microbes, then NASA.

David Grinspoon: Planetary biospheres are complex entities whose histories are fraught with contingency, accident, and luck. Therefore, the time it took for complex life to arise on Earth is probably much faster than some and much slower than others.

We can’t stand a mystery without a chief suspect, so we pin the rise of complex life on the rise of oxygen. This may well have factored in, but as Chris pointed out, there is no reason to believe that oxygen rose on Earth as quickly as it might have elsewhere. The rate of plate tectonics is one variable that will change atmospheric history - there are countless others. For example, if Earth had formed less rich in iron, then oxygen would have risen much more quickly because there would not have been as much iron to devour the oxygen.

So in other planetary systems that are less metal-rich, creatures might have evolved to levels far beyond our current state.

Peter Ward: On Earth, evolution has undergone a progressive development of ever more complex and sophisticated forms leading ultimately to human intelligence. Complex life – and even intelligence – could conceivably arise faster than it did on Earth. A planet could go from an abiotic state to a civilization in 100 million years, as compared to the nearly 4 billion years it took on Earth.

Evolution on Earth has been affected by chance events, such as the configuration of the continents produced by continental drift. Furthermore, I believe that the way the solar system was produced, with its characteristic number and planetary positions, may have had a great impact on the history of life here.

It has always been assumed that attaining the evolutionary grade we call animals would be the final and decisive step. Once we are at this level of evolution, a long and continuous progression toward intelligence should occur. However, recent research shows that while attaining the stage of animal life is one thing, maintaining that level is quite another. The geologic record has shown that once evolved, complex life is subject to an unending succession of planetary disasters, creating what are known as mass extinction events. These rare but devastating events can reset the evolutionary timetable and destroy complex life while sparing simpler life forms.

Such discoveries suggest that the conditions allowing the rise and existence of complex life are far more rigorous than are those for life’s formation. On some planets, then, life might arise and animals eventually evolve – only to be soon destroyed by a global catastrophe.

Frank Drake: The Earth’s fossil record is quite clear in showing that the complexity of the central nervous system -- particularly the capabilities of the brain -- has steadily increased in the course of evolution. Even the mass extinctions did not set back this steady increase in brain size. It can be argued that extinction events expedite the development of cognitive abilities, since those creatures with superior brains are better able to save themselves from the sudden change in their environment.

Thus smarter creatures are selected, and the growth of intelligence accelerates.

We see this effect in all varieties of animals -- it is not a fluke that it has occurred in some small sub-set of animal life. This picture suggests strongly that, given enough time, a biota can evolve not just one intelligent species, but many. So complex life should occur abundantly.

There is a claim that "among the millions of species which have developed on Earth, only one became intelligent, so intelligence must be a very, very rare event." This is a textbook example of a wrong logical conclusion. All planets in time may produce one or more intelligent species, but they will not appear simultaneously. One will be first. It will look around and find it is the only intelligent species. Should it be surprised? No! Of course the first one will be alone. Its uniqueness -- in principal temporary -- says nothing about the ability of the biota to produce one or more intelligent species.

If we assume that Earths are common, and that usually there is enough time to evolve an intelligent species before nature tramples on the biota, then the optimistic view is that new systems of intelligent, technology-using creatures appear about once per year. Based on an extrapolation of our own experience, let's make a guess that a civilization's technology is detectable after 10,000 years. In that case, there are at least 10,000 detectable civilizations out there.

This is a heady result, and very encouraging to SETI people.

On the other hand, taking into account the number and distribution of stars in space, it implies that the nearest detectable civilizations are about 1,000 light-years away, and only one in ten million stars may have a detectable civilization. These last numbers create a daunting challenge to those who construct instruments and projects to search for extraterrestrial intelligence. No actual observing program carried out so far has come anywhere close to meeting the requirement of detecting reasonable signals from a distance of 1,000 light years, or of studying 10 million stars with high sensitivity.

Donald Brownlee: But how often are animal-habitable planets located in the habitable zones of solar mass stars? Of the all the stars that have now been shown to have planets, all either have Jupiter-mass planets interior to 5.5 AU [1 AU, or astronomical unit, is the distance from Earth to the Sun] or they have Jupiters on elliptical orbits. It is unlikely that any of these stars could retain habitable zone planets on long-term stable orbits.

On the other hand, many of the stars that do not have currently detectable giant planets could have habitable-zone planets. But even when rocky planets are located in the right place, will they have the "right stuff" for the evolution and long term survival of animal-like life? There are many "Rare Earth" factors (such as planet mass, abundance of water and carbon, plate tectonics, etc.) that may play important and even critical roles in allowing the apparently difficult transition from slime to civilization.

As is the case in the solar system, animal-like life is probably uncommon in the cosmos. This might even be the case for microbes: how can scientists agree that microbial life is common in our celestial neighborhood when there is no data? Even the simplest life is extraordinarily complicated and until we find solid evidence for life elsewhere, the frequency of life will unfortunately be guesswork. We can predict that some planetary bodies will provide life-supporting conditions, but no one can predict that life will form.

Frank Drake: Only about 5 percent of the stars that have been studied sufficiently have hot Jupiters or Jupiters in elliptical orbits. The other 95 percent of the stars studied do not have hot Jupiters, and just what they have is still an open question. The latest discoveries, which depend on observations over a decade or more, are finding solar system analogs. This suggests that 95 percent of the stars- - for which the answers are not yet in -- could be similar to our own system. This is reason for optimism among those who expect solar system analogs to be abundant.

David Grinspoon: I think it is a mistake to look at the many specific peculiarities of Earth's biosphere, and how unlikely such a combination of characteristics seems, and to then conclude that complex life is rare. This argument can only be used to justify the conclusion that planets exactly like Earth, with life exactly like Earth-life, are rare.

My cat "Wookie" survived life as a near-starving alley cat and wound up as a beloved house cat through an unlikely series of biographical accidents, which I won't take up space describing but, trust me, given all of the incredible things that had to happen in just the right way, it is much more likely that there would be no Wookie than Wookie. From this I do not conclude that there are no other cats (The Rare Cat Hypothesis), only that there are no other cats exactly like Wookie.

Life has evolved together with the Earth. Life is opportunistic. The biosphere has taken advantage of the myriad strange idiosyncrasies that our planet has to offer. Not only that, life has created many of Earth’s weird qualities.

So it is easy to look at our biosphere, and the way it so cleverly exploits Earth’s peculiar features, and conclude that this is the best of all possible worlds; that only on such a world could complex life evolve. My bet is that many other worlds, with their own peculiar characteristics and histories, co-evolve their own biospheres. The complex creatures on those worlds, upon first developing intelligence and science, would observe how incredibly well adapted life is to the many unique features of their home world. They might naively assume that these qualities, very different from Earth’s, are the only ones that can breed complexity.

Read more articles on this topic:

Are We Alone? Life in the Universe