Summary: When the book "Rare Earth" was published two years ago, it raised a great deal of controversy among astrobiologists. Written by Peter Ward and Donald Brownlee, the book's hypothesis suggests complex life is rare in the universe, and may even be unique to Earth. This debate, a 5-part series, will cover a variety of topics prompted by the Rare Earth hypothesis.
When the book "Rare Earth" was published two years ago, it raised a great deal of controversy among astrobiologists. Written by Peter Ward and Donald Brownlee, the book's hypothesis suggests complex life is rare in the universe, and may even be unique to Earth. If life does occur elsewhere, the authors contend, it will only be in the form of single-celled microbial life such as bacteria.
This debate, a 5-part series beginning today, will cover a variety of topics prompted by the Rare Earth hypothesis. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.
Michael Meyer: Thank you for joining the first in what we hope will be a series of Great Debates. Before delving into the vagaries and specifics of planetary and biological evolution, and into a discussion of whether we are unique or common, it might be useful to set a baseline for at least one prerequisite for complex beings -- life itself. This leads to the first question:
Other than on Earth, is there life in our stellar neighborhood?
Peter Ward: There is a cultural assumption that there are many alien civilizations. This stems in no small way from the famous estimate by Frank Drake -- known as the "Drake Equation" -- that was later amended by Drake and Carl Sagan. They arrived at an estimate that there are perhaps a million intelligent civilizations in the Milky Way Galaxy alone.
The Drake and Sagan estimate was based on their best guess about the number of planets in the galaxy, the percentage of those that might harbor life, and the percentage of planets on which life not only could exist but could have advanced to culture. Since our galaxy is but one of hundreds of billions of galaxies in the universe, the number of intelligent alien species would be numbered in the billions.
Surely, if there are so many intelligent aliens out there, then the number of planets with life must be truly astronomical. But what if the Drake and Sagan estimates are way off? If, as could be the reality, our civilization is unique in the galaxy, does that mean that there might be much less life in general as well?
In my view, life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake and Sagan envisioned. However, complex life -- animals and higher plants -- is likely to be far more rare than commonly assumed. Life on Earth evolved from single celled organisms to multi-cellular creatures with tissues and organs, climaxing in animals and higher plants.
But is Earth’s particular history of life -- one of increasing complexity to an animal grade of evolution -- an inevitable result of evolution, or even a common one? Perhaps life is common, but complex life -- anything that is multi-cellular -- is not.
Chris McKay: There is no solid evidence of life elsewhere, but several factors suggest it is common. Organic material is widespread in the interstellar medium and in our own solar system. We have found planetary systems around other sun-like stars. On Earth, microbial life appeared very quickly -- probably before 3.8 billion years ago. Also, we know that microbial ecosystems can survive in a variety of environments with liquid water and a suitable chemical energy source or sunlight.
These factors suggest that microbial life -- the sort of life the dominated Earth for the first two billion years -- is widespread in the stellar neighborhood.
David Grinspoon: It is always shaky when we generalize from experiments with a sample size of one. So we have to be a bit cautious when we fill the cosmos with creatures based on the time scales of Earth history (it happened so fast here, therefore it must be easy) and the resourcefulness of Earth life (they are everywhere where there is water).
This is one history, and one example of life. When our arguments rest on such shaky grounds, balancing a house of cards on a one-card foundation, we are in danger of erecting structures formed more by our desires than the "evidence."
Frank Drake: I think this is an occasion where that old principal of good science, Occam's Razor, is helpful. Apply Occam's Razor to the question of the origin of life on Earth. We look at the Earth, and with regards to that origin, as best we know, no special or freak circumstances were required. It took water, organics, a source of energy, and a long time. Deep-sea vents are the current favorite and a reasonable place for the origin.
But even if they weren't the culprits, the chemists have found a multitude of other pathways that produce the chemistry of life.
The challenge seems to be not to find the pathway, but the one that was the quickest and most productive. The prime point is that nothing special was required. There will be a pathway that works, on Earth and on similar planets. Then, by Occam's Razor, the origin of life on Earth is nothing more than the result of normal processes on the planet. Furthermore, life should appear very frequently on other Earth-like planets. There will be microbial life nearby the solar system.
Donald Brownlee: While there is hope and even expectation of nearby extraterrestrial life, the goal of "Rare Earth" was to point out that the universe is fundamentally hostile to life. Most planets and other places in the universe clearly could not support any type of Earth-like creatures. The universe is vast, so there may be many Earth-like places, but they will be widely spaced, and if they are too widely spaced they will be isolated from each other.
What fraction of stars harbor Earth-like planets with Earth-like life? Is it one in a hundred, one in a million, or even less? Even the most optimistic have to admit Earth-like environments must be rare.
In our book "Rare Earth," we suggest that extraterrestrial life is likely to be near but that complex animal-like life is rare and will probably not be found close to us in space. A major question about life relates to the environments needed for its formation and long term evolution. Unfortunately Earth is our only successful example. Predictions of life elsewhere are problematic; presently there is no detectable life elsewhere in the solar system.
David Grinspoon: I am not convinced that the Earth’s carbon-in-water example is the only way for the universe to solve the life riddle. I am not talking about silicon, which is a bad idea, but systems of chemical complexity that we have not thought of, which may not manifest themselves at room temperature in our oxygen atmosphere. The universe is consistently more clever than we are, and we learn about complex phenomena, like life, more through exploration than by theorizing and modeling. I think there are probably forms of life out there which use different chemical bases than we, and which we will know about only when we find them, or when they find us.
An obvious rejoinder to this is, "But no one has invented another system that works as well as carbon-in-water." That is true. But to this I would answer, "We did not invent carbon-in-water!" We discovered it. I don’t believe that we are clever enough to have thought of life based on nucleic acids and proteins if we hadn’t had this example handed to us. This makes me wonder what else the universe might be using for its refined, evolving complexity elsewhere, in other conditions that seem hostile to life as we know it.
Frank Drake: All evidence of the most primitive steps in the first 700 million years of chemical evolution on Earth is apparently lost. We grope towards understanding of that profound gap in our knowledge by working backwards, hypothesizing that there once was an RNA world based on self-catalyzing RNA. But this system evolved from something else, and led to the esoteric DNA-protein world.
As David Grinspoon rightly points out, we are not remotely smart enough to hypothesize ab initio the system of the DNA-protein world, or even the RNA world. It was handed to us on a silver platter. This should be a strong warning that we are over our heads when predicting what might have taken place on other worlds.
Give us knowledge of another independent origin of life in space, and the doors to great progress in this field may open.