Saturday, May 24, 2008

Panspermia Expelled

If you saw the movie Expelled you will remember that Ben Stein had some fun at Richard Dawkins’ expense when Dawkins brought up the theory of panspermia. Stein took him to mean that aliens might be responsible for the origin of life on Earth and played it for a laugh. The theater audience at the screening I attended obligingly snickered and poked each other in the ribs. But panspermia is a serious scientific theory, although it has the misfortune to carry a name which evokes middle school giggles. I would suggest that if you don’t want people to giggle at your theories (e.g., intelligent design) then it is bad form to giggle at their theories. The irony is that advocates of panspermia, while admittedly few, are actually allies of a sort with creationists and others who doubt the plausibility of traditional evolutionary theories. This is one reason why panspermia is often greeted with derision by mainstream scientists. To admit this theory has any reason to exist is to admit there are doubts about evolution as normally understood. So let us examine briefly what panspermia is all about.

First let’s review a little background on the theory of evolution. It is important to realize that evolution is not actually a theory about the origin of life but the origin of species. It states that natural selection operating on random genetic variation will produce, over time, new species better adapted to their environment than previously existing species. This process requires self-replicating organisms on which to operate, so it cannot explain how life began. You need another theory for that. The first organisms would have been much simpler than current life forms. Exactly how simple is a matter of some debate, but they couldn’t be too simple because they would need some sort of genetic code and the ability to both replicate and mutate.

Since no one knows how simple a self-replicating organism can be, no one knows how long you might have to wait for one to form by spontaneous chemical reactions in the presumed primordial soup. And no one knows how many mutations would be required for it to evolve into the simplest single-celled creatures we observe today. Therefore, no one knows the time required for such a chain of events to occur by random processes. It seems undeniable, however, that it would take a very, very long time. It is therefore convenient that modern scientists believe the Earth is very ancient indeed – about 4.5 billion years old. Of course, if you believe in a young Earth based on Genesis there’s no time for anything but a miracle. But for the sake of argument, let’s accept the old Earth chronology since we are seeking to understand competing naturalistic theories of origin.

Geologists have discovered fossilized cyanobacteria in rocks which they estimate are 3.5 billion years old. This gives a window of about a billion years for the origin of life. However, it is thought that due to heavy meteor bombardment in the early years the Earth might not have been habitable until about 3.9 billion years ago. The very oldest sedimentary rocks also contain some chemical evidence of photosynthetic life forms existing about 3.8 billion years ago. This narrows the window considerably, to as little as 100 million years. That’s still a very, very long time, but it seems uncomfortably short when you contemplate how many improbable steps might be required to produce cyanobacteria. These bacteria are the simplest life forms in existence today, but they are still exceedingly complicated.

There are three possible responses to such a short window. You can either 1) take it as evidence that life spontaneously develops quickly and easily given the right conditions, 2) assume we got really, really lucky, or 3) start casting about for another theory. Most mainstream scientists accept some form of 1) or 2). Advocates of panspermia take the third position. They ask, “What if life originated elsewhere in the universe and was seeded here on Earth?” These scientists still believe in a naturalistic origin of life occurring somewhere in the universe. They just think the odds are low it happened here in only 100 million years. The universe is estimated to be about 13 billion years old. If life originated elsewhere then the window opens back up to almost 10 billion years. This hypothesis bolsters the plausibility of natural origins in two ways. First, it considerably increases the time available for life to originate, because there are presumably many solar systems in the universe which are billions of years older than ours. Second, it means that life only needed to develop on one planet out of millions or billions. Then it could spread to all the others.

Contrary to Ben Stein’s humorous questions, panspermia doesn’t imply intelligent aliens arriving in flying saucers to farm the ancient Earth. Rather, it hypothesizes that some forms of microscopic life were hardy enough to hitch a ride on meteors and cosmic dust. They survived for many millions of years before falling by chance to the Earth and finding here a hospitable environment in which to reproduce. This strikes most scientists as very far-fetched, but not quite as far-fetched as you might think. There is evidence that dormant bacteria and spores can survive incredibly harsh conditions for thousands of years. There is also evidence that rocks blasted from the surface of Mars and the Moon by meteor strikes have landed on the Earth as meteorites. However, it seems much less likely such material would wind up on Earth from a distant solar system, or that even a hardy spore could survive such a long trip. The times and distances are a million times greater.

So in the competition between origin theories, scientists have to decide whether it is more plausible that life originated and evolved on Earth in 100 million years, or that organisms traveled interstellar distances and survived millions of years. Most hold the former position. You can see now why panspermia proponents are in a strange way allies with creationists. They do repudiate the divine origin of life, but they also challenge the plausibility of the traditional evolutionary timeline, and on the latter point I am in complete agreement. At the present state of knowledge it is impossible to put evolution on a quantitative basis. A very large, unknown sequence of improbable events had to occur to produce complex life forms. Proponents of evolution are reduced to little more than hand waving when it comes to justifying the assumption that all these unknown events could reasonably have occurred by chance in the time available, or even in 10 billion years on one in a billion planets.

In the face of such uncertainty we must fall back on our presuppositions. If you are philosophically convinced that life must have naturalistic origins you say to yourself, “Gee, I don’t know the mechanism and I can’t calculate the probability, but it must have happened or we wouldn’t be here, right?” On the other hand if you are a Christian like me you have another alternative. I believe that God created life and created us. I’m not sure exactly how he did it but I know he wasn’t limited to naturalistic processes. So I can look at the postulated sequence of improbable events and comfortably conclude it is more plausible to believe in miracles.

[Update - July 4, 2008] This recent article reports the discovery of chemical evidence for life on earth as early as 4.25 billion years ago. Mineralogists have discovered ancient rocks in Australia which contain an anomalously high ratio of carbon-12 to carbon-13, normally considered an indicator of organic matter. If true (and it's really only suggestive) this puts an even greater squeeze on the time available for life to have evolved on Earth.


Electric Monk said...

Well-written and well-thought-out article. I have a friend - she is extremely intelligent with a Master's in planetary geology - who is firmly convinced that life began on Mars and, through panspermic processes, came to Earth. Effectively, we've seen the Martians, and they are us. The only problmem is that, while the process by which this could happen might be understood, there is precisely zero evidence that it's what did happen. We've found Martian meteorites, but beyond that, the evidence that this is what actually occurred is non-existent.

Bill Hensley said...

That’s very interesting. I wonder why she would be so sure life came from Mars to Earth, rather than the other way around.

As you well know, one of the major reasons NASA has invested so much in the robotic exploration of Mars is to look for evidence of life, whether past or present. At the very least they are looking for whether Mars would ever have been habitable (i.e., thicker atmosphere, more water, moderate temperatures, etc.) One motive is simply to answer the fundamental question, “Are we alone?” But I think another motive is to bolster the plausibility of current theories of origin. If it happened twice on adjacent planets it suggests that life forms quickly and easily under the right conditions. On the other hand, if life on Mars and Earth appear related, it makes it easier to believe that life might have spread through the galaxy from a single point of origin. Either way, naturalistic origins seem more plausible.

But there's a fallacy in that line of reasoning. It requires that you start with the assumption that life must have occurred through a naturalistic process. If that’s true, then finding it in two places makes the initial assumption seem more plausible. But of course that’s a circular argument. As long as there are two possible origins of life, natural and divine, finding a second instance of it does nothing to bolster one possibility over the other. What’s needed to bolster naturalistic theories of origin is to uncover more evidence for a specific mechanism by which life might have arisen. This is where I have been critical of current theories. There are several wildly different theories of origin, none of which are particularly specific or detailed, and none of which have any particular evidence to support them. Of course, having a second instance of life might shed light on the question. That's another reason there's so much interest in Mars.

BTW, have you noticed how so many otherwise thoughtful atheists are tempted to resort to theological arguments against the divine origin of life? They would claim that finding life on Mars argues for natural origins, because why would God want to create life somewhere else if we’re so special? But I think none of us, least of all perhaps our atheistic friends, are qualified to speculate on the motives and reasoning of Almighty God. That’s why I have always found this counterargument amusing, if somewhat exasperating.

Anonymous said...

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Anonymous said...

I have recently read a lot of Fred Hoyle, the renowned astrophysicist. He not only subscribed to Panspermia but also believed that due to the necessity of certain physical constants needed to produce the exact proportions of carbon and oxygen required for life, that the entire universe was a "setup job".
As for myself, I believe that aside from creationist or evolutionist beliefs there is a third possibility that few subscribe to. Namely, that humans are and always shall be incapable of understanding their exact origin. This has nothing to do with a lack of intelligence. To understand our universe, we would need to be able to observe it from the outside. However, logic, mathematics and physics as we understand them, possibly exist only within our universe. This formally excludes any form of absolute knowledge ever becoming obtainable.