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.