Life may have been the result of intelligent aliens sending bacterium to Earth. This theory is called “directed panspermia”. It was proposed thirty-five years ago by Francis Crick, co-discoverer of DNA, and Leslie Orgel, a highly respected British chemist. I found the original paper they published. Here I examine it and provide some comments[1].
A common objection to the theory that aliens brought life to Earth is the problem of infinite regress: If life on Earth was created by aliens, who created the aliens? And who created those creators? And so on. Crick and Orgel get around this problem by speculating that some planets may have chemical properties that make the origin of life much more probable than it is on Earth. Although it’s not mentioned, I think it’s possible that there are forms of life more likely to originate from non-living matter (and without the guidance of an intelligent designer).
Citing the work of astronomer Carl Sagan, they conclude that life traveling on a meteor would probably be destroyed by radiation long before it would arrive at Earth. But what if an alien civilization designed a special radiation-proof microorganism-carrying ship? Then the “life-from-space” proposal would once again be plausible.
In the paper, Crick and Orgel say, “[I]t is quite probable that planets not unlike the Earth existed as much as [6.5 billion years] before the formation of our own solar system.” This allows life to originate, evolve and spread before Earth even existed.
They go on with arguments supporting their theory:
“Infective theories of the origins of terrestrial life could be taken more seriously if they explained aspects of biochemistry or biology that are otherwise difficult to understand. We do not have any strong arguments of this kind, but here are two weak facts that could be relevant.
The chemical composition of living organisms must reflect to some extent the composition of the environment in which they evolved. Thus the presence in living organisms of elements that are extremely rare on the Earth might indicate that life is extraterrestrial in origin.
Molybdenum is an essential trace element that plays an important role in many enzymatic reactions, while chromium and nickel are relatively unimportant in biochemistry. The abundance of chromium, nickel, and molybdenum on the Earth are 0.20%, 3.16%, and 0.02%, respectively. We cannot conclude anything from this single example, since molybdenum may be irreplaceable in some essential reaction – nitrogen fixation, for example. However, if it could be shown that the elements represented in terrestrial living organisms correlate closely with those that are abundant in some class of star – molybdenum stars, for example – we might look more sympathetically at ‘infective’ theories.”
Crick and Orgel’s second argument for their theory is the genetic code. As you may know, the genetic code is universal[2]. It is the same in plants, animals, and bacteria. Crick and Orgel believed this means all life on Earth shares a common ancestor with a complete and fully developed genetic code[3]. They reasoned that if an alien civilization sent life to Earth, then it would have developed an organism with that same genetic code. On the other hand, if life originated naturally, the common ancestor of all living things may have simply had a primitive genetic code which coded for only a handful of amino acids (later on, this species would have split into several different lineages which had genetic codes that coded for more types of amino acids using the codons which originally did not code for anything). In this scenario, all life would have genetic similarities, but would also have significant differences. The moral of the story is that directed panspermia would show a universally shared genetic code. A non-panspermia origin could produce a single, universal genetic code, or many codes with significant similarities).
Admittedly, both of these evidences are very weak, yet it’s still an intriguing hypothesis.
With that said, the case for a natural origin of life on Earth is much stronger. For example, researchers have determined that the last universal common ancestor of all life (“LUCA”) used proteins constructed with amino acids, and these amino acids can easily be produced in experiments meant to recreate early Earth conditions[4]. Note: the last common ancestor was not necessarily the very first living thing. It probably evolved from an even simpler organism.
Another way of describing this is by the Miller experiment: in 1953, Stanley Miller attempted to recreate the early Earth’s atmosphere[1]. In the end, he produced several amino acids. Dozens of experiments like his have been done since then, many of them correcting mistakes that he made (for example, he assumed the atmosphere was composed of methane and ammonia, while recent evidence suggests that early Earth atmosphere was mainly composed of carbon dioxide and nitrogen[6]). The amino acids that these experiments produce in abundance are the most frequently used amino acids of the LUCA. This is surely to be expected if life arose naturally from the chemicals on the early Earth. Later organisms could’ve evolved ways to synthesize amino acids which were not readily available in the environment. Yet the very first organisms would not have had the ability to synthesize just any amino acid. They would have had to use what was readily available – the simple amino acids generated by Miller-type experiments. The fact that the LUCA predominantly used these simple amino acids means it likely evolved from an organism which used these simple amino acids exclusively (or almost exclusively). This is what we would expect if this form of life originated naturally.
In conclusion, the simplest explanation is that life on Earth arose naturally. This explanation should be preferred because of its simplicity, because it is less speculative than “directed panspermia”, and because of the evidence mentioned above concerning the frequency of amino acids used by the last universal common ancestor. Yet directed panspermia is still intriguing. I believe that we cannot completely rule it out, and any rational person ought to admit that it’s a possible explanation for the origin of life on Earth.
REFERENCES
[1] FHC Crick and LE Orgel, “Directed Panspermia” Icarus 19 341-346 (1973)
Accessed 5/14/09
[2] Or nearly universal. There are a handful of slightly different genetic codes which are clearly derived from the standard code.
[5] http://en.wikipedia.org/wiki/Miller_experiment
Accessed 5/18/09
[6] http://www.scientificamerican.com/article.cfm?id=primordial-soup-urey-miller-evolution-experiment-repeated
Accessed 5/18/09
