Inferences about Life in the Universe

Recently, Brendon Brewer posted an interesting piece titled “Life in the Universe” on how the information that life appeared early in Earth’s history affects one’s inferences about whether it is easy to form life or if life is rare. Its an interesting inference problem based on a single piece of evidence. His basic conclusion was:

The basic conclusion is that the early formation of life does provide some evidence that life is “easy to form”, but not enough to convince a skeptic who thinks life is extremely rare.

This has led Brendon to carefully consider Radford Neal‘s article titled “Puzzles of Anthropic Reasoning Resolved Using Full Non-indexical Conditioning” at arXiv:math/0608592 [math.ST]. This is something I will have to read and consider.

In the meantime, I posted a tangential reply, which I repeat (with some edits) below:


Hi Brendon. The questions regarding the origins of life on Earth and life in the Universe are compelling. Perhaps this is a bit off-topic since one of the main aspects of the paper you are considering is how to reason in the face of the anthropic principle and a paucity of information.
Here are some references to other works as well as my half-baked thoughts related to life in the universe.

Robert Zubrin has an excellent article making the case for panspermia

R. Zubrin (2001), JBIS, 54, 262-269 Interstellar Panspermia Reconsidered

where he notes that the fact that life arose in such a short time after the bombardment phase, and that there are no pre-bacterial forms present (along with arguments about time scales required for panspermia and the ability of bacteria to remain dormant in space for tens of millions of years) suggest that life arose elsewhere.

While, David Thomas later notes that much of Zubrin’s argument relies on the missing evidence of pre-bacterial life forms, which may either be represented by the nanobacteria discovered in the last decade (and possibly present in Martian and other meteorites), or by organisms that have not yet been detected on Earth (alive or in the fossil record), I find a few other observations by Zubrin to be compelling.

First, one would expect (and perhaps this expectation is wrong) that it is far more difficult to create a bacterium out of an organic soup than it is to create a multicellular organism out of a colony of single celled creatures. If this expectation is correct, then it is shocking that bacteria appeared on Earth within 300 million years after the bombardment phase, but then took 2 billion years to evolve into eucaryotes with simple plants and animals appearing another 1 billion years later. This could be explained by the fact that at the sub-bacterial level the relevant time scales for replication, experimentation and exploration are rather short, but in the case of plant and animal reproduction, the time scales are much longer slowing the evolutionary rate. Regardless, some prior information here may be of use in performing the inferences intended.

Another consideration is that (aside from viruses, which may be relevant) there is no evidence that abiogenisis has happened here since the formation of the first life forms. It may be that a biotic environment tends to prevent the establishment of any newly abiotically generated life. Despite this, one might expect a dramatically different abiotically generated lifeform to not directly compete with bacteria and establish a foothold. Either way, a biotic environment is simply an environment and both the climactic and biotic aspects of Earth’s environment have gone through dramatic changes over the last 4 billion years. It may be that the fact that abiogenisis has apparently only happened here once might be able to put some reasonable constraints to the time scale at which one would expect abiogenisis to occur in general.

This suggests to me that it is indeed very possible that the abiogenisis characteristic timescale mu is very long. This could mean that Earth was lucky with life evolving very early on, or life arrived at Earth a short time after the bombardment period.

If the abiogenisis characteristic timescale is relatively long, this would imply that the generation of life in the universe is slow. However, the probability of panspermia may actually be appreciable, which means that life would only have to take hold in a few places to populate the galaxy since infection is exponential. For example, Zubrin estimates that if an average star system with a planet like Earth could infect one other planet in another star system every 100 million years (that is 35 planets in 3.5 billion years) and this process was repeated, then there would be 2^35 = 34 billion planetary systems with life. Three to five nucleation points occurring 3.5 billion years ago could conceivable populate a fair portion of the galaxy by now.

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