Review: Life might be rare despite its early emergence on Earth – a Bayesian analysis of the probability of abiogenesis

For the 8th Astronomy Twitter Journal Club we tackled a recent paper that attempted a Bayesian analysis of the probability of abiogenesis.

The paper was suggested by Matt Burleigh. He thought it would be a good paper to discuss as it “has been getting a lot of publicity for an unrefereed paper…”.

You can also read our preview and view the full unedited Tweet archive.

On to the discussion.

Note: some of the tweets were sent without the #astroJC hashtag – so if you attended the meeting some of the details in this review may be new to you!

Bayesian Analysis

The discussion was slow to get going as we realised there wasn’t a Bayesian statiscian in attendance! Fortunately a late comer saved the day:

Sorry #astrojc, late. Glanced at paper. Priors on the Bayesian analysis seem to be reasonable, but the model is overstating the case basically, unless you have all of model space covered (volume of universe, definition of life) stats won’t tell you much. – LeonBaloo

When dealing with so many unknowns, Bayes=magic. Any personal bias is numerated 🙂 I’m more of a frequentist usually- LeonBaloo

But as we only have one data point (that life exists on Earth):

with Bayesian statistics one data point is not going to lead to a strong constraint! – astronomyjc

basically they point out that we need a second data point – life on mars, an exoplanet or another kind of life on Earth – Matt_Burleigh

If you want to learn more about Bayes’ Theorem:

For you #astrojc‘ers @telescoper has written a nice blog on Bayes’ Theorem here: http://bit.ly/oidovV – LeonBaloo

How the analysis differs from the Drake Equation

The Drake Eqn has too many free parameters to even make a rough guess. This paper makes the rough guess first #astrojc – LeonBaloo

It’s saying that (fig 1) we’re heavily influenced by what we assume but (fig 2) finding life on mars increases probability of life being common very little, but NOT finding life on Mars decreases the probability a lot! – LeonBaloo

DNA/RNA based life and Panspermia

Question:

If we find DNA based life elsewhere in Solar System, does that count as a second data point or not? Thoughts? – kashfarooq

Answers/Discussion:

My first thought would be ‘panspermia’ – petehague

It seems like it should. Problem is the paper wants proof that life arose *independently* there, so panspermia not good – astronomyjc

I think finding DNA (contamination ruled out) elsewhere would be the strangest and most unbelievable observation. Ever. – Harrison_Peter

DNA based on Mars or an exoplanet would be very interesting though. Raises possibility life didnt arise on Earth – Matt_Burleigh

But it probably restricts us to only be able to say “Life happened in the Solar System” – kashfarooq

I think it would provoke a right old argument. But makes life in wider galaxy more common, surely? After all, if life arose on Mars and travelled here on meteorites, could it travel to other solar systems? We’ve had plenty of close encounters with other stars in Sun’s lifetime – Matt_Burleigh

Too big a distance, surely? – kashfarooq

No, pioneers will reach pass star in that direction in 40,000 years or so – Matt_Burleigh

Question:

Say DNA life found on Mars or Europa. Where did it originate? In one place or independently? – Matt_Burleigh

Answers/Discussion:

In one place, surely? – kashfarooq

How would you know? – Matt_Burleigh

I guess you’d look for DNA sequences & plot evolutionary background. Need an evolutionary biologist @harrison_peter?  – kashfarooq

Tough one. I mean, if it was like DNA or literally was DNA would make a big difference. I think that’s just far too weird. If the DNA in the organisms was similar in it’s arrangements (think genomes) to what we have, I’d say that’s evidence pointing to panspermia – harrison_peter

Question

Would you be able to place the organism in the tree of life if it were DNA based? – kashfarooq

Answers

Well, that might be too hard if the event happened very early. But if cellular mechanisms are the same for example, it’s fishy. But I think DNA itself would be fishy and I’d put money on panspermia or contamination. If aliens visited us from millions of light-years away and also had DNA (like ET lol) then I’d eat my hat twice – Harrison_Peter

DNA may be ace but I bet there are other ways to do something we might call life. – @jaclong

Are there a large number of possibilities other than RNA and DNA? – Matt_Burleigh

More on this subject of DNA at the end of this review.

Non-DNA based life

I spoke to an astrobioligist recently – he would prefer to find non-DNA based life. As that makes life more likely elsewhere in the Universe. – kashfarooq

Responses:

But how then do you define ‘life’ – astronomyjc

Are there a large number of possibilities other than RNA and DNA? – Matt_Burleigh

More on this subject of DNA at the end of this review.

Overall thoughts on the paper

I liked the paper! It at least shows a discrepancy in the models on the basis of posthulated information. Makes nice prediction for finding life on the basis of discovery/non-discovery of life on Mars – LeonBaloo

I think it’s interesting — my feeling is that we still don’t understand any of the priors. Bayesian guessing = still guessing! – DrMRFrancis

This Bayesian conclusion was at least suspected by mathematical astrobiologists. Problem is that no-one has explained this to astrobio community, which is v. multidisciplinary w/ varied maths skills – dh4gan

In no way should this be read as “proof life is rare” – Matt_Burleigh

Correct. It should be read as “Most parameters are unconstrained” – just like Drake, but more subtle. Of course, whisper “aliens” and the press are all over you – that’s been my experience. – dh4gan

More on DNA

I’m going to end this review with #astroJC tweets that were tweeted from Harrison_Peter after the meeting was over (he has studied evolutionary biology and I thought it would be a good way to conclude this discussion):

I’d just like to add that DNA has a history. If we find organisms with DNA we can see how they use it, how their genetics work etc. It’s not just morphology that’s shows examples of seemingly intelligent design, and also unintelligent design. Our genetic systems have workarounds simply to fix the flawed design. What organisms can do with DNA in order to replicate is fantastic, but still has many flaws. History on earth has left us with DNA, with genetic systems, and with organisms with many unique quirks due to evolution. If conditions on earth had been different, life would have been different.

So finding anything the SAME should be suspicious.

As for the question; what other options are there besides our DNA and RNA? This could be like considering possible languages of advanced aliens and asking, what possible languages could they speak besides Earth languages like English and French? If we find them elsewhere, with all the same quirks, mistakes, solutions, and results because of generations of gradual evolution, I could only respond that something creepily improbable is happening.

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