I would also point out to anyone tempted by the “earth is special” arguments that earth is actually NOT optimized for our kind of complex life. Most organisms on earth can exist only in very limited regions of the planet, and would quickly die miserably if transported anywhere else. Earth is also too close to the sun, sitting on the inner edge of the habitable zone instead of the middle. As a result, our planet will go the way of Venus in 0.5 to 1.0 billion years. If it had been a little further out, it would have been habitable for several billion years more.

It isn’t even certain that we are orbiting the right kind of star. Our sun will only spend 10 billion years on the main sequence, and its gradual warming will cook us long before then. M-dwarfs don’t warm with time, and will live ten times longer.

We simply don’t know enough about any of the variables to say anything intelligent on this subject. All we can say is that the conditions here allow for life to occur. We might end up being in the one of the least optimized environments for life, for all we know.

“However, science sees this as a process that can be recurrent.”

There is no evidence that it can be recurrent. Scientists have not been able to reproduce abiogenesis in the laboratory, nor has life been proven to exist anywhere except on Earth.

“By your own assumption of uniqueness you can conclude that it is unique.”

No, you are misconstruing what I am saying. I do NOT assume that life is unique on Earth (indeed I would be very, very happy if that was not the case). I am simply saying that there is NO convincing evidence to the contrary. Those are really two vastly different positions.

“We can still tell that abiogenesis is easy; easier than complex life forms such as ourselves. (Instead anthropicity is, ironically, probably luck.)”

Of course, complex life forms have to follow abiogenesis in order of time. Other than timing I don’t know what evidence you are suggesting which tells us which part of the process was easy and which part of the process was difficult. If process A took twice as long as process B then was process A necessarily twice as difficult as process B?

“Your argument was that several locales that “should” have life has not.”

Again, complete misconstrue – I claim there is NO evidence that those locales have life, not that I have evidence that they don’t. You do see the difference there, don’t you?

“It was based on a statistical sample of a vast biosphere under billions of years. Notably all life stems from a single LUCA that obeys evolution. Where are the vast amount of contenders that is expected if other biological processes are more efficient – or more likely, why aren’t we all such life?”

A vast biosphere but all descended from one LUCA. I believe you’ve answered your own question – due to the Founder Effect in many ways all of life on Earth is just one data point, or as Carl Sagan might say, one voice in the Cosmic Fugue.

“I see that some suggest you bone up on statistical processes. Maybe that is a good idea.”

Yes, that comment came from someone who thought 35,000 data points was a statistically small sample. LOL.

@ Al Viro:

references?

The Last Universal Common Ancestor: emergence, constitution and genetic legacy of an elusive forerunner (Biology Direct, July 2008):

LUCA does not appear to have been a simple, primitive, hyperthermophilic prokaryote but rather a complex community of protoeukaryotes with a RNA genome, adapted to a broad range of moderate temperatures, genetically redundant, morphologically and metabolically diverse. LUCA’s genetic redundancy predicts loss of paralogous gene copies in divergent lineages to be a significant source of phylogenetic anomalies, i.e. instances where a protein tree departs from the SSU-rRNA genealogy; consequently, horizontal gene transfer may not have the rampant character assumed by many. Examining membrane lipids suggest LUCA had sn1,2 ester fatty acid lipids from which Archaea emerged from the outset as thermophilic by “thermoreduction,” with a new type of membrane, composed of sn2,3 ether isoprenoid lipids; this occurred without major enzymatic reconversion. Bacteria emerged by reductive evolution from LUCA and some lineages further acquired extreme thermophily by convergent evolution.

“The methods used in to root the tree are vulnerable to and with the present data we can’t even exclude the possibility of Eukarya being paraphyletic wrt Archea and Eubacteria” != “we think that Eukarya *is* paraphyletic wrt Archea and Eubacteria”,

Thanks, now I had to get paraphyly finally.

Yes, that claim is true if I understand correctly, but the authors seem to have some actual support for their ideas rather than claiming a possibility from undecidable data.

not to mention any statements about nucleus being present in the root node…

They present several potential nodes, but seem to prefer a RNA LUCA with a nucleus:

If LUCA had a RNA genome and DNA synthesis was invented twice [11,60], we could consider the possibility that a membraneous compartment (let us call it a protonucleus) had already formed autogenously around the primeval RNA genome, so that the nucleus itself would not necessarily have emerged twice independently. Such compartmentation could very early have played a capital role in protecting RNA, in ensuring its correct partition at cell division and in separating replication and editing from protein synthesis. An interesting model [61] suggests that proteins of nuclear pores and coat vesicles (thus components of the endocytotic apparatus) could have been formed endogenously from defined protein structural modules. This model makes the emergence of the nucleus much less of a mystery than before and links nucleogenesis to the emergence of phagocytosis (see further). Moreover, as pointed out by P. Forterre in his comments on this paper, RNA “nuclei” still exist today since RNA viruses recruit host membranes elements to form compartments in which their replication apparatus is surrounded by one or two membrane layers with an opercule for communication with the cytoplasm [62].

It seems that, on the whole, the model of a protoeukaryotic RNA LUCA is in keeping with current evidence. In particular, the antiquity of an already complex spliceosomal mechanism, appearing to have evolved before the last ancestor of living eukaryotes, is not easy to reconcile with eukaryogenesis by merging of prokaryotes. Rather, the LUCA itself may have been that ancestor, already endowed with the forerunner of the eukaryotic nucleus.

The occurrence of nucleus-like structures in some Planctomycetes, in Poribacteria (and perhaps some Archaea as well [27] is a striking feature that must be accounted for in evolutionary scenarios centered on LUCA. The possibility of early endogenous nucleogenesis by a rather straightforward mechanism [61] places the origin of the Planctomycete nucleus in a new perspective. The nuclear body of Gemmata, with its double membrane and its pores, is presently the closest approximation of a eukaryotic nucleus outside of its traditional Domain. There may be a relationship between this eukaryotic-like structure and the capacity for sterol biosynthesis, a feature of Planctomycetes [[66] and next section]. It is however not known whether these structures are really homologous nor is it known whether TT-coupling is the rule in this bacterial group; moreover, there are ribosomes in both the cytoplasm and in the Gemmata nuclear body.

Not being a biologist, I would like to hear any comments on the survey and its results, especially as regards “nucleus-like” vs “nucleus”. Also, the authors rely partly on papers which I’ve heard biologists elsewhere note as using unusual or unsound cladistic methods, such as forcing constraints. On the hand it is AFAIU permissible in other fields, where you want to identify possible, or enforce known, constraints and not just rely on “natural” solutions.

@ Tom Marking:

With a statistical sample size of one you can not make sweeping conclusions.

True. However, science sees this as a process that can be recurrent. By your own assumption of uniqueness you can conclude that it is unique. By science assumption of an abiogenesis process that can happen if the environment is habitable, it follows that the sample tells them about the process at some likelihood.

There is also the Anthropic Principle at work here: if life had started very late on the earth, say 4 billion years after the Earth formed, there would not be enough time for complex lifeforms such as ourselves to evolve in which case we wouldn’t be here huddled around the intertoobs debating this subject.

Again true, by those numbers, but we are discussing tens of hundreds of million years against delays of billions. After multicellularity led to body plans (happened independently several times), complex life forms hit on us after half a billion years. We can still tell that abiogenesis is easy; easier than complex life forms such as ourselves. (Instead anthropicity is, ironically, probably luck.)

If you or your astrobiologist associates know of such conclusive evidence for the existence of extraterrestrial life then please provide it.

Your argument was that several locales that “should” have life has not. If you claim no evidence you have no argument.

(To spell it out in detail, the difference is that astrobiologists doesn’t claim that all volumes of space has life, they don’t even base any science on assuming that any body except Earth is known to harbor life. They are trying to find out.]

I believe you are suggesting that life requires complex organic chemistry. I wonder what that is based on.

Read my previous comment for such a base; several in fact.

Again, this is based on a statistical sample of 1 planet.

It was based on a statistical sample of a vast biosphere under billions of years. Notably all life stems from a single LUCA that obeys evolution. Where are the vast amount of contenders that is expected if other biological processes are more efficient – or more likely, why aren’t we all such life?

Measuring a stochastic process is not the same as making a random measurement! I see that some suggest you bone up on statistical processes. Maybe that is a good idea.

Yes, but the Earth’s rotation period is 23h:56m:04s — equatorial rotation velocity: 1674.396 km/h — whereas Mercury’s rotation period is 58.646 (Earth) days — equatorial rotation velocity: 10.892 km/h — and Venus’ rotation period is 243.16 (Earth) days — equatorial rotation velocity: 6.52 km/h. Maybe that has something to do with plate tectonics — more rapid flexing of the Earth’s crust due to tidal force interaction from the Moon.

I must respectfully disagree on that point. As someone has already pointed out (it may have been John) the sun also produces tides on the earth which are about half the size of the tides produced by the moon. Since tidal effects go as the inverse cube of the distance then that means the solar tides on the inner planets are something like:

Venus: ~1.5 times Earth’s lunar tides
Mercury: ~8 times Earth’s lunar tides

If the lunar tides are strong enough to cause plate tectonics on Earth then the solar tides are definitely strong enough to cause plate tectonics on Venus and especially Mercury. Yet there is no evidence that either Venus or Mercury has plate tectonics.

GAry 7

The point is that there is no “narrow range of mass/distance relationship” for the Earth-Moon system since the distance has varied by over a factor of 30 and life has continued throughout that range of distances (unless you want to call a dynamic range of 16dB narrow )

Concerning the Roche limit, I always remember it as being 1.4 times planet radius but maybe that’s from some outdated textbooks. In any case I wasn’t particularly trying to model that in my program so just disregard all distances you consider inside the Roche limit.