The LHB itself does seem very likely to have occurred. The lunar cratering record (with radioisotope rock dating of melted forms) strongly supports a peak in large impacts at this time. There is also an excellent explanation for the LHB from the study of how the orbital architecture of the solar system came to be the way it is. Specifically, at an early stage (about 4 Gyr ago), it seems that there was a reconfiguration of the outer planets (Neptune, Uranus) during which time Neptune in particular underwent strong gravitational interaction with all the remnant planetesimals/asteroids/dwarf planets in the outer solar system. As a consequence Neptune’s orbit expanded, and much of this remnant material of smaller bodies (part of which is now the Kuiper belt) was scattered inwards, with some ending up pelting the Earth-Moon system. This also, incidently, seems to explain the different internal structures of the Jovian moons Ganymede and Callisto – because of heating via large impacts.

The Moon formed earlier, about 4.53 Gyr ago, and the model in which it is formed from the debris following the collision of the young Earth with a Mars-sized proto-planet is still very much the best bet – it neatly explains many things, including the relatively high angular momentum of the Earth-Moon system.

The RNA world idea is also quite compelling, and you’re right, it might well have been the stage at which a core planetary gene set would have begun to ‘assemble’ – although presumably the metabolic processes in such a world might be rather different ?

Do we have any information, or even informed speculation, on whether life dates back to the Late Heavy Bombardment? Or does the LHB itself fall into the category of informed speculation rather than established fact? The reason I ask is that I had understood the Moon’s formation to be known to stem from an impact event. Yet lately I hear that this is possible but not proven.

Another thought: What about the notion that early life was RNA based? Would the existence of a core planetary gene set not be written in RNA, more than in DNA, if the RNA emergence theory was true?

E.O. Wilson wrote about the blossoming of knowledge that occurs when 2 previously separate scientific disciplines meet, in Consilience. Astrobiology sounds like an example of such a meeting.

I’m told Theobald will be blogging on his paper at Panda’s Thumb one of these days real soon now.

http://www.physorg.com/news192882557.html

But according to Theobold, his findings do not preclude the possibility of multiple independent starts to life on Earth.

From the link:
“Let’s say life originated independently multiple times, which UCA allows is possible,” said Theobald.

… we don’t actually know whether the great domains of bacteria, archaea and eukarya (to which we belong) all have a single common ancestor, right now there isn’t enough information to join those final dots.

Time (and research) marches on: we know more about that this week than we did last week when that sentence was written. See Doug Theobald’s new paper in Nature. Doug will be blogging about it on Panda’s Thumb very soon, and Nick Matzke already has a summary up there.

Thanks to Caleb, and Sean for hosting!

My semi-informed speculation is that microbial life is definitely out there and common in the universe. The building blocks of life are found all over, and the physical laws that governed the evolution of life on this planet appear to be the same throughout the cosmos.

It’s a gargantuan visible universe… which may be just an infinitesimally small part of something much larger, or even infinite. In a Cosmos so large and so old, perhaps infinitely old – everything that can happen has and will happen… probably a gazillion times. In an infinitely large universe, even an infinite number of times!

The speculation that there are other intelligent species on other planets is the ultimate application of the Copernican Principle/Principle of Mediocrity. As a layperson, my knowledge is far from complete, but that we are no alone makes sense. It also makes sense however, bearing in mind interstellar distances, that it may be many centuries, if at all before we obtain conclusive proof of ETI. The mean distances between civilisations may be huge!

Astrobiology as you describe it – as a label for (mostly) “traditionally” trained scientists whose work has bearing on the question of life beyond Earth – makes much more sense to me then a completely separate discipline.

So, where am I going with this ? I think it’s all going to hinge on exactly how rich a chemical mix ends up on the surface (or in the subsurface) of a potential harbor like a terrestrial planet (other environments could work too). Rich and complex enough and then at the very earliest stages of self-organizing behavior I could imagine multiple styles of life arising, ultimately ending up in layers of symbiotic and endosymbiotic (fully merged) relationships – to the point where it’s impossible to disentangle the ancestral patterns. I’m hedging my bets – I’d say that very early on then independent beginnings might indeed have occurred, but that these are likely to merge and erase evidence for that history.

Could a separate start to life have occurred later on – after recognizable things like bacteria were already around ? Maybe, but I’d think it’d still have the same RNA mix, or something similar, at its root, I also think it would have a hard time finding a free niche – unless it got very specialized – *or* quickly figured out how to join in the biochemical party with everyone else.

I agree that the details have big implications for whether life is a common or rare phenomenon in the universe.

So the question which interests me most right now is whether life on Earth began only once or whether it had at least two completely independent beginnings. If all our diverse biospheres evolved from the same beginning, then life may be a very rare occurrence despite its ability to evolve and adapt once initiated. I hope I have made myself clear, and I would welcome input from Caleb or anyone else.

What is deep about this (specific) truth? Well, it’s the result of extensive research by many, many people, it’s not an answer that just fell out of the sky, it’s a carefully considered, examined, partially tested, idea that no one had come to just on the basis of applying evolutionary theory and/or horizontal gene transfer. In retrospect I completely agree that it makes a lot of sense…most of the best scientific ideas do, with hindsight.

As for poor old (new ?) astrobiology. Well, it may surprise you that I actually agree that ‘astrobiology’ is not so much a unique discipline, but that it’s a convenient way of labeling an array of research done in different classical disciplines, all of which nonetheless is tackling the common questions of ‘life in the universe’. For example, I do not call myself an astrobiologist – I’m an astrophysicist by trade, who happens to work on exoplanets and the questions relating to how we can go look for life beyond the Earth. One of the reasons we use the term ‘astrobiology’ is because historically it has been hard to get people in different classical scientific disciplines to talk to each other, beyond casual bar room fare. Consider it an experiment to attach this label in the hope that people feel less embarrassed to admit that they’re interested in ‘aliens’. The fact is though that within the classical disciplines (and yes, under the label of astrobiology) there have been amazing advances in understanding extremophilic organisms on Earth – often motivated *precisely* because of what might be happening on Mars or Europa or elsewhere. In astronomy the big drive is to find the terrestrial type planets (NASA’s Kepler mission is designed to provide a statistical measurement of how many such planets exist) – why ? Because the idea of finding a planet like the Earth, that could harbor life is so compelling and interesting. Does this need to be called astrobiology ? No, but that is what it is….

Caleb: “A truly wonderful paper was published a couple of years ago in which Falkowski, Fenchel and Delong laid out the big picture for life on Earth. In essence, they argue that single-celled microbial life is the manifestation of an even deeper truth; the core planetary gene set. This is the set of recipes for metabolism, or how to harvest a planet for energy, and we all rely on them.”

What exactly is so deep about this truth? It seems like a pretty straightforward consequence of the theory of evolution and abundance of horizontal gene transfer.

And why call it “the core planetary gene set”? To make it sound more astrobiologish? The adjective planetary makes little sense, we don’t know for example lunar or comet gene sets, yes, the genes in question are specific to Earth but they are not specific to planets in general.
The “core” also makes no sense, what core? There are plenty of alternative routes to generate energy, there is no core set of such genes present in every organism.
Finally we should distinguish the gene set in question from many others not involved in harvesting energy like those who carry out anabolism or replication.
Taking all this into account a much better description would be “the metabolic gene set.”