As promised, we'll get away from the philosophical stuff (as much fun as it's been) and on to some of the current state of abiogenesis. I will borrow some previous articles I've written on Cassandra's Tears, but I'll try to update them as we go. My vacation is over and I've learned I'll be traveling to New York middle of next week for work.
What is abiogenesis?
Well, a long time ago, some scientists proved that abiogenesis didn't happen and creationists have been arguing about it ever since. Francisco Redi showed that flies lay eggs on meat, but meat does not generate maggots. Lazzaro Spallanzani* showed (sort of) that sterile broth doesn't make critters either. Pasteur made Spallanzani's experiment better and basically proved abiogenesis is impossible.
Oh wait. No he didn't. He proved that spontaneous generation of living things from inanimate matter was effectively impossible. This has absolutely nothing to do with abiogenesis. A point lost on 47% of US citizens.
Abiogenesis is the formation of living things from non-living matter, but how is that different from spontaneous generation? Well, Redi and the others were looking at modern organisms appearing without having parents. Mice born from sacks of grain, that sort of thing. Abiogenesis is the idea that the first 'living thing' on the planet was produced via chemical reactions. Once a replicator exists, then it can spam the environment with copies of itself. Presumably, the organism copied itself imperfectly and the idea of species were born.
I'll say up front that, most likely, we will never know exactly how life on this planet came to be. Many of the ideas I'll describe here are not incompatible, so it could have been one method or multiple methods, each contributing a piece to the puzzle. This, of course, is where we get into the determinism vs. contingency discussion again. We won't, but this is where it would go. 🙂
Given that though, it would be trivial to falsify the concept of abiogenesis. If any experiment showed that a critical step was chemically impossible, then the concept of abiogensis would be in serious trouble. However, at this point there are numerous pathways for several major chemical groups, so even if one path were falsified, it is likely we could find an alternate path that would work just as well.
So, what are our options?
The first, and probably the most popular, is the RNA World hypothesis. In general, this hypothesis suggests that because RNA can be both a carrier of genetic information and act as an enzyme to speed up chemical reactions, that RNA came first. This hypothesis is supported by the relative ease by which some nucleic acids are made from non-biologic sources. The famous Miller-Urey experiment didn't form nucleic acids, but then this paper successfully overcomes some of those issues.
Our findings suggest that the prebiotic synthesis of activated pyrimidine
nucleotides should be viewed as predisposed30. This predisposition
would have allowed the synthesis to operate on the early
Earth under geochemical conditions suitable for the assembly
Now, there are some issues with the RNA world. To date (as far as I'm aware) there has only been one case of a self-reproducing RNA developed. It is not, shall we say, elegant. It would be very difficult to form and a pretty exact sequence of nucleotides is needed. Unfortunately, neither of the authors (Tracey Lincoln or Gerald Joyce) see their work continuing along these lines.
The beauty of the RNA world is that it takes care of a lot of the details that we think of when we think of life. Reproduction, evolution, heredity, and chemical metabolism (kinda) are all there. These are the pieces of life. What it lacks is a a couple of critical pieces that are missing are that we haven't built self-replicating RNAs that are simple enough to assemble spontaneously.
Here, I'd like to insert a possibility that is not very popular, but it is relevant. The idea of panspermia. Now, this isn't the older "aliens seeded the Earth with life" notion. But it is known that the conditions in space produce large volumes of organic compounds, including nucleic acids. So, that's another method that sufficient quantities of organic compounds could appear on Earth.
OK, the next big idea is that of the Iron-Sulfur World. This concept is significantly different from that of the RNA world in that this is a 'metabolism first' idea rather than information first idea. Iron sulfide minerals have been found to have some very interesting catalytic properties. The other major difference is that this would take place in the deep ocean, near undersea volcanic pipes. Similar to the Miller-Urey experiment, if you put a bunch of volcanic products in really warm water in the presence of iron and nickel sulfides, you end up with some interesting compounds. Like the methyl thioester of acetic acid and thioacetic acid which are simple analogues of acetyl-CoA.
As far as problems with the Iron-Sulfur world, what it doesn't have is reproduction or heredity. It does have a powerful metabolism and the ability to synthesize a wide variety of organic compounds. So, we're missing a key element, genetics.
There are a number of other hypotheses within this area of research. The clay models, the lipid world, the PAH World, etc. The important thing to note here is that very few of all these different concepts are mutually exclusive. Which leads me to my preferred explanation...
All of them.
Why not? You get the metabolism from iron-sulfur world, the genetics from RNA-world, the long chain molecules from the clay model, and combine them. Hence my thoughts on the contingency of life. What if the Earth was too cool to have volcanoes? Then we would lose a big chunk of the ability to get organic compounds from inorganic sources. There are a lot more 'ifs', 'ands' and 'buts' here. But I think this is a good way to go. Not thinking like a reductionist "this hypothesis is supported" but like a lumper "look at all the hypotheses that are supported".
We have some good ideas for how cells came to be and homochirality too. More on that later.
I know this is a very brief look at these. There is A LOT to explore in this area. I've done some research blogging on a few of the articles I've mentioned here.
If you guys have any questions or concepts you'd like to explore, let me know and I'll try to get some material and put something together for it.
* Spallanzani's name always reminds of me of Dr. Lizardo... 5 internets if you can remember what movie that name is from.