Friday, February 26, 2016

Symbiosis and the Origin of Life

Everyone who works on the origin of life seems to be concentrating on how to get that first cell to form. There are many good suggestions, but as of yet, no experiment that forms one. Maybe the process takes too long. Or maybe that's not the right concept.

In a recent post, the essentials of origination were laid out in clear steps. Basically, there has to be a first something that replicates, and a way for it to get energy and building blocks, the predecessor chemicals that the replicating thing, like a molecule, makes into a copy of itself. At the end of the post, it was discussed that maybe it was a two-stage process, with some molecule or loosely bound group of them that made one component of what another molecule or loosely bound group of them needed. The first one was upstream, and the second one was downstream. Or they both inhabited the same pore of rock face and shared what they received and produced. Perhaps the second one made copies of both the first and second.

This is essentially symbiosis on a chemical level. Symbiosis usually has meant two things living together, like lichen being algae and fungus. The algae are photosynthetic, and produce energy for the combination, and the fungus anchors the combination to some surface. Maybe that is an idea that originated in the chemical world of the deep sea vents. One chemical is good at absorbing energy from the outflux of the sea vent, and another one is good at adhering to a rock surface and holding on to the energy chemical. They are not part of the same molecule, but they are somehow locked together geometrically.

If two molecules can fit together, then neither one has to be able to do everything. They have to be able to exchange energy, or some chemical component, at least in a one-way transfer. In another post it was noted how adhering to a surface in a flow greatly increases the available chemicals. A molecule floating in the stream has a short interval to grab onto whatever it needs, but something attached to a wall with the flow passing it can just wait and grab and wait some more and grab some more.

How might one molecule prevent another molecule of a different type from being dragged away by the flow of the current. If it were shaped like a cage, it might do that. The cage has to have porous walls so that energy-carrying chemicals could get inside, and also there could be no complete interference with the provisioning of chemicals needed to replicate the two molecules.

The type of replication would not have to be the same. The cage-like molecule could just add on holes by extending whatever passes for walls in this molecule, and every once in a while, a piece, capable of extension with the right provisioning, chemical energy source and conditions breaks off and adheres somewhere else. The other one, which might just be a molecule which is metastable, taking energy from some simple chemicals in the flow and then absorbing energy, later to give it to the cage molecule, could be able to reproduce itself or have the cage molecule do it. If there are strong chemical similarities between the cage chemical and the other chemical, there could be some joint mechanism whereby a copy of the second chemical is formed.

Lichens are land creatures but there are other examples of symbiosis. Humans, like most animals, are symbiotic with bacteria living in the intestine. They perform various chemical tasks for us, and in return are provided a stable environment, mostly, and a flow of nutrients. A bacteria is immensely more complicated than a molecule which is metastable, but the basic concept of one molecule providing the stable environment and flow of nutrient-containing water is analogous.

One of the big difficulties of figuring out the origin of life is that there are no simple creatures left. Somehow whatever process that originated life is no longer making the very simple things that it used to. Everything seems to have gotten complicated by a few billion years of evolution. Until recently, it was thought that sea sponges were simple creatures, perhaps the fundamental multicelled creature. Then, more examination of them showed they are rather complicated creatures, and their genome is of the same order of complexity, measured by number of genes, as most of the rest of the living things on planet Earth.

One way to express that is to say that any living organisms with DNA coding its genetic structure, gets complicated and there has been enough time for virtually everything we can find to get complicated. No one at any time has found some simple cell, with practically no detailed features, whose cell membrane was encoded by only a short stretch of DNA. Not too long ago, the microbiological community decided that microbes should be divided into two kingdoms, instead of just one. One is the old bacteria, and the newer one was named archaea. Perhaps the name, archaea, was meant to indicate that these creatures were the predecessors of the other kingdoms, but it turned out, they were not.

Some very clever and interesting chemical analysis was done of the cell membrane of archaea, and they were found to have the middle section with the opposite twist of the molecule comprising it that bacteria and higher organisms. It seems like the primitive precursor of everything went along and invented, via evolution, cell membranes, but not one, but twice, the two versions being enantionmers. The two enantiomers would operate the same, and this indicates that there is one good way to make a cell membrane, that works better than any other form, and it just happened that two super-primitive molecular structures came upon this idea, in stereoisomeric fashion.

If it is possible to originate life via a series of simple steps, why is it not being originated now, and producing wonderful evidence showing each step of the way. Perhaps the more developed things just eat the primitive, newly evolved and newly originated stuff, so that absolutely nothing can be found. In other words, nowhere on the world is free from predatory organisms that feast on these primitive structures that would otherwise have begun, all over again, to evolve. Or maybe there is another explanation.

No comments:

Post a Comment