May 25 2009

RNA World

The origins of life on earth remains a daunting scientific challenge. The difficulty is in trying to find evidence to infer what chemical reactions took place billions of years ago. There may ultimately be no way to settle the issue, but that does not mean the question cannot be addressed scientifically.

Of course, the enemies of science (creationists and their ilk) exploit this fact to argue that science cannot understand life origins, and therefore we must invoke supernatural explanations. They often further confuse the question of life origins with evolution – the subsequent change in life over time.

Despite the claims of creationists, there actually is a rigorous scientific discipline exploring questions surrounding the first stirrings of life on earth. Recently researchers took an important, if incremental, step in understanding how non-life became life.

Powner, Gerland, and Sutherland published a paper in which they explore how RNA (ribonucleic acid) could have arisen on the early Earth.

For background, the question that is being addressed is how chemical processes alone could have given rise to a system that could reproduce itself – a system we call life. By purely chemical processes this means we cannot invoke enzymes to catalyze reactions or any process associated even with the simplest cells – just the chemical themselves floating around in the “primordial soup”, or perhaps drying on clay beds or some other plausible early Earth environment.

One of the leading contenders for the first self-replicating molecule is RNA, which still serves as a fundamental basis for the genetics of life. Many viruses, for example, use only RNA. RNA is like a single-strand version of DNA, and even moderns plants and animals still use RNA as part of the genetic cycle – shuffling information from the DNA to the protein factories in the cells, for example.

Once you have a strand of RNA you are a long way toward creating life. You still need the other components of a simple cell – the RNA has to direct proteins to manufacture themselves and other proteins. But that is a subject of other research. This current research focusses just on how you get to RNA itself.

The problem with the RNA hypothesis of life origins is that we don’t know how the chemical reactions necessary to make RNA could happen spontaneously – without the infrastructure of life already in place. From the recent article:

In particular, although there has been some success demonstrating that ‘activated’ ribonucleotides can polymerize to form RNA, it is far fromobvious howsuch ribonucleotides could have formed from their constituent parts (ribose and nucleobases). Ribose is difficult to form selectively, and the addition of nucleobases to ribose is inefficient in the case of purines10 and does not occur at all in the case of the canonical pyrimidines.

In other words, we don’t know how some of the reactions necessary to make RNA could happen. What the researchers set out to do was explore possible chemical pathways that could spontaneously lead to RNA in “prebiotically plausible conditions.” They did not set out to determine what did happen 4 billion years ago on Earth, but what could have happened. Part of the point of this research is to remove the objection from the RNA hypothesis that it is no possible or plausible.

Their results were encouraging. Here is their conclusion:

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 sequence. Although the issue of temporally separated supplies of glycolaldehyde and glyceraldehyde remains a problem, a number of situations could have arisen that would result in the conditions of heating and progressive dehydration followed by cooling, rehydration and ultraviolet irradiation.

The chemical details are spelled out in the paper, for those who are interested (there is no link to the full paper, as you need a subscription to download it, but I will give the full reference below).

What this all means is that it is plausible that RNA could have arisen in a prebiotic environment  – without the machinery of life already in place.

Researchers are slowly putting the pieces of life origins together – mainly by exploring what plausibly could have happened. The argument from creationists that life arising from non-life is not possible was never credible – it was nothing more than confusing unexplained with unexplainable. They would have thrown up their hands and given up on this question, so that they could just assume a supernatural explanation.

But dogged scientists see unexplained not as an excuse to give up, but as a challenge. While there is still a long way to go, they are making progress in answering one of the toughest scientific questions out there – what chemical reactions led to life on Earth.

Reference:
Matthew W. Powner, Beatrice Gerland & John D. Sutherland; Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions. Vol 459| 14 May 2009| doi:10.1038/nature08013

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