Читать книгу IN THE BEGINNING - Welby Thomas Cox Jr. - Страница 14

But not everyone agrees that life began in deep sea hydrothermal systems. Armen Mulkidjanian at the University of Osnabruck in Germany says there are several big problems with the idea, one being the relative sodium and potassium ion concentrations found in seawater compared to cells.

Mulkidjanian invokes what he calls the chemistry conservation principle – once established in any environment, organisms will retain and evolve mechanisms to protect their fundamental biochemical architecture. He says therefore it makes no sense for cells that contain 10 times more potassium than sodium to have their origins in seawater, which has 40 times more sodium than potassium. His assumption is that protocells must have evolved in an environment with more potassium than sodium, only developing ion pumps to remove unwanted sodium when their environment changed.

Mulkidjanian thinks life could have sprung from geothermal systems, such as the Siberian Kamchatka geothermal fields in the Russian Far East. ‘We started to look for where we could find conditions with more potassium than sodium and the only things that we found were geothermal systems, particularly where you have vapor coming out of the earth,’ he explains. It is only pools created from vapor vents that have more potassium than sodium; those formed from geothermal liquid vents still have more sodium than potassium. A handful of such system exist today, in Italy, the US and Japan, but Mulkidjanian suggests that on the hotter early earth you would expect many more.

David Deamer of the University of California Santa Cruz in the US has been studying macromolecules and lipid membranes for over 50 years. He comes to the field from a slightly different angle, which some have called ‘membrane first’. But, he says, ‘I’m pretty sure that the best way to understand the origin of life is to realize that it is a system of molecules all of which work together, just as they do in today’s life.’ The location ‘comes down to a plausibility judgement on my part’, he muses.

One of the biggest arguments against a deep-sea origin is the fact that so many macromolecules are found in biology. DNA, RNA, proteins, and lipids are all polymers and form via condensation reactions. ’You need a fluctuating environment which is sometimes wet and sometimes dry – a wet period so that the components mix and interact and then a dry period so that water is removed and these components can form a polymer,’ says Mulkidjanian. ‘There is no way for this kind of a thing to happen in [a deep sea] hydrothermal vent because you cannot have wet–dry cycles there,’ adds Deamer. Wet and dry cycling occurs every day on continental hydrothermal fields. This allows for concentration of reactants as well as polymerization.

The assumption that natural selection is incapable over 4 billion years of coming up with an improvement I think is mad (WTC)

Deamer has been trying to create his own protocells in the lab – by mixing lipids and RNA components adenosine monophosphate and uridine monophosphate. When dried, the lipids self-assemble into membrane-like structures, and if nucleotides are trapped between lipid layers, they will undergo esterification to produce RNA-like polymers. Over multiple wet–dry cycles the yield increases to 50%.6

Deamer has confirmed the presence of these polymers inside the ‘protocells’ by direct RNA sequencing techniques. ‘We really do have single-stranded molecules that are in the size range of biological RNA,’ but Deamer cautions that it is not RNA as it is in a biological organism. He created a mixture of RNA, some with phosphate groups bonded as they are in nature, but some bonded ‘unnaturally’, which he concludes then ‘must have been subject to selection and evolution in these little protocells’.

But the deep-sea hydrothermal vent camp is not ready to throw in the towel just yet. Barge says the vent environment could allow for concentration of reactants and condensation reactions. ‘You have gels all over the sea floor, you have minerals that absorb things and in the [chimney micropore] membrane itself there are gels, so you can have dehydrating reaction conditions even though the whole system is aqueous.’

Lane also rebuffs the idea that potassium or sodium ion levels might fix future metabolic processes. ‘The assumption that natural selection is incapable over 4 billion years of coming up with an improvement I think is mad,’ explains Lane. ‘In my view, selection drives intracellular ion balance.’ He thinks life would have been quite capable of evolving in a sodium-rich environment and over time developing the ion removal pumps that create the current potassium-rich cells.