mollwollfumble said:
mollwollfumble said:
I’ve now hand written my best idea for the origin of life.
But this keyboard is too small to type it.
Twelve and a half hand-written pages long. I suppose I’d better start typing.
This is most of it. Enough to get a good overview.
Origin of Life
The two predominant hypotheses about the origin of life on Earth are hydrothermal vents and panspermia. But there is an alternative, Miller-Urey’s primordial soup.
Let’s start at the beginning, the origin of the solar system. When the proto-Earth was forming, the solar system was full of hydrogen gas. It’s the drag of this gas that forced all the planets to orbit in the same plane and circularised their orbits. Why circular in the same plane? Because that’s the configuration that minimises energy for given momentum, gas drag causes and energy loss but doesn’t cause a loss of momentum. All this is well known.
So the proto-Earth’s primordial atmnosphere, because it was in equilibrium with the interplanetary gas, must have been dominated by hydrogen, like the atmospheres of Jupiter and Saturn today. Jupiter and Saturn were able to hold onto their primordial atmospheres. Earth was not.
The genes of the last universal common ancestor (LUCA) of all extant life have been partially determined. Some genes show that LUCA used molecular hydrogen as an energy source, so we’re on the right track.
The hydrogen-dominated atmosphere of Jupiter contains large amounts of methane CH4, ammonia NH3 and water H2O. The uppermost clouds are white, with a transition in turn to yellow, red and finally black down through the atmosphere. The black layer was revealed when the linear comet Schumaker-Levy 9 (SL9) slammed into Jupiter. This may seem to be a digression, but the colour sequence clear to yellow to red to black was also observed in the Miller-Urey experiment as the liquid darkened oiver months. It’s a sign of the abiological creation of largish organic molecules.
It is also well known that the Miller-Urey experiment produced a primordial soup containing amino aacids and and othger simple organic molecules. A recent re-analysis of Miller’s original broth found more amino acids and more simple organic molecules.
There are two things about the Miller-Urey original experiments that are not well known. One is that the resulting broth was hydrolysed before analysis. Hydrolysing destroys bio-organic polymers including proteins, reducing them back to amino acids, and other polymers such as complex carbohydrates if any were present.
A second less-known fact about the original Miller-Urey experiment is that a semi-solid goo built up on the electrodes. This would prove to be important more than 20 years later.
The immediate reaction to Miller-Urey was howls of protest from geologists. Methane and hydrogen, they insisted, did not exist in the atmosphere of primitive Earth. I have already explained that the geologists were wrong. Methane and hydrogen must have existed in the atmosphere of proto-Earth. I have not yet explained why the geologists of that era were wrong. They were wrong because they had no way of knowing, back then, exactly how the Earth lost its primordial atmosphere.
Because of the protests by geologists, the immediate follow up experiments to Miller-Urey used CO or CO2 in the atmosphere, or used a weaker energy source such as UV light. As a result, for the next 20 years or so, published reruns of Miller-Urey produced a lower yield of amino acids and other organic molecules.
Another experiment from that era produced some nucleic acids from formaldehyde and hydrogen cyanide. Other experiments produced mixed sugars and lipids.
In one early paper it was calculated that the ocean of proto-Earth could have been covered by organic molecules in a layer of thickness 20 metres.
In a little known paper of 1973-4, the Miller-Urey experiment was redone with atmospheric concentrations optimised. On analysis of the goo around the electrodes under an electron microscope, enclosed structures were found that were the size of bacteria and had bi-layer membranes. Contamination was ruled out, and other structures of similar size were open or had one or three surface layers. I consider this paper to be of extreme importance as it means that bacterial envelopes and membranes could have developed long before such complicated molecules as RNA. Unfortunately, the paper only did a cursory chemical analysis of the bilayer membrane, enough to confirm that it was composed of organic molecules.
The sea surface of the proto-Earth would be stirred by convection. It would have polymerisation by evaporation at the top and collect surfactants such as certain types of proteins at the bottom. The top would collect medium-sized organic molecules produced in the atmosphere by lightning.
The proto-Earth had much more volcanic activity than present day Earth, so the ocean bottom would have had the combination of high temperatures and catalyst minerals for producing large organic molecules from water-soluble precursors. The ocean margins would have played a role as well, with rapid hydration-evaporation cycles.
At this stage, the proto-Earth had lipids, protein-like chemicals containing amino acids, membranes equivalent to those found in bacteria, some nucleic acids, surface-active organic chemicals, and quite probably some large organo-metallic compounds, and aromatic compounds.
While the proto-Earth still had a reducing atmosphere, the Sun’s increasing UV light cleared the hydrogen out of interplanetary space, and following that the ptoto-Earth’s atmosphere slowly lost hydrogen. As the hydrogen content of the atmosphere fell, the new molecules that formed started to have a higher ratio of oxygen to hydrogen. The sugars and phosphates formed.
The above is hard solid fact. What follows is speculation.
I speculate that life as we know it developed while while proto-Earth still had a significant molecular hydrogen content in its atmosphere. This most easily explains why the last universal common ancestor (LUCA) of all extant life used molecular hydrogen as an energy source. As the hydrogen dissipated, also life first learnt to get energy by eating abiologically produced sugars. This process slowly improved in efficiency to become what we now know as respiration. Once eating sugars was established, life forms learnt to polymerise these sugars into complex carbohydrates for energy storage for later, to survive food shortages.
Energy storage using phosphates is more subtle than from carbohydrates. So further down the time line, early life forms produced sugar-phosphate polymers as an energy storage for when times were tough. These sugar-phosphate polymers evolved into RNA by the addition of nucleic acids.
RNA turned out to have excellent catalyst properties. Some turned out to be able to catalyse its own production and some to catalyse protein production. Life as we know it had well and truly arrived.
Recently, the simple chemical di-ammonium phosphate, with just 13 atoms, has been found to catalyse both RNA polymerisation and DNA polymerisation.
What is totally uncertain about the above speculation is the timescale.
The Earth has suffered a large number of extinction events in its history and, as a general rule, the further back in time the more severe the extinction event for life on Earth. The Permian-Triassic extinction event was severe. Before that snowball Earth, where the oceans were frozen solid down to the bottom of the ocean even on the equator, was more severe. We know for a fact that life on Earth survived both the Permian-Triassic and snowball Earth extinction events. Before snowball Earth, the “late heavy bombardment” event was even more severe.
Many people have speculated that life on Earth could not have survived the late heavy bombardment of Earth by asteroids. The late heavy bombardment caused the resurfacing of the near side of the Moon, the lunar mares. I think life could have survived the late heavy bombardment era. For starters, it wasn’t a single event but a large number of events over many millions of years, and life would have had a chance to recover after each one. The worst one would have caused a lava province say 3,000 km wide, with associated volcanic gases and climate change. The far side of the crust would have been largely geologically unaffected. Unicellular life could have survived that.
Before the era of late heavy bombardment was the impact that formed the Moon. I need to talk about this in some detail. All of the interplanetary atmosphere had been blown out of this part of the solar system before the Moon formed. The strongest evidence for this is the lack of xenon in the Earth’s present atmosphere.
The second piece of evidence that the hydrogen had by then vanished from this part of interplanetary space is the existence of the Moon. If the interplanetary gas had still been around at that time then the drag from that gas on the fragments would have caused them to spiral back to Earth rather than coalesce into the Moon.
The impact that formed the Moon was neither a central impact nor a glancing blow, but half way in between. Half of the Earth was pulverised. The rest remained on one piece but was horribly distorted and mostly melted.
Earth’s atmosphere was jolted loose and driven out of the solar system by the solar wind and ionising radiation.
Could life on proto-Earth have survived such an incredible catastrophe? The knee-jerk reaction is “no”, and this is what led geologists to strongly protest about the Miller-Urey proposal back when it was first published, but they were thinking about the destruction of life by a central impact. I agree that if the impact that formed the Moon had been central then life could not have survived, but because the Moon exists we know that the impact was not central, and that leaves two ways in which life might have survived.
Suppose we consider life in a thick oily carbon-rich mat opposite the pulverised side of Earth. Then such a mat would have been able to float on liquid rock. The underside of the mat would have charred like the heat shield of a re-entering Apollo capsule, forming a superb thermal insulator. That thermal insulator could have protected the life metres above the liquid lava until the lava had cooled to basalt.
The other way would be if the thick oily carbon-rich mat was on the side of the Earth that was pulverised. Big slabs of living matter would have been thrown into space. And come back down days or weeks later. Even now, some bacteria can survive the vacuum of space for that length of time.
But wait! Wouldn’t the life from proto-Earth have then been deposited on the Moon? Yes and no. Because the Moon formed at the same time, life would have been deposited ‘in’ the Moon, and been destroyed be being encased at depth in molten rock for a long time. On the surface of the Moon, life from proto-Earth would have been present in a concentration of only about 1 part in 100,000, and even then would largely have been destroyed by volcanic action. Life from proto-Earth would have been deposited ‘on’ the Earth but ‘in’ the Moon.
etc.
