(Permalink) Now this chapter assumes that we passed all the previous conditions, so that we speak of planets with suitable conditions for life and enough time to evolve to civilisation. But first, this chapter seeks to determine the probability of life to actually appear when suitable conditions are gathered. The next will seek to determine the probability that this like evolves to a civilisation, but the two studies are somewhat interlinked.
The timeline of appearance of life on Earth is:
-4.5 billions years: formation of the Earth. The first water appeared only some millions years after the accretion (see next subchapter on the solidification of the Earth crust).
-3.9 billions years: first DNA bacteria (procaryote). Genetic studies tend to show that the DNA system appeared at this moment. Isotopic signatures, graphite and bacteria floor mates confirm this date.
-3.5 billions years: earliest known fossils are the stromatolites, showing DNA bacteria.
So, 600 million years were needed to create the DNA, leading to the first procariote bacteria. This era is called the Hadean age, the first 600 millions years after the accretion of Earth.
Today how life appeared is still a mystery. We still have no idea on how raw chemical elements were able to assemble into complex cells with DNA. There is a huge gap of six hundred millions years in our knowledge, between the formation of Earth and the earliest known DNA bacteria. Worse, there is a huge complexity jump between raw matter and the complex DNA computer of a cell, thousand times more than anything which occurred since (indeed there is not only the DNA, but also the code, the genes marking system, the ribosomes, etc. all elements which had to appear all together). This making this epoch thousands times more creative and mysterious than any further stage of Evolution, in the strange Hadean world where natural nuclear reactors sometimes shone in blue.
It is generally accepted that the first bacteria appear in a few time after suitable conditions exist. This makes very likely that many planets with bacteria exist, and we still have some chances of finding alien bacteria in our own solar system (Mars, Enceladus, Europa...). However we saw that bacteria does not at all make sure that a civilisation appears, since much stricter conditions are required for this. This makes that all the other planet in our system are doomed to remain at the bacteria stage. (this was accounted with in our previous estimate of ne)
A debated theory is panspermia. It starts from the fact that organic molecules has been found about anywhere in space where they can remain stable. Assuming that life came from space gives some magical aura to the origin, but it is not really an explanation, since it just rejects further the origin of life. In reality it is very unlikely that any significant amount of spatial organic matter survived the heat of the accretion, or of meteorite impacts. Not to speak of life appearing in space itself: nowhere in space the conditions are gathered, and by very far.
But organic matter can easily appear on Earth itself, or on any planet with similar conditions, as demonstrated by the Stanley Miller experiment. So that a space origin is not needed at all. As a matter of fact, basic organic matter has been found on Titan and Pluto, where it is cold enough to remain stable. Safe enough evidence that it can appear on any planetary body, and it very likely appeared on Earth in the same way. Only difference is that on Earth it had water to dissolve in, instead of accumulating as dust on Titan or Pluto.
There are today several scientific theories on the appearance of life. It would be tedious and useless here to comment them all, so I arbitrarily select the most likely, in order to find if at least one mandatory path exists. The main theory is that organic matter accumulated into the oceans (or at least some propitious places) into a «primordial soup». This mixtures does not rot, since no life and no oxygen exist yet. It is known, since the Stanley Miller experiment, that natural chemistry into such an environment swiftly creates many molecules which are the basic building blocks of living bodies: amino acids, fatty acids, etc. And still, these experiments were made only with a very reduced set of elements, not including trace elements (salts, phosphorus, iron...) or catalytic elements like clays (which after the link above may have played an essential role).
The difficulty appears when we try to organise this matter into living cells. However many studies shed some patches of light, so that we can attempt to draw a sketchy image.
(Permalink) Stanley Miller-like experiments including phospholipids showed that they can form membranes in a matter of hours (a variant is with protein membranes). These membranes form small pouches of water enclosed in a spherical membrane, called a liposome. These phospholipid membranes are still our today cell membranes. These liposomes are in no way living being, but they can «reproduce» while mechanically breaking in two when they become too big (this can result from waves action, but also from chemical forces). This is important, as it makes a natural selection evolution process possible as soon as organic matter dissolves in water, in a matter of some days, favouring the invasion of the ocean by liposomes with more stable structures!
These liposomes may contain some of the varied molecules of the surrounding water. But if a self-catalyst molecule appears at random, producing a better membrane, then one liposome becomes more stable than the others. So it can «reproduce» as described above, and colonize all the available space, and replace all the others. This is how works the natural selection process. Thanks to it, random accumulation of different self-catalyst systems may lead to further complexity, with each time the new systems replacing the ancient ones. So we get something resembling a bacteria, that scientists call protobiontes or protocells, able of synthesizing its structural materials with enzymes, but still without DNA. We may find this on Europa or Enceladus. This may be very puzzling to identify, without DNA (or with a different system). Probably such beings may have many more catalytic molecules than the simplest DNA bacteria.
Such protobiontes will soon meet a challenge: their cytoplasm is a soup with a lot of different enzymes and a complex chemistry. This situation creates a selective pressure toward any organisation system able of coding for the information on all these molecules, in order to synthesize them only when they are needed. So this led to the formation of the DNA. We have no idea on how it happened, how such a complex structure could appear from nothing. But we can still try some possible steps, among others, for the passage from raw chemistry to DNA-controlled chemistry:
- Catalytic molecules including nucleic acids (basic building block or RNA). They have the catalytic properties on proteins, but without coding for information. These first molecules created a huge evolutionary pressure toward proteins and RNA. Later, some of them could evolve in the ribosomes (molecules able of decoding the DNA or RNA, which still contain triplets of nucleic acids able of recognizing the genetic code).
- RNA existing as many short free floating strands, expressing more or less at random. These already are genes, each as an individual strand. The cells containing them would not have any defined genetic identity, even not a defined specie, instead inheriting at random of such or such strands. RNA viruses and the transfer RNA may be remnants of this epoch (since it was too difficult for the RNA ribosomes to switch to DNA, DNA is transcoded instead).
- Last step being probably passing from RNA with two codons to DNA with three codons. This probably also included the packing of many short random RNA strands into a much more efficient single DNA strand. The first such DNA strand may be the true prime ancestor of all the further beings. Also, the single DNA strand introduced the notions of individual genome, and of specie.
To be noted that the reverse transcriptase (coding RNA into DNA) probably was what allowed the first DNA to get the information from the RNA (and even this primitive transcriptase probably was what transcoded two codons RNA into three codons DNA, a conceptually very difficult and unlikely operation, which could be done only at a special occasion like this one). But after, it went useless for the DNA bacteria, which dropped it. But today some viruses still have the reverse transcriptase.
Other note, the geneticians call LUCA the most ancient common ancestor to all forms of life. We can identify it to the very first DNA strand seen above (although there may be several such prime strands, which appeared and evolved simultaneously, and exchanged genes later). And the genetic studies may have effectively found it. So that the proposed date of -3.9 billions years for LUCA would really be the date of the first individual DNA strand.
Can genetics go farther in time than this first DNA? Technically yes, with applying its methods to the ribosomes, individual genes, molecules, catalysers. However this first DNA appears as a fantastic bottleneck: a single individual was selected, so that the genetic clocks were nearby all reset. So, it would be interesting to search if there would not still be today such beings without DNA, into very special niches. This was supposed by some authors, but is not scientifically proved, see the idea is not considered seriously. Such a discovery would however open a fantastic window on the Hadean, and on the intermediary steps between pure chemistry and the DNA.
Well, the process explained in this sub-chapter is certainly speculative, and it may be false on several points. However it has a huge advantage: it break the lock on the «impossible» appearance of life, not from magical or mysterious processes, but in a series of explainable and likely steps, led by a Darwinian Evolution.
And once this process of DNA evolution started, 3.9 billions years ago at the end of the Hadean age, any arbitrary level of complexity could appear, provided that it has enough time to do so:
(Permalink) Probably the appearance of DNA was bringing so huge advantages that it replaced any previous life forms in a very short time.
Although some scientists speculated that non-DNA life may still exist today in special niches, but not identified as such, by lack of DNA precisely. Its much lesser energy requirements (no need to maintain the DNA) still gives it some Darwinian advantage in places with very low energy resources, like deep rocks. We may also find this on Mars and other niches in our solar system. This challenges the usual definition of life, based on the existence of a DNA system, since these beings don't have any.
Once DNA exists, it seems to evolve naturally into a rich ecosystem with more and more complex and varied beings:
-First DNA bacteria 3.9 billions years ago
-first photosynthesis, 3.5 billions years ago
-About 2 to 2.7 billions years ago, the primitive cells (procaryotes) evolved in more efficient cells containing a nucleus (eucaryotes) and other structures (organites). The procaryotes are still here today, though, but they were unable to take the further steps.
-About 1.5 billions years ago, multicellular beings started to form, ancestors of today fungus, animals and plants. These precursors still exist today: myxomycètes (slime moulds), ciliates, and others.
-However it is not before the Cambrian explosion, 600 millions years ago, that large organized beings appear, simultaneously in the already separated plants and animal lineages.
-The appearance of a nervous system, about 500 millions years ago, led to the appearance of organized behaviour (caring of offspring, nesting, mating, feeding, etc.) and started the fantastic adventure of consciousness (emotions and awareness of the world, see chapter V-2)
-The appearance of intelligence some hundred thousands years ago led to spirituality (mastery of the functioning of the brain, chapter V-10 and chapter V-12) science (reasoning, knowledge of the world) and technology (mastery of environment conditions).
What is striking with these dates is the huge times needed for each step to happen, while the conditions are already present. Highest discrepancy maybe is the today human brain, Einstein's or Gandhi's brain, which exists since 150 thousand years, and seems to awake only today (and yet still tottering in some strange drunkenness). But the huge 3.3 billions years to pass from primitive cells to multicellular beings is a much worse challenge to the probability of finding civilisation on a planet: this planet must be at least as old as that, and in more have constantly maintained very specific conditions during all this time.
There are several reasons for these times, some non biological, that we are to study in this chapter, and some biological, that we shall study in the next chapter VIII-5.
(Permalink) One of the challenges of the evolution of life on Earth is that, in the beginning, the sun was 30% cooler than today. In the future, this heat will increase until life is unsustainable. Since all the stars have their energy output increasing with age, this is a very common problem, not specific to our planet.
This makes that the early Earth would be frozen, not allowing for life to appear. However this obviously did not happened. In a more general way, with the exception of some ice ages, the temperature of Earth remained nearby the same, and anyway within favourable limits.
The reason for this is understood today, it is the regulation of temperature by greenhouse effect of the carbon dioxide. This gas is present within the mantle of Earth (and most probably of all the planets). It is slowly released in the atmosphere by volcanoes. However, in liquid water, this carbon dioxide reacts with alkaline metals released from rocks by the erosion. From here, it forms carbonates (limestone, dolomite) which are relatively insoluble, and accumulate in sometimes very thick layers. If the temperature increases, the limestone formation increases, absorbing more carbon dioxide. If the temperature lowers, this absorption stops, until the volcanoes release enough carbon dioxide in the air to raise again the temperature. This regulation process seems at work on Earth since the beginning, maintaining it in the same temperature range. This, and some other clues, gave rise to the Gaia hypothesis (Earth as a simple living organism able of some basic regulations).
This regulation may also work on many other planets. So that, when statistics on the temperature of planets will be available, we may find an excess of Earth-like temperatures, signing for the existence of rocky planets with water and emerged lands, where such a regulation is operating. Although this is not a direct evidence of life, the concerned planets should receive closer scrutiny in this search. Especially if they are also in the excess of 20% oxygen level seen in chapter VIII-3.
To be noted that this temperature on Earth is anthropic, in the weak sense (chapter IV-6).
A curious consequence of this regulation effect, is that it probably selects the forms of life. Indeed, a carbon based life may be possible at higher temperature, based on different proteins or nucleic acids. Speculative silicon based life forms may be possible at even higher temperature. But with stars like the sun, these life forms would not enjoy the right conditions for long enough for civilisation to appear. Even such bacteria have much less time to evolve, and so we may find much less or even no high temperature life (which we did not counted in ne anyway)
On red dwarves however, the temperature change is much slower, so that the regulation is not needed, and we may expect higher temperature life forms here. Anyway, with worlds evolving quietly since as much as 12 billions years, the far suburbs of our galaxy may conceal very strange things.
(Permalink) The oxygen produced by plants is an indispensable factor for the appearance of brains, and thus for the appearance of consciousness, intelligence, technology, spirituality. The reason is that our neurones need a lot of oxygen, that only breathing a highly oxygenated air can provide. (Actually, our organism could store oxygen under the form of oxydizing compounds, just as it stores fats. But the inconvenience would be that we would literally be walking bombs). Muscles have the same oxygen requirements, and they also needed for technology and communication (speech).
Actually, fishes have brains and muscles functioning with much less oxygen. However, this limitation makes that they work by short impulses, instead of continuously like ours. The state of consciousness of a fish would be like us when we try driving a car while being very sleepy: we have moments of unconsciousness, but startle each time the car begins to shake from running on the side of the road (not a safe experiment to try!). So that clearly fishes cannot work or meditate like us, or even not learn or do engineer work.
So that clearly an oxygen rich atmosphere is needed for any kind of civilization to appear.
Problem is that, on Earth, high enough oxygen appeared only 600 millions years ago, that is only 13% of the age of Earth! Strictly speaking, its appearance was gradual:
-From 3.4 to 2 billions years ago, oxygen produced by algae was absorbed by the oxidation of free iron
-From 2 billions years to 0.6 billions years, oxygen stayed at a relatively low level of some 5%. It is not known why, probably some other chemical reaction was clamping its level at 5%. This ratio may be the signature of such a reaction: oxidising ferrous iron to ferric, oxidizing pyrites or volcanic sulphur, etc.
-Only 0.6 billions years ago, oxygen reached levels comparable to the today 20%. It is at this time that superior animals started to evolve from the multicellular beings which appeared 1 billion years before. This evolution made brain and muscles possible, allowing the further appearance of today civilisation.
The important conclusion to bring of this is that there are two chains of conditions for civilisation to happen:
- The evolution of life itself
- The modification of the planet toward more suitable conditions (oxygen, clean water, food, climate regulation...).
Then civilisations can appear only when both chains of conditions come to a suitable state.
Seeing the timetable of Earth evolution, we may conclude that the absorption of oxygen was the limiting factor, which delayed the Evolution of life for roughly 2 billions years. However we shall see in the next chapter VIII-5 that the Evolution itself also had important limiting factors to overcome, and it also took about the same time. This may even have delayed the solution of the oxygen problem (because non-evolved plants were less efficient at emitting it). We can still safely conclude that, with better conditions than on Earth, planets as young as 3 billions years may also host intelligent civilisations, and thus can be targeted by astronomy and SETI. Unfortunately, it can also happen that the whole process takes much more time. So that we can assume an average of 4.5 billions years (the only value we know).
To be noted that, on a carbon planet, oxygen would be absorbed endlessly, forever delaying the appearance of large multicellular beings. Unless plants save it under the form of oxygen compounds, which then form the main food for animals and conscious beings. This was discussed in chapter VIII-3, and we saw that this solution is also possible, but less probable.
(Permalink) I stated above that life may have appeared very fast after the accretion of Earth. A common prejudice however, even among scientists, is that our Earth would have remained molten for hundred of millions years, some say up to 600 millions years after the formation of Earth. This is called the Hadean age, which ended 3.9 billions years ago, together with the late heavy bombardment by large meteorites. Little remaining rocks of this age are known, contributing to the idea that there were no solidified rocks in this age. The very name, Hades, speaks of an inferno.
I allow myself to frankly disagree with the vision of an ocean of lava lasting for 600 millions years: a planet is not a star, and it cannot produce so much energy for so many time. What we see in lava lakes in volcanoes is very different: a solid crust forms in a matter of hours, quickly bringing the surface temperature bellow the red glow. What happens however, as observed in the Nyiragongo volcano, is that this denser crust is regularly sinking into the molten rock under, bringing again incandescent molten lava in the surface, with a period of some hours.
I state that this is a small scale plate tectonics, that the plate tectonics started in this way on Earth, at this scale of some tens of metres, and that it continued since uninterrupted, just changing progressively of scale, with the increasing thickness of the crust and hardening of the mantle, up to its today thousands kilometres scale.
Indeed if we look at the plate tectonics, especially in subduction zones, what we see is exactly what happens in lava lakes: old cold crust sinks, dragged to the bottom by slab of denser cool rocks, while warm rock recovers it on the surface. There is just a change in scale.
Added on October 31, 2017: We have a recent observational confirmation of this theory, at an intermediate scale between the Nyiragongo and the whole Earth: Astronomers found that the Loki volcano on Io is in fact a huge lava lake, more than 200km wide!! Just as the Nyiragongo lava lake, the molten lava is covered with a thin solid crust, which overturns when it becomes dense enough. The overturning propagates at about 1km per day, and it happens every 2-3 years. Astronomers explicitly compare this to a superfast version of the spreading of a mid ocean ridge on Earth.
Earth cooling time can be estimated: After the Stefan-Boltzmann law, Earth entirely at 1200°C, assuming a 800J/Kg°C specific heat and 5.97x1024kg mass, would require only 1228 years to cool off! (This is only the starting speed, though. Actual cooling would take more time, but still in the same order of magnitude). Buffon, in the very first astrophysics simulation, found 70,000 years, extrapolating from heated iron balls. So in any case this is much less than 600 millions years.
However such an ultra-fast cooling is certainly not what happened, as the interior of Earth is still hot today. What certainly happened is that a crust formed readily, in a similarly short time of some years, and that at once this first crust immediately insulated Earth and avoided its interior to cool off. This is the process leading to the formation of a solid crust over yellow hot rocks. Today newly formed oceanic crust takes its full thickness in only some millions years. A similar time is logically to be expected for the first Earth crust.
What controls the speed of plate tectonics is the fluidity of the mantle material where it is taking place. More fluid, and the speed is high, bringing a strong cooling, which in turn brings an augmentation of the viscosity. Less fluid, and the movements slow down, until the heat produced at the bottom heats the rock and makes it more fluid again. So that it is ultimately the heat produced which determines the fluidity of the mantle. This fluidity in turn controls the size of the convection cells, and the quantity of cold crust this mantle can bear (avoid it to sink). This makes that a strong heat production will allow for a thin crust, fluid mantle, and a fast turnover, like in a volcano lava lake. A lesser heat flow allows for a thicker crust, viscous mantle, and a slow turnover. In both cases anyway, the heat brought at the surface is exactly the heat produced in the depth, so that there is no temperature change anywhere. It is a dynamic equilibrium.
When Earth formed, it was very hot and very fluid, from the accretion heat. So the crust was very thin, with probably a plate tectonics at a hundred metres scale and a turnover of some hours. But if so, the heat radiated was many orders of magnitude larger than today, bringing a strong cooling of the whole mantle.
So the formation heat was evacuated very fast, thousands or millions times faster than today where heating and cooling are at equilibrium. A rock at 1000°C emits 250 times more energy than at room temperature. And there was a billion time more lava surface exposed than today. This made that the fast plate tectonics of the beginning quickly vented the accretion heat off. In doing so, it quickly made the mantle more viscous, and the convection went on larger scales, and the crust thicker and thicker. This makes that Earth had a solid and cool enough crust to bear a stable ocean, maybe only thousands years after the formation, certainly in less than some million years after. Exact figure can be found only with a global simulation, thought.
Also, once the accretion heat released, which was quickly complete, the heat production rate was not much larger than today: only twice from uranium, and less (zero) from core solidification. Aluminium 26, the main heat source in asteroids, was the only heat source able of heating the Earth significantly more than today, and it may have delayed the formation of an ocean. But only some millions years, over 600 millions. So that the Earth of the Hadean age quickly took its today appearance of a cool oceanic planet, with basaltic volcanoes here and there, maybe only 10 millions years after the final accretion. Volcanic islands appeared soon, in patches or in arcs, looking like today Azores, Japan or Aleutian. (Large continents appeared only later, from a different process) These islands in turn allowed for water erosion, which in turn dissolved alkaline metals, allowing for the C02 greenhouse temperature regulation. So that in only some millions years in the Hadean, we had blue sky and rain, with temperatures similar to today. Just the sun was less fierce, as in a today end of winter afternoon.
Also, the craters of the late heavy bombardment were certainly large but this was far not enough to erase all the crust of the Earth! The ones on the Moon (Aitken basin, 2500kms) or Mercury (Mare Caloris, 1550kms) were barely able to punch through the crust into the mantle. As to the ocean, we can imagine a 100kms high tsunami, but even so it would not be felt by bacteria into the water.
In passing, it makes ridiculous the common idea of prehistory men in a volcanic landscape. Even in the Hadean there was not much more volcanoes than today.
(Permalink) The oldest known minerals found to date (2011) are zircones 4.35 billions years old (possibly 4.4). Zircones are components of trachyte/granite rocks. And these rocks can form only in lava chambers several kilometres deep, which require a thick enough and cool enough crust for them to become solid at such a depth. So, these zircones show that Earth already had a thick crust at the beginning of the Hadean, 4.35 billions years ago, only 180 million years after the accretion! This thickness is comparable to its today thickness, well consistent with the similar heat production. We are very far from a molten surface... More, such a thick crust was perfectly able to bear water and oceans, well before these rocks formed. This ocean may have started only some millions years after the accretion, and it, just like today, much contributed to a thick and cold crust, through hydrothermal circulation.
But even further, granites form from subduction magmas, themselves forming from water soaked ocean silts, just like today. (This is consistent with the (debated) water found in the same zircones). This tells that these silts formed in a deep ocean, several more tens of millions years sooner, still repelling the date of the first proven rains, at worse 100 millions years after the accretion.
So the date of the oldest rocks known today (4.35 billions years) does not mark the solidification of Earth crust, which occurred much sooner. It tells the date where started the differentiation process of the continental rocks of the granite family, when the basaltic crust became thick and cool enough to allow for their formation in deep lava chambers. This differentiation process took its full strength later, forming our continental mass (although it may be still going on today at a much smaller rate).
We may chance on even more ancient granitic Hadean rocks, but in small and scarce deposits, deeply buried into the continents. The older, the rarer and smaller deposits. But probably the most ancient rocks are from the basaltic or ophiolitic families. They may be already known, but not properly dated. Identifying and dating such rocks is usually very difficult, as most of the time they were deeply altered by further processes. Only zircones are known to withstand these processes, but they are not present in basalts and ophiolites.
A possible definitive evidence of water however would be the detection of nuclear wastes of early natural nuclear reactors. These can happen only when water erosion of granite concentrates uranium-bearing minerals, and water moderates the neutrons. On the early Earth, with 24% of U235, even very small uranium mines would ignite easily, while it is impossible today. We shall probably never find direct traces of such reactors, blurred by folding and metamorphism. But finding isotopic traces (nuclear wastes) may be possible, telling for sure the date of the first water erosion of granite.
The interesting conclusion of this is that nearby the entire 600 millions years of the Hadean were available for the evolution of life from raw matter to DNA cells (which appeared in the end). This makes the appearance of life much less magical and mysterious, as the chemical data processes of the cell nucleus had all this time to appear.
(Permalink) This chapter and the next are about biological factors.
We can assess now the 4th factor fl (fraction of suitable planets where life actually appears). The value is quite close to 1, since, once the conditions are gathered, the chemical processes which lead to primitive cells seems mandatory. And it happens in a relatively short time (Less than 600 millions years on Earth). Only on planets undergoing radical catastrophes this process is stopped, but this was accounted in ne. On Earth, even the late heavy bombardment did not stopped it.
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