Now that we have life, this chapter seeks to determine the probability of this life to evolve toward civilization, or the time required to do so.
(To be noted that, in the chapter VIII-3, in order to determine ne, we used the result of this one, more precisely an estimate of the needed time, based on the only real data we have: 4.5 billions years for Earth.)
We note that huge delays were necessary for some steps of the evolution of life. This is, of course, because the necessary mutations have very little chances to happen. However they happened, at least on Earth. This series of very unlikely mutations induced many to think that we just were extremely lucky, like winning many time in a row to the lottery, and that intelligent life would be rare. I say that, on the contrary, these mutations were mandatory, mechanically bound to happen, and also bound to happen in about the same sequence and delays in any other planet with suitable conditions, and even in different condition than Earth. And I demonstrate this, in this chapter, thanks to a biological model of evolution that we have at home, and which exists in so many instances that we can easily make statistics (and many were done, so that I just have to draw the conclusions). To the contrary of planets, where we have only one instance (which may even not be representative of the average).
(Permalink) I point out that (this is my personal contribution to the puzzle, and a rather important one) we have a model which closely resembles the Evolution of life on a planet. It is not a nice model, but we see it at work daily, and it regularly produces the same deadly results: the evolution of a cancer.
It is well known today that cancers are lineages of cells (monoclonal) which started from a genetic mutation in a single cell. But further, cancers need several mutations to become invasive and deadly. The starter mutation will allow the cell to escape the control of the organism; but several others are needed, so that the tumour becomes able to build its own blood vessels, or that its cells become able to «walk» around or travel into the blood stream, in order to form metastases. It also needs several other mutations to escape the constant attempts of the immune system to eradicate it. So that this is really a Darwinian evolution process! At a much more modest scale admittedly, but bound to reproduce the same statistical laws that the larger Evolution of life.
Each of these mutations is very unlikely, but the fact is that they happen regularly. Why? How? Simply by brute force: if a mutation has one odd over a billion to happen, it becomes statistically mandatory, once the tumour also reaches one billion cells. If even the tumour has only one million cells, the mutation will still appear at random on one person over a thousand! This still makes millions of instances, in a billions population. This is the way things would work in cancers. This is just the well known laws of probabilities: winning the prize in a lottery is very unlikely for a single individual, but there are so many players that each draw gets a certainty to see several winners.
Let us discuss this in more details.
Cancers do not appear abruptly. They often start with proto-cancers: small clump of cells, smaller than a millimetre, containing some cells to some millions. It even seems that we have much more proto-cancers than actual cancers, but most of them never evolve in a deadly disease. A good example is the naevus, the well known benign skin tumour which often disappears spontaneously (when the immune system finds a way to do so). But it is sometimes the start of a deadly melanome. We clearly have here a two stages process leading from a sane cell to a deadly cancer. (Happily, only a very small proportion of naevuses evolve into a cancer, so that we do not really need to bother about them.) So, the immune system is usually able to contain these proto-cancers, and even to kill them. However if one of these cells finds the good recipe to thwart the immune system, or to create metastases, then this proto-cancer will grow out of control until it kills the person.
And these phenomena will take place at varied time scales. If ten years are necessary for a proto-cancer to reach the billion cells size which make a propitious mutation likely, then this will be the average maturation stage of this kind of tumour. These maturation times are, statistically, fairly constant, just depending on the type of cancer. These times range between 5 to 50 years, depending on the type of cancer (the fastest being radioactivity-induced leukaemia. Some are so slow that they appear only in old age, and never reach the final stage).
What is important to understand here, is that when a cancer reaches a given size, the weak probability for an individual cell to mutate leads to a certainty to have this mutation to happen somewhere into the large number of cells, just as to get a lottery winner in a large enough town becomes a certainty. And if lottery winners could bear children who would also be lottery winners, then they would be a new race and they would replace the poor people in only some generations. Similarly, when a cell in a cancer becomes a «lottery winner», meaning that it found a way to escape the immune system, or to be more invasive, then this lineage soon replaces the older cells, and it takes the lead of the growth and spreading.
This is just standard probability laws, applied to the natural selection process: As unlikely an event can be, it WILL happen in a large enough population, or with enough time. If the event is very rare, it will just require a much longer time to happen. This time is just the invert proportion of the probability for it to happen.
The comparison between something as horrible as a cancer, and something as wonderful as life on a planet, is certainly shocking on an aesthetic point of view (If somebody finds a better model, thanks to make it known). But it is very relevant on a mathematical point of view, as both processes obey to very similar statistical laws.
The interesting conclusion of this model is that we can bring the delays needed for the different steps of evolution to a probability for this step to take place. We observe very different delays on Earth, from some hours (liposomes) to hundreds millions years. Much more improbable events, requiring for instance 50 billions years, will still happen in 5 billions years on a tenth of the planets with suitable conditions. This makes still certain to find them somewhere in space. On a SETI perspective, they are just 2.15 times farther, or 13 decibels lower in the radio signals. Not a lot.
On the events which actually happened on Earth, we can conclude that they were forced to happen, and within the observed delays. And on other planets, they will also happen with the same statistical certainty, and in comparable delays.
So yes, the Evolution on Earth won 20 times in a row in the lottery. But with such a huge number of drawings, it was statistically certain to win each time.
(Permalink) I think we can complete the Darwinian theory of evolution with this principle: any event will happen, even very improbable ones. It is just a matter of enough time, or enough tries (number of individuals on a planet, or number of planets). More exactly, the population (or number of tries, or number of generations) multiplied by the probability of this thing to happen. If this value becomes larger than one, then the result is certain to happen.
So let us apply this to the timeline of Evolution on Earth:
- 4.5 billions years ago: first life chemistry
- (unknown date): first catalytic molecules
- (unknown date): first RNA molecules coding for one gene each
- 3.9 billions years ago: DNA system complete, first bacteria
- 2.7 to 2 billions years ago: first eucaryotes (modern cells)
- 1.5 billions years ago: first multicellular beings.
- 0.6 billions years ago: oxygen revolution, large organized beings appear.
- 0.5 billions years ago: first nervous systems and brain, consciousness. Life starts to colonise emerged lands.
- 0.2 billions years ago: mammals, flowers
- Today: intelligence, allowing for spirituality (mastery of our emotions and functioning of the brain, chapter V-10 and chapter V-12) science (reasoning, knowledge of the world) and technology (mastery of environment conditions). This is the SETI definition of civilization.
We note that the Evolution advanced at a relatively regular pace, save at some occasions:
-Passing from the procaryotes to the eucaryotes.
This happened thanks to endosymbiosis, when a bacteria enters in another, but is not destroyed, so that both benefit of the association. Our modern cells show the traces or many such events, where the absorbed cells became organelles in the host cells. The most important are the chloroplasts and the mitochondria. Thanks to them, our modern eucaryote cells have much better performances and capacities, as of a modern organised factory compared to artisans workshops.
Symbiosis is relatively common, but to make of it a genetically single being requires that genes of one of the two partner passes to the other partner. But as soon as only one gene is transferred, the two cells become a single entity, and separation is no longer possible, and they evolve together. But even after two billions years, the transfer is not complete, and our today mitochondria and chloroplasts still have a plasmid (circular strand of DNA) containing some of the genes of the original organism.
So that we can safely infer that such gene transfers are very rare, explaining the long delay of 1.5 to 2 billions years between the procaryotes and the eucariotes. Modern plants also needed 2.9 billions years to incorporate the cyanobacteria which became the chroroplasts. This is how the lineage of the plants was separated from the lineage of fungi and animals.
-Passing from monocellular beings to multicellular beings.
The most evolved monocellular beings face a considerable problem: they need to change their chemistry to adapt to changing conditions. Some even ended to gather larger genomes than an Human! Having several cell types performing different functions has interesting advantages. However this requires considerable reorganisation of the genome.
That is, something complicated, needing to change a lot of genes in a single operation. So, just as for the appearance of the birds (chapter IV-6), the process certainly decomposed into several logical steps, each making the next possible. So, we should find a quantity of such intermediate beings, for each step. And actually, not only we find them, but in more many are still existing today, allowing to write a precise History:
1) Cells must be able to exchange chemical messages. A cell encountering a specific condition emits a substance, and the receiver cells react by expressing a corresponding set of genes to perform a given task. The oldest bacteria can do this.
2) Cells must be able to move. This is a common occurrence in some bacteria, and many are able to move all together for food. This is still how modern mushrooms are able of growing much faster than plants. In the today human bodies, we still have several cell types which have to crawl from their origin place to their functional places.
3) Cells must specialise in different types. This process is called cell differentiation, and it requires a complex set of genes and control zones allowing for the cell to activate and deactivate vast parts of the genome and change its functioning. In the beginning, these activations and deactivation are reversible, but as we enter further in this system, the advantages of reversibility are lost to more specialized and exquisite cell types. Today human cells do not naturally show reversion, but it can be induced in the laboratory (for creating stem cells).
4) Cells must place themselves in precise positions the one to the other. This alone probably needed several sub-steps, the main being the appearance of a set of genes reacting to a gradient of an hormone. The first discovered such system is the homeobox, a chain of adjacent genes. It emits a substance from the future head of the early undifferentiated embryo. This substance then diffuses all along the embryo, reaching different concentrations. Then all the cells of the embryo react to the concentration it detects, activating each gene of the homeobox, in the order they are on the chromosome. Then each of these genes starts the formation of a body part: head, arms, thorax, abdomen, legs, tail. Then other homeoboxes create the more detailed shapes of bones, muscles, nerve paths, etc.
We imagine that such a fantastic system is complex, that there are several, and they could work properly only after many trials, probably needing several hundred millions years, explaining that modern organized bodies took so long to appear.
Actually on Earth several appeared, giving each a different body organisation: bilaterals (us), 5-fold symmetry (starfishes), and some others.
The simple conclusion is that the extra-long delay of the appearance of multicellular beings decomposes in facts in several delays of ordinary duration, where the Evolution of the genomes was still advancing at a constant speed.
Hence the name of «invisible steps» for these reorganisations of the genome without important modifications of the body shapes.
The oxygen lock
We see the result of 1 or 2 billions years of evolution toward multicellular beings, appearing abruptly 0.6 billions years ago, in an event called the Cambrian explosion. This happened when the appearance of enough oxygen allowed for much larger organisms to grow and make fossils. However the coincidence of these two events is suspicious, and I think that all the life forms which appeared «suddenly» at the Cambrian explosion were already existing several hundred millions years before, but that they were too small to make detectable fossils. The only remains are the genetic traces of evolution which happened before this event, and they actually show a much more progressive story. Probably the overall evolution of life just remained stuck at this stage, by lack of oxygen.
The role of the thermophiles
The farther we go in the past of life, the more we meet bacteria able of withstanding high temperatures, sometimes as hot as boiling water. This makes that today (2015) a common hypothesis is that the first life forms appeared in warmer oasis, such as volcanic springs. This makes sense, as they were the only source of energy and food at this time (before photosynthesis). But this situation may have slowed the Evolution, in two ways:
-scarcity of inhabitable places and resources
-need to switch to lower temperature but more efficient enzymes.
Data management in the genome?
There may be some other locks to Evolution, in the very structure of the genome. Or on the contrary something suddenly made Evolution faster:
-System for correction of DNA errors, allowing the requested reliability for the building of complex bodies
-Deliberate mutations, as seen in some bacteria, and still existing in our immune system. Although it is not clear in which extend this can accelerate Evolution, or on the contrary make it uselessly run in circles.
-The nanomachines which reproduce the DNA code would have improved their reliability. In doing so, the cells became able of storing much more data with enough reliability to form a complex body (monocellular bacteria, even complex, do not need such a reliability). This may have allowed for the multicellular revolution to start.
-A recent hypothesis is the discovery of an upper layer to the DNA code, allowing for a kind of data compression: proteins are no more formed from a single gene, but from a set of basic building blocks requiring much less data. However these proteins require a complex set of instructions (the «introns») and several other nanomachines, in order to assemble a working protein from the building blocks. The evolution of such a complex system may have required several invisible steps (without apparent changes of the organisms) explaining the extra-long time required for multicellular organisms. And the human genome would be really by far the most complex, much more than any other bacteria, but coded in MP3 instead of WAV.
Added in May 2023: These control sequences that I supposed were positively identified. Article. Source: Corld Spring Harbor Laboratory.
My preferred guess is that the multicellular revolution happened much sooner than the Cambrian explosion, and in several progressive steps, as of any other Evolution. Thus it took longer, only due to its complexity and the required number of invisible steps involved: many more steps to pass from a simple bacteria to the earthworm, than passing from the earthworm to the Human! So this evolution was not very visible, as most of these steps were abstract, about the organisation of the genome, rather than just adding visible body functions.
So that this invisible evolution in facts happened at the same pace as the other visible steps of evolution! And the first multicellular beings could appear only when all these data management steps were complete, 1 billion years ago.
And the sudden appearance of large beings, 600 millions years ago, would not be a direct consequence of this evolution, but of the sudden availability of oxygen. So there would not be a radiation (appearance of multiple lineages in the same time) 600 millions years ago, but much sooner, and the famous Burguess shales would in fact show only an isolated snapshot, near the end of the story, due to very lucky conditions here, a very rare fossilisation of soft bodies. Much more ancient fossil could exist, showing similar beings, but much harder to recognise because of their smaller size.
(Permalink) We saw in chapter IV-6, on the example of the birds, how an «impossible» evolution could indeed happen, by serendipitous small steps. So, not only the «impossible» flight appeared, but in more it appeared several times: Insects, pterodactyls, birds, bats, and it is still taking place under our very eyes today: «flying» squirrels, flying fishes!
The lesson is that the evolution-selection process transforms a very unlikely series of events into a mandatory series of events, leading to mandatory results. So, the same can go for the whole evolution of life on a planet, from liposomes to intelligent creatures. Once the suitable physical and chemical conditions are gathered, the process starts, and it continues as long as these conditions are maintained, until the mandatory result appears: intelligent beings and civilisation. And it mechanically does the same thing on all the planet with suitable conditions.
So, not only civilisation appears on all the planets with suitable conditions, but in more it does so in similar timeline, from organic chemistry to civilisation.
A second more subtle lesson of the cancer comparison comes from the fact that we have a virtually infinite number of mutations able to lead to a cancer. So each cancer should be different. However, we only have some tens of types of cancer. This is extraordinary. Similarly, despite the variety of possible planetary conditions and composition, we may expect to have only a relatively small number of biosphere types, of genetic systems, basic organism organisation, etc. For instance the genetic code may result from a small set of efficient catalytic reactions. If so, an extraterrestrial genetic code may look very similar to ours.
(This does not imply that cross-breed would be possible, as this would require an exact match of not only the code, but also of the mapping of all the genes into the chromosomes. Without communication, this is just impossible. But still, grafting extraterrestrial genes on Humans would be possible).
Conversely, the bilateral organisation, see humanoid, should be the most common, without however excluding others.
(Permalink) What we see above is that the evolution of the genome (including its formation during the Hadean age) seems to advance at a relatively regular pace of average 0.2 billions years per major step, including several such steps when an «invisible» reorganisation of the genome is needed.
However this walk is occasionally slowed down by scarcer events:
-endosymbiosis, which seems to occur at a pace of 2 billions years.
-the oxygenation event, which timeline is determined by the chemical conditions on the planet. On Earth it took 3 billions years.
These special events look like slowing the Evolution, as they happen less often. However they often unlocked situations where the sole Evolution of the genes was leading nowhere.
So, these statistic considerations show that Earth is very far of being an exception, or even a very lucky case. Of course, depending on local conditions, a planet may need 3 billions years, or 6 billion years, instead of our 4.5 billions, to obtain the same result. But we cannot expect planets where life evolved in very different time frames, like 100 millions years, or 40 billions years.
To be noted that the today appearance of psychoeducation (the capacity to bend the material working of our brains to the requirements of our consciousness, chapter V-10 and chapter V-12) may also unlock the evolution of our civilisation. But this does not yet count in the evaluation of the 5th factor fi of the Drake equation, where intelligent life and civilisation simply appears, without prejudice of what happens next.
(Permalink) This chapter and the previous are about the biological factors.
The 5th factor fi is the fraction of planets with life, where intelligence appears.
The conclusion of this analysis of Evolution is that this evolution is statistically forced to advance at a regular pace, so that intelligence is bound to appear, and at first glance we are forced to give a value of 1 to fi.
However this step takes a lot of time. The full timeline of the Evolution of the genome, without locking events, was about 3 billions years on Earth. Since this delay results from many steps with a statistically forced duration, we can assume that the total is relatively constant. However the locking events can bring other delays, so that these 3 billions years are in practice a minimum. We have no idea of the larger bound, though, but they result from harder and harder conditions (less water, more iron...) so that increasing values become more rare. Anyway these hard conditions were accounted in ne.
Practical conclusion is that we can start to search for intelligent life in systems older than 3 billions years. No upper limit is provided, as civilisation, or even life itself, may be appearing just today in 10 billions years old systems.
Strictly speaking, the delays are not accounted with in the Drake equation, which assumes a constant rate of formation of the stars. But we know today that this is not true: 5 to 10 billions years ago, the rate of star formation was tens of times higher. This is why it is important to deal with the age of the stars, what the Drake equation does not. So that, strictly speaking, when counting possible civilisations we are forced to apply a correction factor to fi, accounting with the age repartition of the stars. I do not have such data at hand, and in more it depends of the place in the Galaxy. So that I do a rough estimate:
fi = 0.5 (in short, to account with the too young stars where the Evolution is still in progress). This is a pessimistic bound, in the halo of our galaxy the value is much closer from 1.
In practice SETI needs to search only in stars older than about 3 billions years.
These very high value for fl and fi, together with the high ratio of planets in the Goldilock zone, is what prompted many scientists in statements about extraterrestrial life being very common, and even civilisation being widespread. We saw in the previous discussion on ne that some pessimistic factors are reducing the number of planets with suitable conditions for civilisations, but nevertheless it remains high enough to fully justify the search, either by astronomy of by SETI. Our closest neighbour must be at less than 100 light years, and there could be millions of them in our galaxy.
Note: these figures are only about planets having started a civilisation. Planets with only bacteria could be much more numerous.
So that the biological factors are the most optimistic, after all.
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