All Astronautical Evolution posts in 2017:

Scenario Block Diagram Analysis of the Galactic Evolution of Life (Nov.)

Comments by Alex Tolley (Oct.)

Elon Musk’s “Great Martian” (Oct.)

Elon Musk’s Mars Plans: Highlights from His Second Iteration (Sept.)

What is a Supercivilisation? (Aug.)

Quantifying the Assumptions Behind the METI Debate (July)

Five Principles of a Sustainable Manned Mars Programme (June)

Pale Red Dot: Mars comes to Oxford (May)

Back to 2016:

Elon Musk and Mars: Looking for a Snowball Effect (Oct.)

New in 2020:

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AE posts:

2022: What’s to do on Mars?…

2021: New space company Planetopolis…

2020: Cruising in Space…

2019: The Doomsday Fallacy, SpaceX successes…

2018: I, Starship, atheism versus religion, the Copernican principle…

2017: Mars, Supercivilisations, METI…

2016: Stragegic goal for manned spaceflight…

2015: The Pluto Controversy, Mars, SETI…

2014: Skylon, the Great Space Debate, exponential growth, the Fermi “paradox”…

2013: Manned spaceflight, sustainability, the Singularity, Voyager 1, philosophy, ET…

2012: Bulgakov vs. Clarke, starships, the Doomsday Argument…

2011: Manned spaceflight, evolution, worldships, battle for the future…

2010: Views on progress, the Great Sociology Dust-Up…

Chronological index

Subject index

General essays:

Index to essaysincluding:

Talk presented to students at the International Space University, May 2016

Basic concepts of Astronautical Evolution

Options for Growth and Sustainability

Mars on the Interstellar Roadmap (2015)

The Great Sociology Debate (2011)

Building Selenopolis (2008)


Issue 139, 30 November 2017 – 48th Apollo Anniversary Year

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Scenario Block Diagram Analysis of the Galactic Evolution of Life

Presented to the BIS Fermi Paradox Symposium, 28 Nov. 2017


The current ignorance of science concerning the abundance and nature of life on a galactic scale allows a number of possible scenarios to exist. These may be organised using Scenario Block Diagram Analysis. Some of these scenarios produce a Fermi paradox, others do not.

The missing factor in the Drake equation

The alleged Fermi “paradox” has been an evergreen source of speculation for half a century. The purpose of this symposium is “to review the facts as they appear at the moment and to discuss their implications”, and this is what I propose to do.

I am a generalist, not a specialist in any of the various disciplines involved. Therefore my objective in this talk is to set up a broad logical overview of the situation. I trust this will be useful to specialists, who are otherwise focused more on the details.

Firstly it is important to free oneself of the temptation to try to identify one single solution to the problem posed by Enrico Fermi in 1950. The first fact that we must start with is this: the number of planets with surface or subsurface conditions hospitable to life as we know it which are known either definitely to have or definitely not to have indigenous life stands at one, exactly the same number as in 1950. Therefore all we can do at present is to identify a range of possible scenarios consistent with our current knowledge, and then to specify the observations we would need to make in order to falsify each one.

The popular tools for discussing extraterrestial intelligence are of course the Fermi paradox and the Drake equation. In my view these are both flawed and misleading, and a new approach to the problem is needed.

My paper in the June 2014 JBIS talked about constructing a parameter space. Until a couple of days ago I assumed that this was what I was going to talk about today. But I found I wanted something more graphical, something more readily comprehensible. The tool I came up with for today is a form of block diagram analysis.

Scenario Block Diagram Analysis of the Galactic Evolution of Life

The starting point, starting from the left-hand side of the diagram, needs to be the question as to how often life emerges from non-living chemistry. Of all the factors in the Drake equation, this is the one whose value is least constrained by current knowledge. According to Paul Davies, both in his recent book The Eerie Silence (p.37), and as he reiterated as a more recent symposium at the SETI Instutute, the probability of the emergence of life in any given location per unit time may be infinitesimal, or it may be inevitable, or it may be anywhere inbetween [1, 2].

On the other hand, on my recent trip to Germany I was talking to the Swiss biologist Dr Hansjürg Geiger, and he assured me that there is no great leap required to get from the most complex organic chemistry to the simplest living organism. This implies that biogenesis is easy, under the right conditions, and therefore must be a common event. Who should I believe?

The question comes down to this: are we able to judge whether biogenesis is rare or common based on speculation, theoretical models and laboratory experiments alone? Or do we require actual observations of the presence or absence of life on a representative sample of astronomical bodies before coming to a conclusion? I would point here to the complex nature of the subject matter, the fact that the only known instance of it occurred four or more billion years ago, in a location and in a manner which are by no means certain, and the fact that science has the unsettling tendency to throw up complete surprises from time to time.

We should therefore exercise restraint in rushing to conclusions. Without real-world observations of the frequency of extraterrestrial life, speculation, models and labwork cannot give a full answer. Science stands or falls through observation, to which theory must always give way.

The fundamental divide right at the start of our investigations is therefore between scenarios in which single-celled life emerges quickly and those in which it emerges only slowly. Clearly, a range of values is possible, and for simplicity I shall consider here the two extremes, which I shall call Rare Biogenesis and Common Biogenesis.

Rare Biogenesis scenarios

Let us assume firstly that the emergence of life is a rare event, taking place perhaps on the order of once per galaxy per ten billion years.

Is it possible that that rare event took place on Earth? It is logically possible. Many would argue that because life is known to have started so soon after the formation of our planet, such a biogenesis would be inconsistent with the view that it was a natural event. On the other hand, Brandon Carter and Robin Hanson have shown that if the average time required for biogenesis on an Earth-analogue planet is vastly greater than the habitable lifetime of such planets, but if life and intelligence develop on one such planet anyway, then the pattern of life’s development would match what we observe on Earth.

This is Carter and Hanson’s Great Filter argument, as described by Paul Davies. Confusingly, it is quite different from another Great Filter argument also put forward by Hanson, and by Nick Bostrom and others, which is far more speculative.

If biogenesis has so far occurred only once in our galaxy, and if it happened on Earth (or indeed on Mars), then its position right at the beginning of habitability of our planet would make it very strange indeed. In fact, it would be every bit as strange as quantum mechanics and relativity – theories that we accept as fact because the experiments force us to.

A second rare genesis scenario is that life has emerged on multiple Earth-analogue planets, but only after a gestation period of at least 8 billion years per planet. It would then be scattered at rare intervals around the galaxy. It may then be that the star for one such planet produced a nova, which blew viable microbes off into space at a time when that star was close to the star-forming region in which our Solar System formed, thus seeding the planets of our system with single-celled organisms.

On this scenario, a rare genesis event is followed by a rare transference event from the location of biogenesis to Earth. A third rare genesis scenario is possible which completely decouples the origin of life from the evolution of industrial civilisation.

Thanks to experiments carried out on the ISS, we now know: “microgravity has been demonstrated to have profound effects at the cellular and molecular level, including changes in cell morphology, proliferation, growth, differentiation, signal transduction and gene expression” [3]. Given this fact, it is not yet possible to rule out the scenario in which biogenesis requires a different gravity level from technogenesis.

If the origin of life is constrained to be on a low-gravity body, for example subsurface on a comet, then it will certainly be a low-probability event due to the short periods of time when water on such bodies is in the liquid state. In order for technogenesis to occur, there then needs to be a low-probability transfer of viable microbes from the interior of such a comet to the surface of a suitable Earth-analogue planet. In this scenario, Earth could easily be the first planet to have experienced animal evolution, followed by industrialisation.

There are thus three possible Rare Biogenesis scenarios. All three are consistent with the loose constraints given by our present scientific knowledge, and none of the three give rise to a Fermi paradox.

Common Biogenesis scenarios

We now move on to consider Common Biogenesis scenarios. Common Biogenesis is the modern default assumption, based on the fact that microbial life was present very early on Earth, and the belief, which may or may not be correct, that Earth life is indigenous to either Earth or Mars. Given this assumption, the known abundance of planets, the inferred abundance of Earth-analogue planets and their inferred existence for approximately 8 billion years before the formation of the Solar System, the result must be that microbial life is and has long been common throughout the galaxy.

It is an observational fact that industrialised alien species which embark on aggressive expansion from their planet of origin into the rest of the galaxy, on a pattern analogous to European global exploration over the past 500 years, either do not exist in our galaxy, or else, if they do exist, have only emerged very recently, essentially simultaneously with our own emergence.

It is the apparent conflict between the assumption of Common Biogenesis and the fact that human technological and growth patterns are not in evidence beyond Earth which is of course the nub of the Fermi paradox.

If technogenesis is common, then the absence of observed industrial alien societies nearby requires either that technical civilisations are always unstable and destroy themselves once they reach our own level of development, or that they never expand into space and instead always adopt an introverted zero-growth economy. The first of these options is obviously highly unpleasant for us to contemplate, the second contradicts the dynamism still evident in modern human society, and both suffer from the drawback that one is trying to apply sociological arguments to all possible civilisations.

But even if biogenesis is common, technogenesis may still be rare. For what period of time do Earth-analogue planets typically maintain comfortable conditions for land-based surface life? This is the sort of thing people love to model, but we won’t really know for sure until we’ve made a detailed exploration of a representative sample of planets in our galactic neighbourhood.

Planetary habitability may be disrupted in two different ways (shown together in just one box in the diagram). Firstly, the astronomical environment may damage habitability through gamma-ray bursts, changes in the planet’s home star and planetary system, and of course asteroid impacts. Secondly, planetary habitability may be disrupted by internal factors such as volcanism or plate tectonics, or simply by the mathematical chaos inherent in any complex system.

A related issue is that an otherwise Earth-analogue planet may have too much or too little water to allow the kind of surface conditions conducive to development of land-based multicellular animals [4].

These points taken together amount to the Rare Earth argument proposed in 2000 in a book of that title by Peter Ward and Donald Brownlee [5]. A recent spin on the thesis was given by Chopra and Lineweaver, who introduced the idea of a Gaian bottleneck restricting development from microbiota to metazoa [6].

The Rare Earth hypothesis has been highly controversial, and was most recently challenged by Dirk Schulze-Makuch and William Bains in their book, published this year, The Cosmic Zoo: Complex Life on Many Worlds [7].

Again I would like to assert that the subject matter is highly complex and that theoretical models and lab experiments are no substitute for observations in the field. The diagram contains scenario pathways through a box labelled Unstable surface conditions in which, given a planet which is an Earth-analogue in its gross properties of mass, composition and orbit, either Earth-analogue surface conditions do not arise at all, or else they do arise but do not endure long enough to allow an initial endowment of microbial life to go all the way to spacefaring industrial civilisation. So far as I can see, at the current level of knowledge these scenario options remain open, again resulting in no Fermi paradox.

The Fermi paradox

How then can we establish that there does indeed exist a Fermi paradox?

Firstly, it is necessary to demonstrate that biogenesis is a common event, not a rare one, in our galaxy. This will require at least one other genesis of life to be found in our galactic vicinity. It must be analysed in sufficient microbiological detail to prove that it cannot be related to Earth life.

Secondly, it is necessary to demonstrate that favourable surface conditions on Earth-analogue planets commonly endure for periods on the order of four billion years. This will require detailed study of the geology and biology of at least one Earth-analogue planet in our galactic vicinity.

Thirdly, these studies will necessarily occupy the attention of an expanding human civilisation over the next century or more. They will depend upon sufficient mastery of interstellar spaceflight to explore exoplanets to better than the level of detail we have achieved on Mars over the past half century.

Therefore the act of making those observations will in itself demonstrate that industrial civilisations are not universally intrinsically unstable, and not universally confined to their home planets, ruling out the two sociological branches deriving from the realisation of common technogenesis in the bottom-right corner of the diagram.

After we have made all these observations it will have been demonstrated that the Fermi paradox does indeed exist!

Alternatively, if we fail to complete this programme of work, the sociological branches will remain open, and thus the Fermi paradox will solve itself!


1. Paul Davies, The Eerie Silence: Searching for Ourselves in the Universe (Allen Lane, 2010).

2. Paul C.W. Davies, “Bio-Signatures and Techno-Signatures beyond Earth”, presented at the three-day symposium entitled “Exploring Exoplanets: The Search for Extraterrestrial Life and Post-Biological Intelligence”, the John Templeton Foundation’s Humble Approach Initiative, September 2015, held at the Royal Society, Chicheley Hall, near London, UK.

3. Cora S. Thiel et al., “Rapid adaptation to microgravity in mammalian macrophage cells”, Nature, Scientific Reports 7, Article number: 43 (2017); doi:10.1038/s41598-017-00119-6; published online: 27 February 2017 (

4. Y. Alibert and W. Benz, “Formation and composition of planets around very low mass stars”, Astronomy and Astrophysics Letters 598, L5 (2017); DOI: 10.1051/0004-6361/201629671.

5. P. Ward and D.E. Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe (Copernicus, 2000).

6. A. Chopra and C.H. Lineweaver, “The Case for a Gaian Bottleneck: The Biology of Habitability”, Astrobiology 16(1): 7-22 (2016); DOI: 10.1089/ast.2015.1387.

7. D. Schulze-Makuch and W. Bains, The Cosmic Zoo: Complex Life on Many Worlds (Springer, 2017).

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