An article I wrote for Scientific American, which is relevant to my forthcoming eBook The Cosmic Origins of Life
The Special One
With hundreds of stars now known to have families of planets, and hundreds of billions of stars in our Milky Way Galaxy, it may seem natural to assume that life forms like us, capable of technological civlization, are common. But the steps which led to the emergence of our technological civilization passed through a chain of bottlenecks which make it much more likely that our civilization is unique. This makes it all the more important to preserve our unique planet.
Why does intelligent life exist in the Milky Way Galaxy? Our presence is intimately connected with the structure of our home Galaxy, and the Sun’s place in it, both in space and time. I do not consider here the vast number of galaxies beyond the Milky Way, because, as the saying has it, “in an infinite Universe anything is possible.” But in our Galaxy there may be only one technological civilization, our own. The reason why we are here is the result of a chain of implausible coincidences.
The chain begins with the manufacture of heavy elements – everything heavier than hydrogen and helium – inside stars. The first stars were born out of clouds of hydrogen and helium, the residue of the Big Bang, more than 13 billion years ago. But they cannot have had a retinue of planets, because there was nothing to make planets from – no carbon, oxygen, silicon, iron, or whatever. With cavalier disregard for chemical subtleties, astronomers call all elements heavier than helium “metals”. These metals are manufactured inside stars, and spread through space when stars throw off material as they die, sometimes in spectacular supernova explosions. This material enriches the interstellar clouds, so the next generation of stars has a greater “metallicity”, and so on. The interstellar medium from which new stars form is constantly, but slowly, being enriched. The Sun is about 4.5 billion years old, so this enrichment had been going on for billions of years before it formed. Even so, it is made up of roughly 71 per cent hydrogen, 27 per cent helium, and only just under 2 per cent everything else (“metals”). This reflects the composition of the cloud from which the Solar System formed. The rocky planets, including planet Earth and its inhabitants, are made up from that less than 2 per cent. Stars older than the Sun have even less in the way of metals, and correspondingly less chance of making rocky, Earth-like planets and people (giant gaseous planets, like Jupiter, are another matter). This means that, even if we are not unique, we must be one of the first technological civilizations in the Galaxy.
So much for the timing of our emergence in the Milky Way. What about our place in the Galaxy? The Sun is located in a thin disc of stars about 100,000 light years across; it is about 27,000 light years from the galactic centre, a little more than halfway to the rim. By and large, stars closer to the centre contain more metals, and there are more old stars there. This is typical of disc galaxies, which seem to have grown from the centre outwards. More metals sounds like a good thing, from the point of view of making rocky planets, but it may not be so good for life. One reason for the extra metallicity is that there is a greater density of stars toward the centre, so there are many supernovas, which produce energetic radiation (X-rays and charged particles known as cosmic rays) which is harmful to life on planets of nearby stars. The galactic centre itself harbours a very large black hole, which produces intense outbursts of radiation from time to time. And there is also the problem of even more energetic events called gamma ray bursts, which gravitational wave studies have now shown to be caused by merging neutron stars (add ref to Sci Am story). Observations of such events in other galaxies show that gamma ray bursts are more common in the inner regions of galaxies. Such a burst could on its own sterilise the inner region of our Galaxy, and statistics based on studies of these bursts in other galaxies suggest that one occurs in the Milky Way every hundred million years or so. Further out from the centre, all these catastrophic events have less impact, but stars are sparser, and metallicity is lower, so there are fewer rocky planets (if any). Taking everything into account, astronomers such as Charles Lineweaver (https://arxiv.org/abs/astro-ph/0401024) infer that there is a “Galactic Habitable Zone” extending only from about 23,000 light years from the galactic centre to about 29,000 light years – only about 5 per cent of the galactic radius, and less than 5 per cent of the stars because of the way stars are concentrated towards the centre. The Sun is close to the centre of this GHZ. That still encompasses a lot of stars, but rules out the majority of the stars in our Galaxy.
There are many other astronomical features which point to our Solar System as unusual. For example, there is some evidence that an orderly arrangement of planets in nearly circular orbits providing long-term stability is uncommon, and most planetary systems are chaotic places where the stability Earth has provided for life to evolve is lacking. But I want to come closer to home to focus on one point which often causes misunderstanding. When astronomers report, and the media gets excited about, the discovery of an “Earth-like” planet, all they mean is a rocky planet about the same size as the Earth. By this criterion, the most Earth-like planet we know (apart from our own) is Venus – but you couldn’t live there.
The fundamental difference between Venus and Earth is that Venus has a thick crust, no sign of plate tectonics – continental drift and the associated volcanic activity – and essentially no magnetic field. The Earth has a thin, mobile crust where tectonic activity, especially the activity associated with plate boundaries, brings material to the surface in places such as the Andes mountains today (illustration to come). Over the long history of the Earth, it is this activity that has brought ores to the surface where they can be mined to provide the raw material of our technological civilization. Our planet also has a large, metallic (in the everyday sense of the word) core which produces a strong magnetic field which shields the surface from cosmic radiation. All of these attributes are explained by the way the Moon formed, about 4.5 billion years ago, roughly 50 million years after the Earth formed. There is compelling evidence that at that time a Mars-sized object struck the Earth a glancing blow in which the proto-planets melted. The metallic material from both objects settled into the centre of the Earth, while much of the planet’s original lighter rocky material splashed out to become the Moon, leaving the Earth with a thinner crust than before (illustration of Big Splash, poss. ref. to Sci Am article). Without that impact, the Earth would be a sterile lump of rock like Venus. And the presence of such a large Moon has also acted as a stabiliser for our planet. Over the millennia, the Earth may wobble as it goes around the Sun, but thanks to the gravitational influence of the Moon it can never topple far from the vertical, as seems to have happened, for example, with Mars. It is impossible to say how often such impacts, forming double systems like the Earth-Moon system, occur when planets form. But clearly they are rare, and without the Moon we would not be here.
Once the Earth-Moon system had settled down, life emerged on the Earth with almost indecent rapidity. Leaving aside controversial claims for evidence of even earlier life, we have fossil remains of single-celled organisms in rocks more than 3.5 billion years old. At first sight this is good news for anyone hoping to find life elsewhere. If life got started on Earth so soon, surely it got started with equal ease on other planets. The snag is that although it started, it didn’t do much for the next three billion years. Indeed, essentially identical organisms to those original bacterial cells still live on Earth today, so they are arguably the most successful species in the history of life on Earth, a classic example of “if it ain’t broke, don’t fix it”.
These simple cells, known as prokaryotes, are little more than bags of jelly, containing the basic molecules of life (such as DNA) but without the central nucleus and the specialised structures, such as the mitochondria that use chemical reactions to generate the energy needed by the cells in your body. These more complex cells, the stuff of all animals and plants, are known as eukaryotes. And they are all descended from a single merging of cells that occurred about 1.5 billion years ago, two billion after the first cells emerged.
Biochemical analysis reveals that there are actually two types of primordial single-celled organism, the bacteria and the so-called archaea, which got their name because they were once thought to be older than bacteria. The evidence now suggests that both forms emerged at about the same time, when life first appeared on Earth – that however life got started, it actually emerged twice. Once it emerged, it went about its business largely unchanged for about two billion years. That business involved, among other things “eating” other prokaryotes by engulfing them and using their raw materials. Then, around 1.5 billion years ago a dramatic event occurred. An archeon engulfed a bacterium, but did not “digest” it. The bacterium became a resident of the new cell, the first eukaryotic cell, and evolved to carry out specialised duties within the cell, leaving the rest of the host cell free to develop without worrying about where it got its energy. The cell repeated the trick becoming more complex. And the similarities between the cells of all complex life forms on Earth shows that they are all descended from a single single-celled ancestor – as the biologists are fond of saying, at the level of a cell there is no difference between you and a mushroom (Nick Lane Molecular Frontiers, Journal Mol. Front. J., 01, 108 (2017). Of course the trick might have happened more than once, but if it did the other proto-eukaryotes left no descendants (probably because they got eaten). It is a measure of how unlikely this single fusion of cells that led to us was that it only happened after two billion years of evolution of life on Earth.
Even then, nothing much happened for another billion years or so. Early eukaryotes got together to make multicellular organisms, but at first these were nothing more exciting than flat, soft-bodied creatures resembling the structure of a quilt. The proliferation of multicellular lifeforms that led to the variety of life on Earth today only kicked off around 570 million years ago, in an outburst of life known as the Cambrian Explosion. This was such a spectacular event that it is used as the most significant marker in the fossil record. But nobody knows why it happened. Eventually, that outburst of life produced a species capable of developing technology, and wondering where we came from. But even then, there were bottlenecks to negotiate.
The history of humanity is written in our genes, in such detail that it is possible to determine from DNA analysis not only where different populations came from but how many of them were around. One of the surprising conclusions from this kind of analysis is that groups of chimpanzees living close to each other in central Africa are more different genetically than humans living on opposite sides of the world (http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1002504). This can only mean that we are all descended from a tiny earlier population, possibly the survivors from some catastrophe, or catastrophes. The DNA pinpoints two bottlenecks in particular. A little more than 150,000 years ago, the human population was reduced to no more than a few thousand (perhaps only a few hundred) breeding pairs. And about 70,000 years ago the entire human population fell to about a thousand. All the billions of people on Earth today are descended from this tiny population, so small that a species reduced to such numbers today would be regarded as endangered. We don’t need to know how these catastrophes happened to appreciate their significance.
Putting everything together, what can we say? Is life likely elsewhere in the Galaxy? Almost certainly yes, given the speed with which life appeared on Earth. Is another technological civilization likely to exist in the Galaxy today? Almost certainly no, given the chain of circumstances which has led to our existence. Which makes us unique not just on Earth, but in the Milky Way.
John Gribbin, Alone in the Universe, Wiley, 2011
Nick Lane, The Vital Question, Norton, 2016