Human Universe (15 page)

The half a billion years or so from the Cambrian explosion to the present day is, in geological terms, relatively short, and life seems to have marched towards greater complexity ever since. This is a gross oversimplification, and we certainly do not suggest that evolution can be viewed as an inevitable march towards intelligence. One might be tempted to assert, however, that given something akin to a Cambrian explosion, the probability of developing intelligent life may be non-negligible, although there are scientists who will strongly disagree.

There is a significantly longer stretch of time between LUCA and the Cambrian explosion – over 3 billion years – and if we are looking for potential barriers to the emergence of intelligence we should investigate the vast expanse of time before complex, multi-cellular life appeared. Why did single-celled organisms remain ‘simple’ on Earth for so long? Most biologists would point to at least two crucial evolutionary innovations that were necessary, though not sufficient, to trigger the Cambrian explosion. The first was oxygenic photosynthesis. An oxygen atmosphere is probably a necessary precursor for the development of complex living things. All multicellular animals today breathe oxygen. This is not a coincidence or a biological fluke; it is chemistry. We release the stored energy from our food by oxidising it – a chemical reaction that is around 40 per cent efficient in the presence of oxygen. Food can be oxidised by other elements such as sulphur, but these reactions typically have an efficiency of 10 per cent or less. If a food chain is to be supported, with predators eating prey that eat plants and so on, then oxygen is probably essential. Without it, the energy available for predators would diminish by 90 per cent at each step in the food chain. This wouldn’t simply mean that an oxygen-starved planet could be full of grazing animals like sheep and cows but no predators such as cats or sharks or humans. The arms race between predators and prey was a vital evolutionary driver towards living complexity on Earth; eyes, ears and brains offer a survival advantage whether you are the hunter or the hunted, and if predation had been impossible for energetic reasons it is far less likely or perhaps impossible that complex animals would have evolved.

Photosynthesis has been around for a long time. The 3.5-billion-year-old Western Australian microbial mat structures are bacterial and they were probably early photosynthesisers, using light from the Sun to grab electrons off hydrogen sulphide and force them onto carbon dioxide to form sugars. They would not have used a pigment as complex as the green chlorophyll that colours the landscapes of Earth today; more likely they would have used simpler molecules from the same family known as porphyrins, which occur naturally and whose precursors have been found in Moon rocks and in interstellar space. Living things are like electrical circuits – they need a flow of electrons to power their metabolism, and given the ready availability of sunlight and naturally occurring molecules that can be assembled into machines to capture it and deliver electrons, it is not too difficult to see how primitive photosynthesis might have appeared very early in the history of life on Earth.

Given the obvious advantage of using the light from the Sun to power the processes of life, it’s not surprising that some early bacteria used photosynthesis for a different purpose – to synthesise a molecule known as adenosine triphosphate, or ATP, the energy storage system for life. ATP is one of the molecules that all living things share, and must therefore be very ancient, perhaps dating back to LUCA and the origin of life.

The type of photosynthesis found in modern plants, trees and algae is a hybrid of these two processes, with an important twist. Crucially, the electrons are no longer taken from hydrogen sulphide, but from water. The fusion of these two slightly different types of photosynthesis, and the use of sunlight to grab input electrons off water, was the great evolutionary leap that led to the oxygenation of the Earth’s atmosphere. Known as oxygenic photosynthesis, it evolved at some point earlier than 2.5 billion years ago. We know this because at this time Earth started to rust, forming great orange iron oxide layers known as banded iron formations, and this requires the presence of large amounts of free oxygen in the atmosphere. Molecular oxygen is an unstable and highly reactive gas, and must be constantly replenished. Astronomers in search of life on exoplanets would consider the detection of an oxygen atmosphere as a smoking gun for the presence of photosynthesis. Oxygenic photosynthesis is a terrifically complicated process, though; the molecular machinery is known as the Z-scheme, and its operation has only been understood in detail in the last few years. The sugar-manufacturing part alone, known as photosystem 2, consists of 46,630 atoms. The structure of the part that holds water molecules in place ready for their electrons to be harvested, known as the oxygen-evolving complex, was discovered in 2006. It is perhaps not surprising, therefore, that the more primitive forms of photosynthesis were not combined together into the oxygen-releasing Z-scheme for well over a billion years.

Beyond the long timescales involved in the evolution of oxygenic photosynthesis, however, there is another piece of circumstantial evidence that may suggest an evolutionary bottleneck. All the green plants and algae that fill our atmosphere with oxygen today perform their photosynthesis inside structures called chloroplasts. Chloroplasts look for all the world like free-living bacteria, and that is because they were, long ago. The story is that a bacterium, most likely one of the great family of early photosynthesisers known as cyanobacteria, was swallowed up by another cell and became co-opted to perform the complex task of grabbing electrons off water and using them to manufacture ATP and sugars, releasing the waste product oxygen in the process. This engulfing of one cell by another, and the merging of their properties, is known as endosymbiosis, an ability possessed by some cells that allows for step changes in living things through the wholesale merger of capabilities that evolved separately and over vast periods of time in different organisms. But here is the key point: everything on the planet today that performs oxygenic photosynthesis does it using the Z-scheme, and this strongly implies that it only evolved once, most probably in a population of cyanobacteria over 2.5 billion years ago. This tremendously advantageous innovation was so useful that it became co-opted into every plant, every tree, every blade of grass and every algal bloom on the planet, flooding the atmosphere with the oxygen necessary for the Cambrian explosion to populate Earth with endless forms most beautiful. If there were ever a smoking gun for a bottleneck, this is it.

But how on earth does a cell ‘learn’ how to engulf another one and survive? How did endosymbiosis arise? A clue, and perhaps an even more significant bottleneck, may be found in another prerequisite for the Cambrian explosion – the eukaryotic cell. All multicellular organisms are made up of cells known as eukaryotes – cells with a nucleus and a host of specialised structures each charged with performing specific tasks. The eukaryotic cells in every living thing look so similar that an alien biologist, knowing nothing about planet Earth, would immediately recognise that human eukaryotes are closely related to those from a blade of grass. The earliest known eukaryotic cells date from around two billion years ago. Beyond this, simpler cells known as prokaryotes were the only living things on the planet. Bacteria and archaea, the two single-celled kingdoms of life that still flourish today, are prokaryotes. They are simple in the sense that they lack the vast, specialised machinery of the eukaryotes, although as we’ve seen they do possess some vital and extremely complex abilities – photosynthesis being a very good example.

The most striking difference between eukaryotes and prokaryotes is the eukaryotes’ cell nucleus, which contains most of its DNA. In the story of evolution of life on Earth, however, it is the small amount of DNA stored outside the nucleus that is most revealing. Almost all eukaryotic cells contain structures called mitochondria. The word ‘almost’ is used a lot in biology. Unlike physics, there always seem to be one or two exceptions that ruin sentences in books like this. Most biologists believe that even the eukaryotes that don’t possess mitochondria did so at some point in the past, however, so we can take it that these structures are ubiquitous. Mitochondria are the power stations of the cell, and their job is to produce ATP. Around 80 per cent of your energy comes from the ATP produced in mitochondria, and without them you certainly wouldn’t exist. A clue as to their evolutionary origin is contained in their DNA, which is stored in loops and kept separate from the genetic material in the cell nucleus. Bacteria also store their DNA in loops, and this is not a coincidence. The mitochondria were once free-living bacteria.

The obvious question is, how did the bacterial mitochondria get inside the cells of every complex organism on the planet? The answer is through endosymbiosis, just as for the chloroplasts, but there is not universal agreement on the detail, and the detail matters a great deal. What is not in question is that the mitochondria are bacterial in origin. The debate surrounds the nature of the original host cell. One camp of biologists believes that the host cell was already a eukaryote, which over many millions of years had evolved an ability called phagocytosis – the ability to ingest other cells. This is a traditional Darwinian explanation – one in which complex traits evolve gradually over time via mutations and natural selection. If this is true, then it is possible to view the eukaryotic cell as just another evolutionary innovation, albeit a very important one, that might crop up anywhere given enough time. The other possibility, which is favoured by many biologists, has different implications. The idea is that the swallowing of the proto-mitochondrial cell was the origin of the eukaryotic cell itself. There was no such thing as phagocytosis or the eukaryotic cell before this singular event, and this ‘fateful encounter’ changed everything. Recent DNA evidence suggests that the host cell was probably an archaeon, one of the two great prokaryotic domains. Somewhere, in some primordial ocean, this simple prokaryote managed to swallow a bacterium – a trick that neither cell possessed before – and against terrific odds the pair survived and multiplied. The archaeon gained a huge advantage – a previously unimaginable energy supply from the bacterium’s sophisticated ATP factory. The bacterium also gained an advantage – it was protected and, over aeons, could specialise and concentrate entirely on producing energy for its host. If this theory is correct, the origin of complex life on Earth was a complete accident. Without access to the energy supply from the mitochondria, all the complexities of the eukaryotic cell, which are absolutely necessary for complex multicellular life, would never have evolved. Earth would be a living planet today, but a planet of prokaryotes, and certainly not home to a civilisation.

I cannot tell you which of these two theories is true. If it were obvious, then all academic biologists would agree. But my impression is that the fateful encounter is currently the more widely accepted theory, and if it is correct then this has very important consequences for estimating the probability of the evolution of intelligent life. Eukaryotes are absolutely essential for intelligence. There is no biologist who would suggest that the prokaryotes, for all their ingenuity in developing photosynthesis and mitochondrial machinery, would have managed to construct radio telescopes given enough time and a following wind. Without eukaryotes, there would be only slime.

I think these are very important points to consider in the Drake Equation. If it is correct that at least two of the necessary foundations for the emergence of complex multicellular life on Earth arose from barely credible accidents, then they might be seen as potential bottlenecks in the evolution of intelligence elsewhere in the Milky Way.

So where are we in our attempt to estimate the chances that, given the origin of life on a planet, intelligence will arise? This is where we move from science to speculation and opinion, and with these caveats, let me give you my personal view.

Given the eukaryotic cell and an oxygen atmosphere, life on Earth became diverse and complex relatively quickly. It is almost certainly no coincidence that the Cambrian explosion followed soon after a rapid rise in the oxygen content of the atmosphere. Whether it is possible to claim that intelligence on the scale necessary to build a civilisation is likely given the right biological building blocks and enough time – half a billion years, let’s say – is another question. We simply don’t know, and the very specific conditions in the African Rift Valley that led to the emergence of early modern humans only 250,000 years ago might suggest that civilisation-level intelligence is a rare development, even given animals as sophisticated as primates, never mind a eukaryote and an oxygen atmosphere.

An optimist would assert that there are billions of potential homes for life in the Milky Way, and that since life emerged on Earth pretty much as soon as it could at the end of the violence of the Hadean, then the Milky Way must be teeming with life and therefore civilisations. I would agree that the Milky Way must be teeming with life – I think there is a sense of chemical inevitability about it. Even accepting this line of argument, however, a pessimist would surely point to the evolution of the eukaryotic cell and oxygenic photosynthesis as being potential bottlenecks. On Earth, it took life over three billion years to get to the eve of the Cambrian. That’s three billion years of planetary stability – a quarter of the age of the universe. If just one of the necessary steps – the fateful encounter, let’s say – was at the fortunate end of a probability distribution, then one can easily imagine that the 20 billion Earth-like worlds in the Milky Way could all be covered in prokaryotic slime. A living galaxy, yes, but a galaxy filled with intelligence? Given what we know about the ascent from prokaryote to civilisation on Earth, I’m not so sure.

Let’s take one final journey back to Green Bank in 1961. Drake and his colleagues, with far less evidence than we have today, concluded that our galaxy seems remarkably conducive to life, full of Earth-like worlds warmed by the glow of benign stars. They too believed that a good fraction of these billions of worlds must be home to life, and given that Darwin’s law of evolution by natural selection must apply across the universe, they concluded that intelligence must have emerged on at least some of these planets. As I’ve argued above, I’m not so sure about intelligence, but we must at least consider the possibility that potential evolutionary bottlenecks like the eukaryotic cell and oxygenic photosynthesis aren’t as bad as they might appear. In this case, the final term in the Drake Equation becomes all-important. Perhaps it is L, the lifetime of civilisations, that is the fundamental reason for the great silence. This is a sobering thought. The reason we have made no contact with anyone is not because of a lack of stars, or planets, or living things; it’s because of the in-built and unavoidable stupidity of intelligent beings.