Issues Magazine

Astrobiology: Building a More Unified Account of Reality

By Charley Lineweaver

The new science of astrobiology is part of the human quest for self-knowledge.

After fertilisation, gastrulation and birth, we fully-bipedal apes go through a period of risky behaviour as we try to find ourselves. Our teenage years are often an identity crisis, not knowing who we are, what we want to be, where we come from or whether we should get up in the morning. We get tattoos and pierced noses and we experiment with drugs, sex and guitars. To help answer the question of who we are, we look in the mirror, draw self-portraits, take selfies and make maps of the world. Somewhere near the centre of our maps we scratch “I am here”. Such identity crises usually resolve themselves but can linger for decades.

Over the summer holidays, I visited the “Mapping Our World” exhibition in the National Library in Canberra, including Ptolemy’s World Map from about 2000 years ago. It was published about 10 years before Columbus’ first voyage.

Europe is in the upper left, China and the Malay Peninsula are on the far right, and Ptolemy’s home, Egypt, is in the middle. The world looks different depending on where you are standing, which way you are facing and, of course, when you are standing. We all struggle at some time in our lives to understand the big picture. How did we get here? Is there a purpose to any of this? Where did all this stuff come from? How did life originate?

Traditionally, these questions have been answered by creation myths. Like language and smiling, creation myths are universal among humans. We seem to need them. If anthropologists ever discover a new tribe in the most remote regions of the Amazon or New Guinea, that tribe (like all other human tribes) will have a creation myth peopled by gods and supernatural forces that gives the members of the tribe a special place and tells them how they fit into the universe.

Astrobiologists study the origin of life on Earth and try to find naturalistic scientific answers to such big questions as “How did life originate?” and “Is life on Earth the only life in the universe?” We are trying to replace creation myths and supernatural gods with a more naturalistic scientific understanding of our origins.

The word “astrobiology” is a combination of astronomy and biology. The basis of astrobiology is a combination of the main concepts of these two disciplines. Darwinian evolution is the most fundamental idea in Big Bang origin of our universe is probably the most fundamental idea in astronomy. In 1973, Russian geneticist Theodosius Dobzhansky wrote, “Nothing in biology makes sense except in the light of evolution” (American Biology Teacher 1973, 35, 125–9). And since the 1965 discovery of the cosmic microwave background, nothing in astronomy makes sense except in the light of the Big Bang. The Big Bang and evolution are the two central tenets of astronomy and biology, and their combination provides the context within which astrobiological research is taking shape.

In earlier reincarnations, astrobiology was known as exobiology or bioastronomy, and since we had no evidence for alien life, it was ridiculed as the only scientific discipline without a subject matter.

In 1871, Darwin didn’t think much about investigating the origin of life: “It is mere rubbish thinking at present of the origin of life; one might as well think of the origin of matter”. Notice that Darwin wrote “at present”. Things are different now. Phylogenetic trees tell us much about the common ancestor of all extant life and even hints about the origin of life. We can address the issue from the top down (we can get information about the common ancestor of all species alive today by looking at their genomes), and from the bottom up (from an understanding of the chemical environments of the early Earth we can deduce the ingredients available for molecular evolution and the self-sustaining chemical reactions from which life could have emerged). We are making progress. Not only can we profitably investigate the origin of life on Earth, physicists even have testable ideas about the origin of matter and the origin of the universe.

Are We Alone?

Astrobiologists are trying to find out if we are alone. Does life exist beyond the Earth? One way to start with the question of whether life exists elsewhere in the universe is to estimate the number of habitable Earth-like planets in orbit around other stars. We have thought for a long time that planet formation should be a natural part of star formation. The latest data strongly supports this idea, and suggests that rocky planets similar to the Earth are common. As our technology gets better, and the more carefully we look, the more rocky planets we see. Thus, every star you can see in the night sky probably has some kind of planetary system in orbit around it. Many of these planetary systems (and possibly most of them) will have a planetary system somewhat like ours; that is, with small rocky planets (like Mercury, Venus, Earth and Mars) in close orbits, and much more massive gaseous planets (like Jupiter, Saturn, Uranus and Neptune) further away from the host star. We have found many exceptions to this expectation, but the exceptions may be in the minority.

After determining how common rocky Earth-like planets are, we want to know what fraction of them are in the habitable zone of their host star; that is, how many have the right temperatures and right surface pressures to keep water in liquid form on their surfaces. If the surface temperature is too high, all the water will be water vapour in the atmosphere, as on Venus today. There will be no surface water. If the surface temperature is too cold, all the water will be ice in the ground or in polar caps, as on Mars today. If the pressure is too low, like on Mercury or Mars, liquid water is not possible. At low pressures, if there is any ice, it becomes vapour without going through a liquid state.

We are most interested in Earth-like planets in orbit around host stars that have been there long enough for life on their planets to have evolved into something we might be interested in. The Sun formed about 4.56 billion years ago when an over-dense clump, in a molecular cloud in the plane of our galaxy, gravitationally collapsed. It took another ~ 50 million years for the Earth and other rocky planets to accrete from the debris in orbit around the Sun.

The mass of a star determines its main sequence lifetime. The more massive the star, the shorter its life. For example, if the Sun were twice as massive as it is, it would have a lifetime of less than two billion years and would have then turned into a red giant and engulfed the Earth three billion years ago, when life on Earth was bacterial. After only two billion years of evolution on Earth, there were no eukaryotic life forms (animals, plants, fungi or single-celled paramecia).

So if we are looking for complex life that takes more than a few billion years to evolve, we don’t need to look at planets around stars much more massive than the Sun.

Origin of Life on Earth and Extraterrestrials

One of the most important ways to find out whether extraterrestrials exist is to investigate the origin of life on Earth. This is because knowledge about the emergence of life on Earth lets us estimate how probable that emergence was. We ask ourselves, “If we replay the tape of the formation of the Earth, how likely would it be that life emerges again?” If our understanding of early Earth environments points to quirky improbable events that are closely related to some unique property of the Earth, then the origin of life is likely to be a stupendously improbable event. And we wouldn’t expect there to be much life in the universe. However, if the Earth doesn’t seem to possess any unique properties compared to other Earth-like planets, (which it doesn’t), this suggests that the emergence of life is a cosmic imperative (Christian de Duve, Cosmic Dust, 1996) and we should expect extraterrestrial life to be plentiful. Alternatively, extraterrestrial life may be so different from what we expect that maybe we have already detected extraterrestrial life and we don’t even recognise it (Charley Lineweaver, Life As We Know It, 2006).

Although water may be a prerequisite for our existence, it is an effective inducer of planetary amnesia. Over the 4.5 billion year history of the Earth, rains, floods, tides and erosion have washed away cliffs, hundreds of mountain ranges and impact craters. Life needs the fresh nutrients that come from planetary resurfacing: volcanoes recycle volatile elements while earthquakes and continental drift dredge up mountain ranges and, like a plough, bring nutrients to the surface so new crops can grow. Nothing grows in old soil leeched by millions of years of rain. Bob Dylan asked, “How many years can a mountain exist before it is washed to the sea?” The answer is about 100 million years, depending on rainfall. With all that erosion resurfacing our planet, how are we going to find our oldest footprints? How are we going to find the traces of the oldest life on Earth? It’s difficult to find out where we came from on such an active planet. The hydrological cycle of the Earth brings life and forgetfulness.

One solution to this hydrologically induced planetary amnesia is to go to the Moon. The Moon is the Earth’s “bombardometer”, reminding us of what the surface of the Earth would look like without our wet troposphere. There is no water on the Moon, no rain, no wind. And the Moon has been close enough to us to serve as our attic. Earth laboratories have dozens of meteorites that were blasted off the surface of the Moon, so the Moon must have a record of the emergence of life on Earth that is even richer and better preserved than ours on Earth. For 4.5 billion years, whenever a large impactor hit the Earth, pieces of the Earth’s crust were thrown into space and occasionally the Moon gravitationally collected some of the debris and preserved it on its (almost) changeless surface. We may find the earliest evidence for life on Earth when we start looking for fossils on the Moon.

Habitability is a tricky concept. How can we discuss habitability if we don’t know what kind of life we are talking about? As we learn more about life, its definition has expanded to include extremophilic bacteria, small and large viruses, prions and parasites, macroscopic superorganisms (like ant colonies), whole ecosystems and, most controversially, the whole earth (Gaia). As we look for life elsewhere we would like to have a definition, or at least a conception, of what life is that isn’t quirky and limited to the Earth. We want a definition of life that has a chance of being universally valid, but our standard definitions of life are based on terrestrial life and are probably not broad enough to be relevant for life beyond the Earth.

It’s hard to imagine what other forms of life could be like. Can we construct a definition of life general enough to give us some confidence in its universality? Our job as astrobiologists is to try to figure out which aspects of terrestrial life are common to life in the universe and which aspects are unique to terrestrial life.

Here is an example of the reasoning. Some features of human beings are unique to our species while other features are common to all mammals. Some features of mammals are unique to mammals while other features are common to all vertebrates. Some features of vertebrates are unique to vertebrates while others are common to all eukaryotes. Some features of eukaryotes are unique to eukaryotes, while others are common to all life on Earth. Some features of terrestrial life are unique to terrestrial life, while others are common to all life in the universe.

Multicellularity, sexual reproduction and brains are not common features of all terrestrial life, so we should not necessarily expect extraterrestrial life to have sex and heads. However, some scientists think that these adaptations are so important that we should expect evolution everywhere to converge on them. Hollywood seems to think so, too.

Like our definition of life, our definition of the universe has expanded. It went from an Earth-centred world to a heliocentric solar system. Then our universe became a galaxy of 100 billion stars. And now we know there are 100 billion galaxies in the observable universe. Recently, modern cosmological Columbuses even talk of a multiverse in which our universe is just one of an infinite number of others.

Whether we are alone in the universe or whether we live in a multiverse with an infinite number of other universes, on the largest scales we (like all of our ancestors and all of our descendants) will always be lost. Astrobiology will never give us a complete story, but physicists are starting to incorporate biology into physics in the form of observer-dependent effects in quantum mechanics and the anthropic principle. “We will not cease from exploration, and the end of our exploring will be to arrive where we started and know the place for the first time,” said T.S. Eliot.

This is the ambitious agenda of astrobiology: to build a more unified account of reality; to return to our origins and know ourselves for the first time.