Do we live on a , so exceptional that it is pretty much alone in hosting a rich diversity of life, with almost all other planets being home to simple microbes at best? Or are we in a universe teeming with living things as complex as those here, meaning that we exist as part of a vast, cosmic zoo?
Debate on this rages on, but we say it’s time to accept that the latter is very likely.
To date we know of and there are out there. We do not know how commonly life arises on them, but many scientists think that it .
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With that in mind, we head to our central question. If a planet does host life, what is it like? Our hypothesis is that if a life-bearing planet remains habitable for long enough, then complex living things will arise. It might take a long time – for example, oxygen was required for the development of animals on Earth, and it took a billion years after the element started to accumulate in our atmosphere before animals appear in the fossil record. But it will happen, eventually and inevitably.
How can we claim this? Surely the path that Earth took was unique, full of extraordinarily unlikely events, such as the vast impact early in its history with another protoplanet? It left Earth with an over-sized moon, a big molten iron core, a mantle that can support plate tectonics, lots of water (but not too much), and many other specifics upon which life depends.
Rare earth contenders are right that our planet is unique, our solar system unlike any other we have found, just as there was only one Bach and one Schubert. But that does not mean that composers are incredibly rare. Other composers from other histories, other traditions have created music, from Beethoven to boy bands.
Life’s many roads
We are not concerned with the specific examples of life on Earth, but what life does; growing large structures, walking, thinking. For example, , but similar tissues designed to feed growing embryos have evolved in scorpions, cockroaches, lizards, sharks and snakes. The mammalian eye is unique, but eyes have evolved probably a dozen times. Each specific path is a one-off, but there are many paths to each of these complex functions.
So what does it take to make a complex organism, such as a Beethoven or a birch tree? There are thousands of specifics, but evolutionary biologists have identified a few key innovations that take life a major step along the path from its simplest form to the diversity we see today.
Some of those steps are evident in the long hard road to the evolution of a brain: before you can evolve a smart brain you have to evolve nerve cells, and that means evolving a way to run a complex genetic program in an organism, one that directs cells to form different tissues in different places and at different times. Many of the key steps are actually relatively basic ones even if the end result is far from basic.
The key innovations turn out to be light capture (to provide energy), oxygen manufacture (to create a widely available and potent energy source for life to spread, ), complex cell architecture of the type we see in eukaryotes, multicellularity, genetic structures more sophisticated than a bacteria’s simple DNA, a way to run a complex genetic program, and intelligence. Oh, and sex.
Looking at these major innovations from the simplest life to the most complex, we find nearly all of them . So while these are big steps towards complex life, they are not highly improbable, because there are many paths toward higher and higher levels of complexity.
Technological intelligence
As such, if and when humans visit one of the other inhabited planets, we expect life there could have taken many courses. The biochemistry may be different and it will certainly have a different anatomy. We may not even recognise the more complex forms as animals or plants, but their functions and what they do is likely to be comparable to the more complex species on Earth.
Does this mean there would be technological civilizations comparable to ours? Intelligence is quite common on Earth. Tool use, playing, problem solving and the ability to learn new tricks and pass them on has arisen independently in many animals such as octopuses, parrots, dolphins, apes (including ourselves) and elephants – lineages which have been around for a very long time. So why has just one species followed the evolutionary path towards technological intelligence?
This may be a bottleneck of evolution, what . If so, it would explain the Fermi Paradox, our failure to find any evidence of extraterrestrial technology. Of course, that search is just beginning. Thirty years ago we did not know of any exoplanets, so who knows what the next generation of ¿ìè¶ÌÊÓƵ readers will discover.
Within decades, the first interstellar probes – planned by NASA and – may be on their way to the nearest habitable examples. This would be the best way to look for the presence of complex life and test the cosmic zoo hypothesis. Ultimately, though, we may have to accept the idea that the galaxy is a zoo with few visitors, and to find other talking, travelling technologists like ourselves we will have to search in a galaxy far, far away…
Dirk Schulze-Makuch is a professor at the Technical University of Berlin, Germany, and an adjunct professor at Washington State University and Arizona State University, and Williams Bains is an astrobiology researcher. They are co-authors of The Cosmic Zoo: Complex Life on Many Worlds (Springer Praxis)
