Throughout the history of humanity, people have looked to the heavens and wondered what's out there. But what about the creatures looking back at us? What kind of weird and wonderful forms might they take?
Most efforts to search for life elsewhere in the universe focus on organisms with a very similar chemistry to our own - carbon-based, using water as a solvent and something similar to DNA or RNA to carry genetic instructions (chemists can't think of anything else that would do it as well). That approach makes sense - it's easier to look for what you know. We know that this kind of life is possible, we know roughly where to look (planets like Earth), and what kind of chemical signatures to watch out for.
But what else might be possible? Could you get gaseous life? Superconducting life? Quantum life? Life on neutron stars, brown dwarfs or in interstellar space? My interest in this was triggered back in 2003. I was a news editor at New Scientist, commissioning stories for the physical sciences beat, and I saw a bizarre-looking paper in a journal called Chaos, Solitons and Fractals. It was by two physicists in Romania, and barely written in English.
Mircea Sanduloviciu and Erzilia Lozneanu at Cuza University claimed to have created self-organising blobs of gaseous plasma that could grow, replicate and even communicate. Fascinated, I asked reporter David Cohen to write a news story about the work.
The researchers were simulating lightning strikes by sending sparks between two electrodes in a low-temperature plasma of argon (a plasma is a gas in which some of the atoms have been split into electrons and charged ions). These electric sparks caused the ions and electrons in the plasma to form spheres.
Each sphere had a boundary made up of two layers - an outer layer of negatively charged electrons and an inner layer of positively charged ions. Trapped inside the boundary was an inner nucleus of gas atoms. The spheres ranged in size from a few micrometres up to three centimetres across, depending on amount of energy in initial spark.
The spheres could replicate by dividing into two, and grew by taking up neutral argon atoms and splitting these into ions and electrons to replenish their boundary layers. They could even apparently communicate information by emitting electromagnetic energy, which made the atoms within other spheres vibrate at a particular frequency.
Sanduloviciu thinks such plasma blobs could have kick-started the origin of life by forming the first cells. They would have formed at high temperatures in electric storms in the early Earth's atmosphere, but he says they can persist at lower temperatures, the sort of environment in which normal biochemical interactions occur.
Perhaps life really did begin with a spark of electricity. But I don't think there's any need to invoke such a far-fetched idea - after all, fatty acid molecules spontaneously form spherical vesicles in water, with membranes very similar to those in biological cells. Harvard's Jack Szostak has shown that these vesicles can grow, divide, compete with each other, and even support replication of an added-in DNA template.
However the work does suggest that these plasma spheres form very easily. So could they be common in the universe, and given the right conditions, could they ever form the basis of a new kind of "plasma life"?
I was reminded of all this on Monday, when I attended a Royal Society discussion meeting on the chemical origins of life. There were some fascinating talks on how the very first steps towards life might have happened - for example Donna Blackmond on solving the mystery of life's "handedness", John Sutherland on how the first RNA molecules could have formed, and Szostak on his protocells.
But one speaker, Martin Hanczyc of the University of Southern Denmark in Odense, was taking a very different approach to life. He is looking for life-like behaviours in drops of oil. I've written a news story about Hanczyc's work for Nature's website, and there are some nice videos of the oil drops here (moving autonomously) and here (responding to a pH gradient).
Hanczyc puts his oil droplets into a watery solution, and feeds them with a chemical "fuel" such as hydrogen cyanide (which would have been around on the early Earth). When the fuel reacts with water at the boundary of the drops it alters their surface tension, which causes them to move. As well as trundling about under their own steam, the droplets can sense and respond to chemical gradients. The droplets can also sense each other, a rudimentary form of chemical communication, and their past actions influence future ones - which you could interpret as a kind of memory. Watery compartments within the oil drops could start to create more complex structures. Hanczyc and his colleagues are now working on getting the droplets to divide and replicate.
While other researchers at the meeting weren't convinced that this has anything to do with how life actually started on Earth, Hanczyc says he wants his work to serve as a reminder about the wide variety of forms that life might be able to take. The behaviours seen in his oil droplets happen extremely easily, he says, just by "throwing things into a pot". And, he points out, many biological reactions happen more easily in oil than they do in water.
Oil-based life forms might exist now on Earth, he says, in parallel with our own water-based life. Or it could have arisen elsewhere in the universe, for example on Titan, where hydrogen cyanide is abundant.
Of course, there's a big downside to both the plasma blobs and the oil drops. The range of behaviours they display is impressive, but critics argue that they both lack any kind of genetic material, and it's hard to see how that could get added.
I can't help thinking, though, that we'd probably say that about water-based biological cells too if we didn't know otherwise. And Hanczyc argues that in any case, demanding that information be encoded within dedicated genetic molecules is a narrow-minded way of looking at life.
Characteristics and structures within the oil drops can be passed on from generation to generation, he says (I think he means things like chemical composition or presence of sub-compartments). In other words, heritable information is there, but it is embedded in the chemistry of the drops themselves, rather than encoded within a genetic molecule. Hanczyc admits that such characteristics would be dependent on the environment and easily lost if conditions changed, but says we should be open to the possibility of a form of life that is intimately entwined with its environment in this way.
I'm not sure... I wonder how complex could life really get if it wasn't able to store instructions somewhere safe like DNA. But what I love about this kind of work is that it forces you to think about the question.
[See related post: Searching for shadow life