Pascale Ehrenfreund: We are the outcome of the evolving universe. It is a crucial exercise for us to understand life as we know it or life as we don't know it by looking at the evolutionary processes of the universe. We have to understand how elements are formed, especially those which are important for building up biomolecules for Earth life or for exotic life. We have to understand how those molecules become more complex, and we have to understand the timescales.

We have to integrate cosmology into astrobiology in our understanding of life -- to understand when the first stars formed, and when the first elements formed. This is a prerequisite for any further investigation. We have set our origin of life at 3.5 billion years ago, the origin of our planet Earth at 5 billion years ago, and we must place that into the context of the universe, which formed approximately 14 billion years ago.

The second important point is to understand abundances and distributions of elements and molecules in the universe. When you think, for instance, about silicon life, you have to take into account that silicon is an order of magnitude abundance below carbon, and it can not build up complex structures.

I think these things do matter, because when you see the processes of how we form stars and planets, they occur the same way throughout the universe. We find very familiar carbon chemistry in space. Some molecules, which have been detected in the interstellar medium or in proto-planetary disks, in particular by radio astronomy or infrared spectroscopy on satellites, including hydrogen cyanide, formaldehyde and ammonia are crucial in modern biochemistry as we know it.

The other thing which is important to note is that, apart from carbon monoxide, which is a very abundant gas in space, carbon in its allotrope forms -- diamond, fullerene, and graphite -- is only present in very small quantities. There is also a small abundance of aliphatic chains. And there is an incredible abundance of aromatic molecules. Probably 70 percent of the cosmic carbon is in the form of some kind of macromolecular carbon of aromatic nature. So this shows you our universe, when you look at carbon, is absolutely aromatic.

I'm just a humble European university professor, but I've heard many American talks now, and as they would say, it's striking and fascinating that the carbon chemistry seems to follow common pathways throughout the universe. Polycyclic aromatic hydrocarbons are the most abundant free molecules in the universe. They have been observed in galaxies, even in elliptical galaxies and in galaxies which pre-date the age of our solar system. We find aliphatic chains in our galactic center and in a lot of other galaxies. So the carbon chemistry follows common pathways throughout the entire universe. I find that very exciting, even as a European.

The current preferred picture of the origin of life is that different sources feed into a pre-biotic soup. There is abiotic synthesis on the early Earth, and organic molecules coming from space. This process may occur on other planets and in other solar systems as well -- the delivery of extraterrestrial material by small bodies was a natural process during the formation of our solar system.

We do not understand the origin of life, and we have a lot of open questions about the steps towards higher complexity. Are we on the right track, using the relevant compositions of molecules? Pre-biotic chemistry experiments in the last 50 years have not been able to reconstruct a cell as we know it in modern biochemistry. We probably have to reflect on the possibility that the real pre-biotic material was based on different components. And when you look at the abundance and the distribution of carbon in the universe, aromatic molecules are an important target.

So in looking at life as we know it, or as we don't know it, we have to follow the basic rules of the universe. We have to understand abundances and distribution. The inventory is strikingly similar everywhere. We see ices and macromolecular carbon distributed all over galactic and extra-galactic space.

The starting material may be similar when we imagine this scenario of having extraterrestrial-delivered carbon, and probably the most abundant material in space, aromatic molecules, would be eventually a good candidate for starting up life. For life as we know it on Earth, we cannot cultivate about 90 percent of prokaryotes. So that is some kind of an alien life. Life has shown a great deal of optimization. Our genetic code is unique. It has been compared to a million other codes, and it works just perfectly. Life is persistent in that form and has adapted everywhere, and shows convergent evolution.

Imagining life as we don't know it, I think we introduce an incredible degree of freedom. But we are very restricted in searching for life as we don't know it. We have not yet detected an organic molecule on our sister planet Mars, not even C2. Space missions are very expensive. We have only a chance in a decade, or probably in 15 years. It will be very difficult to pursue attempts to look for life as we don't know it in the very near future.

Finally, I want to leave you with a statement by Simon Conway-Morris: "Life may be a universal principle, but we can still be alone."

Lynn Rothschild: As the evolutionary biologist here, I'm going to show my hand and argue that life is based on organic carbon. And organic carbon, much as Pascale has been able to show us that there's so much out in the universe, pales in comparison to the amount of inorganic carbon.

And so the first thing that any living organism needs to do is to be able to get an adequate supply of organic carbon. The clever organism is lazy. This is what I tell my students is the laziness principle of life. If you don't have to make dinner, you don't. You eat what's there. But at some point if the organic carbon becomes limiting, the clever organism figures out how to make its own dinner. And so you end up with what we call autotrophs. Well, the next one that comes along says, hey, what if I eat them? And you have what we call herbivores. And the next organism that comes along says, hey, you know, I can eat a herbivore for dinner and be a predator, and then you get the carnivores. And you see this sort of chain arising over and over. And you're going to see these sorts of things in life anywhere, because they all flow back to the fact that we're based on organic carbon.

So let me throw out the first question. If we're going to talk about life as we don't know it, can we first agree on some kind of definition for life as we do know it?

Pam Conrad: Could we first agree on what the heck is organic carbon?

Lynn Rothschild: (laughs) Ok, by organic carbon, I mean something that's not carbon dioxide or carbon monoxide. You're not going to make life out of CO2.

Peter Ward: Plants have a great time with CO2.

Lynn Rothschild: No, you cannot make a body out of CO2.

Peter Ward: Well, you do eventually. My body is nothing but CO2, or hot air, one or the other.

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