There are many definitions of life, but none of them is satisfactory. Is the biosphere alive? What life is depends on how it got started. Nietzsche: only that which has no history can be defined. Life has a history, therefore it cannot be defined. Like the giants of the scientific revolution, we too have made a great stride by realizing that we don't really know what life is.

Despite efforts to disagree, Charley and Jochen agree that deep homology undermines the numerous claims about the "independent" evolution of eyes, endosymbiotic events, complex multicellularity, flight, sabre-toothness or big braininess. How independent? The so-called independence of "convergent features" is not the type of independence that would help us to extrapolate to life elsewhere.

Steve Benner (molecular biochemist). Microbial life occurred rapidly on Earth, therefore it is likely elsewhere. Intelligent life evolved only recently, therefore it is less likely elsewhere. There is both deep homology and convergence, but not the kind of convergence one can extrapolate to life elsewhere. Yes, "Are we alone?" is an important question.

Jack Szostak asks: How did life emerge from the chemistry of the early Earth? Can we identify a series of steps which make the whole pathway easy? He is simulating a hot spring environment with hydrothermal circulation. If astonomers find life elsewhere, then in the lab, we will be more confident that we can find the chemical pathways that led to life. Jack is a big Star Trek fan.

John works on the origin of RNA. The best way to spend $100 billion dollars: $1 billion on chemistry research and the rest on astronomy to find other Earth-like planets. Complexity is in the eye of the beholder. If "complex" molecules self-assemble, they are not "complex". The origin of life and LUCA are two completely separate entities. Having aliveness as a variable is a convenient way of avoiding the endless debate about the definition of life.

Jim Cleaves (astrobiologist, biochemist) would be very surprised if we are alone. He is happy with the Joyce/NASA definition of life. Viruses are not alive because they are parasites on another system that is alive. The chemical pathways that lead to life are not so well-trodden. Spend the $100 billion on astronomy and going to Mars.

Nick Hud (biochemist) studies the origin of DNA and RNA. The chemistry that leads to DNA and RNA is likely to occur elsewhere. The components of proto-RNA were hardy molecules. Wet/dry cycles polymerize monomers to produce RNA. RNA and DNA came from a common ancestor called proto-RNA. All life has a common origin because all life uses the same amino acids and the genetic code is approximately the same.

Nick Lane is an expert in bioenergetics and the earliest metabolisms of life. He imagines that there is plenty of bacteria elsewhere. Cellularization is important to keep the inside in and the outside out because a unit of selection is needed for selection to act. A proton or ion gradient across a membrane was the first energy currency and is as universal as the genetic code. The eukaryotic bottleneck is a solution to Fermi's paradox.

Jeremy England (thermodynamicist) does research focused on the question: What are the physical conditions required or conducive to the origin of life. Regarding the relationship between science and religion, he seems to share Stephen J Gould's ideas on non-lapping magisteria. Physical systems have histories that bear the mark of the environments in which they have dynamically evolved.

Martin Van Kranendonk (geologist/astrobiologist). Stromatilites as the oldest evidence for life on Earth. Shallow marine environments vs hot spring environments. Photosynthesis vs chemical energy of hot spring systems. Wet/dry cycles are needed for the origin of life, so hydrothermal systems are not plausible environments.

Laurie Barge (astrobiologist) is interested in the environments in the Solar System that could plausibly be associated with the origin of life (and ones we will hopefully visit in her lifetime). We all should be interested in microbes.

Mike suspects that microbes will be in many places, but that intelligent life will be pretty rare and short-lived. Life exists to dissipate gradients and use up free energy. To have complex life and telescopes, you need some dry land. A wet rocky planet like ours is like a battery with an output of 1 volt. Life is a fuel cell that uses up the battery's free energy. Life in the universe is a cosmic imperative and it's a continuum from geochemistry to biochemistry.

Kathy is an expert in rocks from hot springs and from undersea cold springs (hydrocarbon seeps). As an environment for the origin of life, both hot springs and hydrothermal vents are in contention. If we had the answer to the question "Are We Alone?" we would be having a holiday on some other planet right now with our warm fuzzy alien friends.

Norm Sleep (geophysicist/astrobiologist) As a boy scout he became interested in the activities of the Earth. Objects between 300 km and 500 km in diameter would sterilize the Earth. The origin of life may be easy because it occurred fast on Earth. The prerequisites for life are common in the universe. The origin of life could have been electrotrophic. Advanced aliens would not be interested in eating us but would be interested in our sociology.

Anthony Poole (bioinformatician/molecular evolution) is an expert in the origin of RNA. He suspects there are multiple ways to get life started. He thinks that something like humans would re-evolve because our generalist strategy might be convergent.

Bill is almost certain that microbes are elsewhere. The chance of intelligent life elsewhere is pretty good. Life is a chemical reaction. We are all descendants of one and the same chemical reaction that took place on the early Earth. The origin of life is the transition from a pre-existing geochemical reaction to one that is compartmentalized and catalyzed in such a way that it makes a copy of itself. Hydrogen and carbon dioxide were crucial.

Betul Kacar (evolutionary biologist/astrobiologist) is interested in understanding the biology of the past. Biology is an historical science. As the only astrobiologist in a department, one can feel alone, but don't be afraid to be the wild child.

Antonio is an expert in the Miller-Urey experiment. He uses genetic sequences to understand the early evolution of metabolic pathways. He thinks there is no other life in the Solar System. We are basically alone as far as life like us is concerned. Animals represent a truly minor component of the huge diversity of life on Earth.

Sarah Walker (astrobiologist) works on the problem of what living systems are. It's a very difficult question. What life forms do with information is important since information is the dividing line between life and not-life. Life forms innovate with information. If you don't know what living systems are, it's hard to say when they originated or how they originated. Viruses, prions, Arizona and the Earth are alive. The Sun is maybe alive a little bit.

Eric says: I don't understand what people want from the question "Are We Alone?" -- the biosphere is rich and interconnected and the history of mankind seems to consist of stubbornly trying to create separations that don't actually exist to make ourselves more isolated than we are. Darwinian evolution presupposes the existence of individuals in a population.

Kim Sterelny (philosopher of biology) thinks we don't have a good model of how life starts. Therefore we don't know whether life here is an incredible fluke or relatively likely on reasonably hospitable planets. The caveat about claims of independent convergence is the question "how independent?" Two bottlenecks in our ability to extrapolate life on Earth to life elsewhere are our ignorance of the origin of life and of the origin of eukaryotes.

Emily Parke (philosopher of science and biology) is interested in whether we can define life. This is relevant to our search for life and to answering the question "Are We Alone?". If we are alone, she thinks the rest of the universe is not a waste of space, because she disagrees with the presupposition that there is a higher purpose to the universe.

Bec, Riley and Murray comment on how science is done by people writing papers, not by omniscient textbooks. Nick Lane's discussion of transmembrane potentials and ATP as the energy currency of life. What is "nitrogen fixing"? How important are the non-HOCNPS elements? Wet/dry cycles from lunar tides do not affect inland hot springs. Riley likes the sliding scale of "aliveness". Is plate tectonics necessary for life?