Most scientists think that simple microbial life is common in the universe. The evidence for this seems to be ignorance of the complexity of microbes. A better guess is that extant organisms at the tips of the SHORTEST branches in the tree of life, are the best guesses for how life elsewhere got started. What fraction of the diversity of life is bacterial?

What can life on Earth tell us about ET life? Figuring out how life DID emerge on Earth may be the most informative way to figuring out how life DOES emerge on other planets. The intimate connections between physics, chemistry and biology on Earth should be the same as they are elsewhere. The basic idea is that the closer you get to the features characteristic of the origin of life on Earth, the closer you are getting to universal features.

LUCA and LECA: the Last Universal Common Ancestor and the Last Eukaryotic Common Ancestor. Over the past 60 years the number of domains of life has changed from 2 to 3 and then back to 2 again. These changes were due to the discovery of the archaea in 1977 and then by the realisation over the past decade that reproductively eukaryotes are a type of archaea but metabolically we are closer to bacteria.

Are viruses alive? A survey of biologists does not lead to any clarity. There are many different kinds of viruses: DNA viruses, RNA viruses, double-stranded and single-stranded, with and without envelopes. The questionable status of viruses (the most abundant organisms on Earth) tells us there is no well-defined boundary to life.

Viruses and the origin of life. In the evolution of life RNA preceded DNA. Ribozymes (made of RNA) can store information like DNA but can also catalyze reactions (like proteins do). A pre-nucleotide world may have preceded the RNA world. Highly conserved protein folding families have been used to create a tree of viruses onto which the tree of cellular life can be grafted. Are cellular life forms a type of virus?

Viroids. Discovery by Diener of the Potato Spindle Tuber Viroid. Viroids are plausible living fossils of the hypothetical RNA world. The relationship between viroids and large non-coding RNAs. The difference between the LUCA of all cellular life and the LUCA of all life (including viruses).

The size range of physical things extends from the size of the observable universe at 10^33 cm to spacetime foam at 10^-33 cm. The size range of biological things extends from about 10^9 cm to 10^-6 cm. What might nano-aliens be and how would you look for them?

Maybe life has no minimum size limit. What life forms on Earth have the smallest genome? Synthetic biologists want to know the minimum amount of information you need to make a living organism. Craig Venter asks: What's the minimum number of genes you need to make a living organism? If you provide a life form with everything it needs, you can make the life form disappear as it becomes the arrangement you have set up for it.

Why did animals evolve so late? Why did it take so long for the Cambrian to explode? Charley thinks this question is based on post hoc expectations about the timing of biological evolution. Jochen, using a train metaphor (and a bit of environmental determinism regarding nutrient availability) thinks the question makes sense.

Charles Marshall (evolutionary biologist) discusses the debate between independent convergence and deep homology. He thinks the universe is teaming with life.

Tim Lenton (Earth system scientist) suspects there are many simple life forms throughout the universe. He thinks complex self-aware life is pretty rare in the universe. He briefly outlines the Gaia Hypothesis.

Simon suspects we are alone in the universe because of the small likelihood of life emerging from non-life. But if life gets started, he thinks the evolution of humanoids is inevitable. He thinks there is such as thing as an intelligence niche and that a Planet-of-the-Apes scenario is plausible. Human intelligence WILL BE a convergent feature of evolution.

Malcolm Walter (astrobiologist and stromatilite expert) explains that stromatilites are microbial reefs that form layer by layer. Astrobiology has been a driving force of planetary exploration for a long time. Astrobiology drives interdisciplinary thinking which is a fundamental good for science. Faced with intelligent aliens Malcolm would ask them: How did life start? and How does life start?

Joe Kirschvink is a fan of the Medea hypothesis that life couldn't give a damn about what it does on a planet. Snowball earth events that threatened life were caused by life. There are more planets with happy stromatilites than with telescopes. If we find aliens maybe they can tell us how to avoid a Medean catastrophe. Joe would use the $100 billion to influence political elections to try to prevent us from killing ourselves. We can't find ET if we die.

Bec asks: How can viruses be the frontier of life if they are parasitic? If we consider viruses to be alive, is there a threshold between life and non-life? Riley's favourite part was Spiegelman's monster. Is it possible that the environment that LUCA evolved in doesn't exist anymore? What are the metabolisms of a viral RNA world? Murray takes Charley's side against Jochen in the debate about biological schedules. Chemotrophy is important for understanding the first steps in the origin of life.

What are the most common misconceptions that students or the public have about life elsewhere? Hollywood aliens dominate our thoughts. Some people think aliens are already here. People don't realize how big the universe is and how hard interstellar travel is. The biggest misconception is that there is a consensus on any issue in science at all.