(1) (a) Using the 4 characteristics of life, consider how they might apply to (i) computers or robots and (ii) grass growing on a lawn. It is clear that grass definitely complies with all of the 4 characteristics of life. As yet, computers and robots are not quite there yet - they do not evolve or reproduce - or even grow - without the help of humans. You MIGHT argue that just means that computers 'live' in a symbiotic relationship with humans. Others argue that computers and/or robots will soon have all 4 of these characteristics of life. But would you say that they are 'alive' -?!
(b) Now consider the textbook's argument that complexity is a better guide for defining life than such rules. Which has greater complexity, a computer (or robot) or lawn grass? I'd say grass because of the complex biochemistry inside. But, on the other hand, grass cannot win games of chess......

(2) (a) What were the ingredients of the "primordial soup"? Which elements? Look up their abundance. Not surprisingly, the main ingredients are also some of the most abundant - Carbon (4th most abundant), Hydrogen (most abundant), Oxygen (3rd) and Nitrogen (6th) - CHON.( The other abundant elements, Helium and Neon, are both inert - they do not react with other elements)

(b) What was the Urey-Miller experiment? What did they do and what did they find? In this early experiement, water, ammonia and methane were put in a flask which was then given electrical discharges. After some time they analyzed the brown "gunk" in the flask and found amino acids had formed.

(c) Electical discharges were used in the Urey-Miller experiment to provide energy in a rapid fashion (to speed up the chemical reactions). What other sources of energy were available for such chemical reactions in the early solar system? (Not necessarily as intense as lightening). There are several possible sources of heat - primordial heat of the warm, early planet; volcanism, particularly in hydrothermal vents in the ocean; the greenhouse gases in the atmosphere retaining solar energy.

(3) (a) How many years after the formation of the solar system (4.5 billion years ago) did the first one-celled organisms appear? (See page 385) The earliest (chemical) evidence of life is in rocks 3.85 billion years old - that would make it 650 million years after the formation of the solar system. The oldest recognizable fossils are 3.5 billionyears old - about 1 billion years after the Earth formed.

(b) What are "stromatolites"? Stromatolites are colonies of bacteria - they form 'mats' in shallow water. They are some of the earliest life forms.

(c) What geological factors "frustrated" early life on Earth? Certainly bombardment by impactors. Also violent geological activity - volcanism and tectonics - perhaps erratic climatic changes too.

(4) (a) Remembering the climate evolution of Earth, Venus and Mars, why might methanogens have been prevanlent in early Earth but not so common now? The early atmospheres of the terrestrial planets contained water, carbon dioxide, nitrogen and sulfur-bearing gases (see page 292). These primative atmospheres are thought to be low in oxygen - conditions which methane-loving organisms would like. But, after billions of years of photosynthesis by vegetation, the Earth's atmosphere not has a substantial fraction of oxygen (nice for us humans) and little carbon compounds such as methane (not nice for the methanogens).

(b)Which of these forms of life (if any!) would you expect to find on Mars and Venus? I would guess life forms which like lots of CO2 and are not so dependant on oxygen as terrestial ones.

(c) How many years after the multi-cellular organisms did humans evolve? (Humans separated from the other apes about 6 million years ago). If multi-cellular organisms formed about 3.5 billion years ago and humans arrived only 6 million years ago, then we took about 3.404 billion years to evolve! If we scale the age of the solar system to 1-day (24 hours), then humans arrived in the last 2 minutes.

(5) (a)Most textbooks say "Life as we know it" is generally taken to mean carbon-based life that originated in a liquid water environment. Why "carbon-based" life? Carbon-based compounds are very varied and seem to make up the basis of all life on Earth. Chemists are skeptical that any other (abundant) element could be the basis of complex biochemistry. But who knows - just maybe there is a totally different kind of life somewhere out there!

(b) What are the physical/chemical reasons for considering a liquid water environment to be important for the origin of life? Liquid water is considered to be an essential element of life. While some organisms can survive in very dry conditions, they can only procreate when water is present. Liquid water allows molecules to mix easily, many molecules dissolve in water, and water has the special property of being polarized which is favorable for some biochemical processes. Water is special stuff!

(c) What are the factors that determine whether a planet has liquid water on its surface? The surface must be between zero and 100 degrees centigrade for water to be liquid, basically. As we have been discussing, the factors controlling the temperature of a planet's surface are distance from the Sun, the planet's albedo and, most important of all, the presence of greenhouse gases in the atmosphere.

(6) (a) What is the evidence for liquid water on the surface of Mars in the past? Earlier we looked at river-like channels, flood channels, mud flows, landslides. Very recent evidence suggests there are sedimentary rocks which could have been deposited in a lake bed.

(b) What did the Viking missions tell us - and NOT tell us - about life on Mars? ( Page 394) The Viking mission involved 2 landers which carried various experiments to look for evidence of biological processes on Mars. They scooped up surface material and tested it with different chemicals. The results were disappointingly inconclusive. There were reactions that occured - but there was no positive evidence that these reactions were really biological - rather than simple chemistry. But they did not rule out the existance of life on Mars, either.

(c) The discovery of ALH84001 was a landmark for exobiology. What is ALH84001? How do we know ALH84001 came from Mars? ALH84001 is a meteorite that was discovered in Antarctica in 1984 and is thought to come from Mars. The evidence for its martian origin is the detailed composition of the gases in the rock - these are distinctly Mars-like and not like terrestrial composition.

(d) What is the evidence of Martian life in ALH84001? There are several lines of evidence for life - the presence of carbon (in large molecules - "PAHs"), the presence of magnetite, the existance of what look like tiny fossils (smaller than the smallest terrestrial micro-fossils) and the presence of carbonates. None of these on their is particularly convincing - but the combined evidence has many people confident of ancient life on Mars.

(7) (a) Mars cooled down about 3 billion years ago. If life developed on Mars at the same rate as on Earth (big "if"), then how complex would life have developed to on Mars when the "big freeze" hit? Not very complex, is the simple answer - probably not more than microbes.

(b) What kinds of fossils could we perhaps expect to find on Mars? Micro-fossils. We are definitely not talking dinosaur bones.

(c) Where on Mars should we go to look for such fossils? My guess would be riverbeds or on the edges of flood channels.