Background

• The main questions: Are we alone or is there life elsewhere? Did life on Earth come from space?
• Need to ask first: What is life? What does it need in order to develop? How can we find out where it did?

What Happened on Earth?

• 4.6 Gyr ago: Formation of the Earth.
• 4.5-4 Gyr ago: Chemical evolution: - At first, atmosphere formed from H, N, C, O degassing; Then, those gases combined into CH₄, NH₃, CO₂, H₂O molecules.
• 3.9 Gyr ago: Heavy asteroid bombardment that resurfaced the Earth.
• Shortly afterward, 3.85-3.65 Gyr ago: Life and biological evolution started. Blue-green algae.
• How did it happen? Possibilities: 1: Energy from radiation, lightning, volcanoes, meteorites led to complex molecules (aminoacids, nucleotides), the basis for life; 2: Complex organic molecules came from space, possibly on comets or dust grains (panspermia) – after all, we do have meteorites from Mars.
• 2.1 Gyr ago: Amoebas, multicellular organisms in the oceans.
• 400 Myr ago: First life on land.
• A few Myr ago: Early Homo sapiens.
• 100,000 yr ago: Homo sapiens sapiens.
• 10,000 yr ago: Agriculture; then cultural evolution, aided by language, toolmaking, ...
• Extinctions: It is generally believed that the K/T extinction 65 Myr was caused by an impact, and others may have been as well; Another astronomical possibility is UV radiation from the Sun after ozone layer depletion by a nearby supernova's gamma rays (has been suggested for the Ordovician extinction 440 Myr ago).

What Do We Know about Elsewhere in the Solar System?

• Summary: There's probably no carbon-based life in liquid water anywhere outside the Earth now, but...
• Mars? Almost certainly not now; Maybe there was microbial life in the past; We have no firm evidence yet, despite the Viking landers and the famous meteorite.
• Europa? Probably has liquid water under the icy crust.
• Enceladus? 8th moon of Saturn; Seems to have liquid water under an icy crust.
• Titan? 12th and largest moon of Saturn; Has an atmosphere, but may be too cold.

  What Can We Find Out from Studies on Earth?

• What could alternative lifeforms be like? Turn to astrobiology and the search of exotic life forms on Earth for clues; The field is rapidly expanding; See NASA's Astrobiology Institute, for example; Could life develop with ammonia instead of water, Si instead of C? Some bacteria found 2 miles below ground derive all their energy from radioactively produced chemicals, and none from the Sun.
• Urey-Miller experiments: In 1953, produced aminoacids in the lab.
• Later experiments: Produced protein-like droplets similar to some fossils.
• Other observations: Study composition of comets and meteoroids.

Is there Life Around Other Stars?

• First condition: Existence of extrasolar planets; We now know there are lots of those.
• Further conditions: Environments suitable for life (not too close or too far from the star, planet not too big nor too small, right chemistry); What is the likelihood that in a suitable environment life will actually arise? How wide is the "Goldilocks zone"?
• How can we find out if there's life there? Look for the simultaneous presence of oxygen (or ozone) and a gas like nitrous oxide or methanes, in a planet's atmosphere.
• Intelligent life? That's a whole different issue...