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Copyright © 2009 Pearson Education, Inc.
Life in the Universe
• The only place we know life exists is here on Earth
• One of humanity’s Big Questions is whether it exists elsewhere
• We can get some clues by considering life’s history here on Earth
• When we do, we can get an idea how likely life “as we know it” is
• We should keep in mind that life as we know it may not be the only kind possible
• But it is the kind that we will be best able to recognize, if it does exist…
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
When did life arise on Earth?
What do these events tell us about the possibility
that life exists elsewhere in the universe?
Copyright © 2009 Pearson Education, Inc.
When did life arise on Earth?
• The first evidence of life
appears ~150 million years
after it became possible
• It is not fossil evidence, but
trace chemical evidence
• The evidence is in the ratio of
carbon-12 to carbon-13
Copyright © 2009 Pearson Education, Inc.
Carbon isotope evidence for life
• In >3.8-billion-year-old
rocks like these in
Greenland there is a higher
than normal ratio of 12C:13C
• Living things incorporate 12C more easily than 13C
• So the higher ratio is taken
as indirect evidence for life
Copyright © 2009 Pearson Education, Inc.
When did life arise on Earth?
• The oldest fossils of living things
date to ~3.5 billion years ago.
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Earliest Fossils
• The oldest fossils of living things
date to ~3.5 billion years ago.
• Fossil stromatolite in 3.5-billion-
year-old rock
Copyright © 2009 Pearson Education, Inc.
Earliest Fossils
• The oldest fossils of living things
date to ~3.5 billion years ago.
• Fossil stromatolite in 3.5-billion-
year-old rock
• This is a living stromatolite
• Stromatolites are layered structures
formed by colonies of bacteria
• They still exist today, typically in
extreme environments like hyper-
salty lakes and lagoons
Copyright © 2009 Pearson Education, Inc.
Earliest Fossils
• The oldest fossils of living things
date to ~3.5 billion years ago.
• Fossil stromatolite in 3.5-billion-
year-old rock
• This is a living stromatolite
• Stromatolites are layered structures
formed by colonies of bacteria
• They still exist today, typically in
extreme environments like hyper-
salty lakes and lagoons
• Here are some in a lagoon in
Australia
Copyright © 2009 Pearson Education, Inc.
When did life arise on Earth?
• So fossil evidence shows that life
certainly existed on Earth by 500
million years after conditions
would permit it to survive
• And chemical evidence suggests
it probably existed much earlier
• But how did it come to be?
• We don’t know
Copyright © 2009 Pearson Education, Inc.
How many civilizations are out there?
Copyright © 2009 Pearson Education, Inc.
The Drake Equation
Number of civilizations with whom we could potentially
communicate
= NHP flife fciv fnow
NHP = total number of habitable planets in galaxy
flife = fraction of habitable planets with life
fciv = fraction of life-bearing planets with civilization at
some time
fnow = fraction of civilizations around now
Copyright © 2009 Pearson Education, Inc.
We do not know the following values for the Drake
equation:
NHP : probably billions
flife : ??? Hard to say (near 0 or near 1)
fciv : ??? It took 4 billion years on Earth
fnow : ??? Can civilizations survive long-term?
Copyright © 2009 Pearson Education, Inc.
Where are the aliens?
Copyright © 2009 Pearson Education, Inc.
Fermi’s Paradox
• Plausible arguments suggest that civilizations should be common…
• So why haven’t we detected them?
Copyright © 2009 Pearson Education, Inc.
Possible solutions to the paradox
1. We are alone: life/civilizations much rarer than
we might have guessed
• Our own planet/civilization looks all the more
precious…
Copyright © 2009 Pearson Education, Inc.
2. Civilizations are common, but interstellar travel
is not, perhaps because:
• interstellar travel is more difficult than we think.
• the desire to explore is rare.
• civilizations destroy themselves before achieving
interstellar travel.
These are all possibilities, but they are not very
appealing.
Possible solutions to the paradox
Copyright © 2009 Pearson Education, Inc.
3. There IS a galactic civilization…
… and someday we’ll meet them.
maybe
Possible solutions to the paradox
Copyright © 2009 Pearson Education, Inc.
• If there are other civilizations as advanced as ours….
• What would they be like?
• What would the “people” look like?
• How would they think?
• Presumably they would have similar science to ours…
• What would their art be like?
• What would their philosophy be like?
• What would their religion be like?
Exocivilizations
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