tree of life

1. Tree of Life
9 September 2016

2. Earth Extremes: possible
analogies to other planets
• Hotsprings
• The deep sea
• Hypersaline environments
• Evaporites
• The atmosphere
• Ice, permafrost, snow
• Subsurface environments
Other Worlds

3. Extremes on other planets
• If we’ve seen life thrive in extreme
circumstances on Earth, why not on other
planets?
• Mars holds most promise
• What about moons in our solar system:
– Europa
– Titan
– Enceladus
Other Worlds

4. Europa
• Life exists w/o
photosynthesis in the
deep ocean
• Europa has a
subsurface ocean
• Life may exist beneath
the surface
Other Worlds

5. Titan
• Airborne micro-organisms?
• Extremes to withstand:
– Dessication
– Radiation
Other Worlds

6. Mars
• Host to several extreme environments
– Deserts
– Ice, permafrost, snow
– Subsurface
Other Worlds

7. Mars: ice, permafrost, snow
• Microbes and algae
exist in frozen
environments on
Earth
• Maybe not thriving,
but microbial
survivors could exist
Other Worlds

8. Mars:
subsurface environments
• Best chance of withstanding
Martian extremes
– No liquid water at surface
– Low pressure
– CO2-rich atmosphere
– Only 43% solar radiation at
Earth
• Subsurface provides
– Protection
– Possible liquid water
– Energy source for
chemolithoautotrophs
Other Worlds

9. Summary
• Earth life arose more than 3.5 billion
years ago. Our oldest ancestors may
have been extremophiles
• Life has become progressively larger
and more complicated, but large
organisms are recent: 650 million years
• Extremophiles live at high and low T, P,
pH; high salinity and radiation
• Earth’s extreme environments resemble
other planets

10. The 3 Domains of Life

11. Origins of Earth life
• Early Earth had abiotic environment
with non-living organic chemicals
• First life known had DNA, RNA,
enzymes and proteins
• All uses same gentic code

12. Key events in life history
• Microbial evolution: first cells were
anerobic and chemotrophic
• Natural selection leads to rapid
differentiation: mutations cause the tree
of life to branch out
• Photosynthesis uses solar energy and
produces oxygen
• O2 increases: crisis! Some evolved,
others went extinct, some went to
anerobic niches, eukarya develop

13. Eukarya
• Cell structures develop and specialize
• Diverse forms, but all life was microbial
until 1.2 billion years ago
• Multicellular organisms lead to the
Cambrian explosion

14. Tree of Life: All life is related
• Evolutionary branches determined by DNA comparisons.
• Bacteria seem most ancient; archaea closer to eukarya.

15. The Cambrian Explosion
• Macroscopic life diversity “exploded” ~540 Myr ago:
– Oxygen levels had risen: A good energy source!
– But O2 attacks organic bonds: many microbes went extinct!
– Soon after several Snowball Earths: coincidence or trigger?
Cambrian sea Trilobite fossil

16. Colonization of the Land
• For macroscopic life, protective
ozone (O3)
layer was vital step to inhabiting
land:
– Shields the surface
from dangerous
ultraviolet sunlight.
• Life generated O2 (through
photosynthesis), which
forms O3, which protects life!

17. Impacts and extinctions
• Played a key role
• Ended species like the dinosaurs
• A giant impact leads to a mass
extinction
• We can see craters on the Moon (no
erosion) and Chicxulub crater in
Yucatan
• Pruned branches of the tree of life and
allowed new species

18. Origin of Life: Summary
1. “Organic soup” vs. dilute solution.
2. Complex organics developed (mineral templates?).
3. “Pre-cells” enclosed complex organics.
4. Natural selection increased RNA complexity.
5. DNA developed within some successful cell(s).
A reasonable scenario, though many details are missing!

19. Summary
• Earth life arose more than 3.5 billion
years ago. Our oldest ancestors may
have been extremophiles
• Life has become progressively larger
and more complicated, but large
organisms are recent: 650 million years
• 3 domains of life: Bacteria, Archaea,
Eukarya
• All Earth life has a common ancestor