Download Systematics/History of Life

This week in lab - trip to Green Oaks!
- dress appropriately (sturdy shoes, long pants, etc.)
Arrive promptly at lab - need to leave quickly
Last day… talking about how phylogenies are constructed
- ended while discussing various types of characters
used to come up with phylogeny (morphological,
developmental, behavioral, molecular)
Mentioned enzyme frequencies, amino acid sequences,
& DNA hybridization…
Phylogeny must be based on overall DNA similarity
(measured by dissociation temperature), not by
number of shared characters
‘Grolar Bear’
DNA sequence data from mitochondrial or nuclear genes
Similar process as
amino acid
sequence
- align sequence,
determine shared
nucleotides
Development of techniques for amplification & sequencing
of DNA leading to explosion of new data and better
understanding of evolution
Whatever technique used, phylogenies give insight into
how organisms evolved, help answer many theoretical
and practical questions
The Origin and history of Life
(or 4.6 Billion years in one hour?)
Evolutionary theory suggests all life could have
originated from 1 common ancestor
Shared traits (esp. common genetic code) indicates it did
Early Earth
- Earth formed about 4.6 billion years ago, extremely
hostile for about 500 million years
- early environment may have been strongly reducing
(O2 scarce), or perhaps neutral, high UV levels,
lightning, etc.
Could even organic molecules form in such an environment?
Stanley Miller & Harold Urey - 1953
- tried to duplicate hypothetical conditions
hydrogen, ammonia, methane, H2O
H2O heated
to near 100°C
Sparks to
simulate
lightning
Cool to
collect
condensation
Many amino
acids & other
molecules
formed
Similar experiments
(including Miller’s!)
made others:
- more amino acids
- purines
- pyrimidines
- ATP
- sugars
The formation of the basic building blocks of life under
abiotic conditions is clearly possible, though how, when
& where assembled into something resembling a cell
more speculative
Few conclusions on origin of life, but many ideas on how
it could have happened…
History of Life on Earth
Earliest fossils about 3.5 billion years old
- appear to be photosynthetic, not likely to be
first organisms
- life may have started significantly earlier, but
earth only solidified about 4.1 billion years ago
– quick start!
Earliest fossils resemble cyanobacteria - still living group
Some cyanobacteria form stromatolites in saline
environments
- common fossils, very successful
Among earliest photosynthetic organisms?
Split H2O, gain electrons for reducing CO2
2 H2O -> 4 H+ + O2 + 4 eOxygen as waste product - concentrations gradually increase
- poison other organisms (anaerobic metabolism)
Other organisms must evolve aerobic metabolism
or avoid O2
- more efficient metabolism
- O2 leads to ozone layer, screens out damaging
UV radiation
Prokaryotes dominated planet for ages (but how long is that?)
Not clear when first eukaryotes evolved
- some evidence hints as long as 2.7 BYA
- earliest fossils about 2.1 BYA (larger cells,
membrane bound structures?), or even earlier?
Fossil - any trace of past life (tissue, shell, tracks,
organic chemicals)
Fossilization is not easy..
- decay removes most tissue
- shells, skeletons much more likely to last
- rapid burial by sediment or ash important;
animals living in sediment most likely, on sediment,
in water column, on land, farther from shore,
increasingly less likely
Number of known fossil species < 2% of probable # of
living species; << 1% of spp. that have ever lived
Minerals impregnate tissue, changing composition
or replacing original material
- subsequently may be deformed or destroyed by
pressure, or exposed & eroded
Relative time inferred by ‘correlating’ common fossils
from different areas
- same spp. deposited in different areas at about
same time
Absolute time (in years, not just ‘earlier’) estimated
by radiometric dating techniques, using ratio of
radioactive isotope & decay product
- measure ratio of isotopes in current sample & in
rocks when formed
e.g. living things  carbon-14, half-life (time for half of
parent isotope to decay) of 5,730 yrs.; older samples
[>75,000 yrs.] dated with other isotopes in rock
Geological time divided
into eons, eras & periods
- earth about 4.6 billion
years old
- fossils scarce throughout
‘Precambrian’ (but some as
early 3.5 billion yrs. ago)
- mostly concerned with
Paleozoic era & later
(starting 542 MYA,
= Phanerozoic eon)
Eons: Archaean,
Proterozoic,
Phanerozoic
(& informally,
Hadean)
Eras: Paleozoic
(ancient animals),
Mesozoic
(middle animals),
& Cenozoic
(new animals)
Periods: Mr. Meanie asks: what period Stegosaurus?
Unenthusiastic student:
Crummy
- Cambrian
Old
- Ordovician
Stones!
- Silurian
(Paleozoic)
Don’t
- Devonian
Care.
- Carboniferous
Period.
- Permian
Mr. Meanie:
Try
Jurassic,
Cretin.
- Triassic
- Jurassic
- Cretaceous
}(Mesozoic)
- Paleogene (‘ancient born’) & Neogene (‘new born’)
periods in Cenozoic
Time divisions originally based on characteristic faunas,
boundaries indicate relatively sudden transitions
- often due to mass extinction of older fauna
Mass extinctions reflect non-uniform conditions on earth
e.g. fluctuations in temperature and humidity
Climate fluctuations due
in part to continental
drift
- shifting continents affect
ocean currents & sea
levels, affecting temp.
& precipitation
Some changes more catastrophic: e.g. mass extinction at
Cretaceous-Paleogene (Tertiary) boundary (K-Pg, or K-T)
- marked by clay layer rich in iridium
Luis and Walter Alvarez
Thought to be caused (in part?) by massive asteroid strike
- wiped out dinosaurs & all large
terrestrial life, 80-90% of
marine spp.