Cladistics
- Similarities and differences in traits can indicate how
closely related species are - Phylogenetic trees reconstruct evolutionary
relationships by linking those with more, and/or more
similar traits, together
Ancestral vs. derived characters
- Ancestral: a character inherited from an ancestor
(aka plesiomorphy) - Derived: an evolutionarily novel trait (aka
apomorphy; shared apomorphies are
synapomorphies) - NB derived characters will become ancestral in succeeding generations! e.g. a vertebral column is derived for the lamprey but ancestral for the leopard!
- BUT
- Are traits are similar because both species inherited
them from a shared ancestor? Aka homologues - Or are traits are similar by chance or because similar
environmental conditions selected for similar traits in
unrelated species? Aka analogues
Mitochondrial dna
- Most DNA is found in the cell nucleus but some is in the cell mitochrondria – this is known as mitochondrial
DNA (mtDNA)
*only from the maternal line - mtDNA is not recombined during sexual reproduction
- mtDNA survives longer than nuclear DNA
- In theory mtDNA can survive to 6,830,000 years at −5 °C (Nuclear DNA degrades at least twice as fast as mtDNA)
- BUT in practice can only be detected much younger: currently oldest ancient DNA (aDNA) extracted is 560-780,000
years old
Relative dating: stratigraphic correlation
- Principle of ‘superposition’: strata are laid
down in order of antiquity - Biostratigraphy: order is determined by
species present in each layer - Lithostratigraphy: order is determined by
geology/rock type/fossils
Relative dating: palaeomagnetism
- The Earth’s magnetic field has reversed
several times in the past - the iron in rocks lines up with the
magnetic field in which it forms - Studying the orientation of iron shows
whether the rock formed in normal or
reversed conditions - The sequence of reversals is well
understood and independently dated - How do you know which period?!
When did homo sapiens start leaving Africa
Roughly 100,000 years ago
What is relative dating
Putting things in order
What is absolute/chronometric dating
Establishing an independent date for something
Typology
A classification based on general type
Oxygen/marine isotope chronology
Oxygen exists in seawater in two isotopic forms:
*18O, a heavy oxygen isotope
*16O, a light oxygen isotope: evaporates readily
During a warm climatic period (interglacial) the 16O that evaporates returns to the oceans via rain and runoff into rivers
* there are roughly equal proportions of 16O and 18O in the oceans
During a cold period (glacial) evaporated water is taken north by winds and falls as snow over the ice sheets and does not return to the ocean.
* The heavy 18O stays in the oceans
Tiny sea creatures called foraminifera build shells out of the minerals and nutrients, including oxygen, in sea water
* In glacials the shells take up predominantly 18O
* in interglacials they take up even amounts of 18O and 16O
When they die deposited on the ocean floor: organic parts decay, shell survives
Drilling a core of sediment from the ocean bed produces a long sequence of sediments comprising these creatures’ shells; the ratios of 18O and 16O are compared at different (time)depths
* High 18O = high evaporation = low sea level = glaciation
*18O / 16O fairly even = return of water to oceans through precipitation, therefore interglacial
Absolute/Chronometric dating techniques
Radiometric dating techniques
* Radiocarbon dating
* Uranium-series
* Fission track dating
* Potassium/Argon dating
Non-radiometric dating techniques
Trapped electron methods:
* Optically stimulated luminescence (OSL)
* Thermoluminescence (TL)
Radioactive decay methods
Based on the speed at which isotopes of an element decay over time in different materials
Carbon in organic materials:
* Radiocarbon: 14C decays into 14N
* Radioactive elements in minerals/rocks
* Uranium-series: 234U decays into 230Th
* Potassium-Argon: 40K decays into 40 Ar
Since we know the rate at which these isotopes decay, by measuring the ratio of one isotope to another we can tell how long it has been since the rock formed and started decaying
Formation of Potassium/Argon often occurs when rocks/fragments/ash are ejected from a volcanic eruption: large parts of Africa were very volcanically active during the period in which humans evolved: fine-grained record
Fission track dating
- Based on analysis of the damage trails, or tracks, left by fission fragments in minerals and glasses containing uranium
- fragments emitted during decay of 238U leave trails of damage in the crystal
structure of the mineral which can be seen under a microscope - The density of fossil tracks correlates with the cooling age of the sample
Problems with radioactive decay methods
- Need to calibrate 14C against tree ring data: the amount of carbon in the atmosphere has varied over time by a few %
- Each technique can only be applied to certain materials e.g. radiocarbon organic or other carbon-bearing materials (e.g. stalagmites)
*14C decays quickly: after c.60,000 years, almost nothing is left - Problem of contamination: early dates can be suspect!
Trapped electron methods: luminescence dating
- Certain minerals (quartz, feldspar, and calcite), store energy from the sun at a known rate.
- This energy is lodged in the imperfect lattices of the mineral’s crystals
- Heating these crystals (e.g. when rocks are
heated) empties the stored energy, after which time the mineral begins absorbing energy again. - TL (Thermoluminescence) dating compares the energy stored in a crystal to what “ought” to be there, thereby coming up with a date-of-last-heated.
- OSL (optically stimulated luminescence) dating measures the last time an object was exposed to sunlight
BP
Before present
MYA
Million years ago
