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ANTC17 – W3.docx

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Genevieve Dewar

ANTC17 – W3 - How do we find fossils of ancestors? Sites? Survery - Chronology: how old are the deposits? Relative and absolute writing - Environment: isotopes - Carbon - Nitrogen - oxygen Tuff Break - You have to be aware of individual bias including your own - He guided chronological ideas in terms of identifying the sample age - Homo Habloae o They are not talking about giant swings of time. Why is it so important that it was older than 2 million years ago? o In their minds, it was one of the early specimens. If it were younger than Homo Erectus then of course you would want your sample to be younger. - What are 2 competing main methods to date methods in Tuff Break? o Argon-Argon/Potassium-Argon dating o Pig teeth method - In the past we had to reply on these methods but there has been 2 things that have been discoveries that transformed the way we think of ages. o Nuclear physics – the fact that some elements have different radio reactivity o Construction of mass spectrometers – they isolate beam of particular atom and bend. If this is a beam of carbon, it will bend on different degrees because they have different atomic rates. It will measure the actual number. - We will talk about these 2 things for the most part of the lecture today Methods Where did our ancestors live? - Where do we find the earliest ancestors of humans? o South and East Africa o We have evidence that bipedalism started not in savanna necessarily but more in the tropical forests. o East Africa – the giant geology – we have exposing sediments that seem to be 4-5 million years of age. o South Africa – we find fossils here because it is based on prospecting. It is famous for mines. Most fossil specimens we have come from minors blowing up sections. Only through the active tunneling deep into the earth will get us one. Archeological visibility ** - Olduvai Gorge o We have 2 different archeological sites  Open site:  best thing is that it represents single event or occupation.  It is easy to radio activate.  Palimpsest  Cave site/rock shelter:  Cluster formation of cave.  Rock shelter is it looks like cave but not. How do we find sites? - Morphological features o Sources of water, firewood, etc. - Field survey o Surface walking o Subsurface investigations  Electrical resistivity survey  Proton magnetometry  Ground penetrating radar o Random testing  Test pitting - We go out there and start waking around in large groups of people. There are morphological features. Everyone needs water, fire, then it will lead to shelter. We start narrowing down the area where you will do your survey. Common way to do survey was to walk around and get samples of ground. - Importantly you have to map and draw everything you come across Chronology - Relative dating methods o Stratigraphy o Cross-dating - Absolute dating o Radiocarbon (C14) o Potassium-argon (K-Ar) o OSL (Optically Stimulated Luminescence) Stratigraphy at Sehonghong Carbon dating - Site A o Layer 1 – small blades o Layer 2 – scrapers o Layer 3 – adzes (C14 1000+- 50 BP) o Layer 4 – large blades o Layer 5 – Handaxes o Layer 6 – No tools - Site B o Layer 1 – small blades o Layer 2 – Adzes o Layer 3 – no tools - Therefore Layer 2 in Site B probably dates to around 1000 +- 50 BP - A good example of stratigraphy - What are 2 major things that can cause sediments to move around? o Burrowing animals o Water – water table can rise. When yellow and black sediments are mixed, they turn grey - You find a sample in one site and same thing in the other site, they should be the same age. It is not necessarily accurate, but it happened. Radiocarbon dating C14 - All forms of life contain carbon o So does food that organisms eat everyday o Living organisms constantly replenish C14 atoms through the food chain - Carbon exists in 3 forms or isotopes: o Carbon 12 is common and stable (99%) o Carbon 13 is rare and stable (<1%) o Carbon 14 is very rare and radioactive - When we actually truly date something from more recent past, we use radio carbon dating. It is the most precise cheap method. Carbon exists in 3 different isotopes: o C12 o C13 o C14 – it is rare but it exists in constant ratio with C12. It is also constantly decaying into nitrogen. C14 is constantly being regenerated into atmosphere. - The proportion of 14C to 12C is constant through the living world o New C14 atoms are constantly created by action of cosmic rays in the upper atmosphere - When an organism dies, it stops eating and stops taking in new carbon atoms o In the remains of the dead organism are a small but it is measurable number of C14 atoms o These begin to decay (slowly) into N14 - C14 atoms have a half-life of 5,730 years - Pick a leaf today: estimate the number of carbon atoms present in the leaf by weight o Calculate the number of C14 atoms o If you re-measure that same leaf 5,730 years from today, there will be exactly half as many C14 atoms. Half will have reverted to stable N14. - We can figure out how old something is or how long it has been dead based on the fact that C12 and 13 has certain ratio. - When we talk about radio carbon dating, you n
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