Dating the Human Past

Introduction by Brad R. Huber Revised, Wednesday, April 01, 2015

1. Archaeologists and Paleoanthropologists study the human past, and base their theories on: 2. Artifacts: a. anything fashioned or made by humans.

b. For example, arrow points, baskets, axes, house ruins, settlements, roads.

3. Fossils: a. the preserved remains of plants and animals.

b. For example, bones, teeth, shells, horns, footprints, frozen mammoths, mummies

1.

The context of artifacts and fossils is important.

a.

The goal is to describe what humans looked like and how they lived.

b.

Archaeologists and Paleoanthropologists go beyond classification.

1.

2.

3.

How are archaeological sites and fossil localities found?

Chance: artifacts and fossils are discovered after an area has been bulldozed Survey: walking, surface collecting, and sampling an area.

Aerial photography, Satellite imagery: airplanes and satellites photograph an area.

a.

For example, the discovery of ancient roads, settlements, and temples on the Arabian Peninsula .

4.

Soil marks

, Raised mounds, and Vegetation: reveal burial sites, ancient roads, settlements, and temples

5.

Oral histories:

Heinrich Schliemann

used Homer's Iliad to discover ancient Troy.

1.

Dating the past

Dating the past accurately is important if we are to have confidence in theories about human evolution and the origins of domestication and civilization.

2.

Two types of dating techniques:

a.

Relative dating: i.

dating methods in which an event, object, or fossil is designated as either older or younger or the same age as another.

ii.

For example, stratigraphy and fluorine dating

a.

Absolute dating: i.

dating methods in which dates expressed in years before present (B.P.) or before common era (B.C.E.) ii.

For example, dendrochronology, radiocarbon dating, and potassium-argon dating.

1.

Stratigraphy: a.

dating artifacts or fossils based upon the strata in which they are found.

b.

First developed by geologists.

c.

Based upon the Law of superposition: i.

older deposits tend to be buried beneath younger ones.

ii.

There is a regional limitation.

iii.

Geological exposures at the Grand Canyon cannot be correlated with Olduvai Gorge in Tanzania by the law of superposition alone.

a.

Also based upon the Index fossil

concept:

i.

Strata containing similar fossils or artifacts must be of similar age.

ii.

This allows for regional correlations.

1.

Fluorine dating: technique for dating fossils based on the amounts of fluorine in fossil bone.

a.

Basic Principle: Older fossil bones contain a greater amount of Fluorine than Younger.

b.

Fluorine combines with calcium over time

c.

Use to expose the Piltdown hoax: i.

In the early 1900s there was an evolutionary theory that brain size increased first, and then humans walked upright.

ii.

The perpetrators of the Piltdown hoax combined a 20,000 year-old human cranium with the jaw of the contemporary orangutan "doctored" to look like ancient fossils.

1.

Dendrochronology: a.

An absolute dating method based on the number of rings of growth in trees.

b.

Each tree ring equals one year.

c.

The width of tree rings varies due to the amount of rainfall and climatic conditions.

d.

Newly fallen trees can be dated easily.

e.

Tree ring patterns in older and older specimens are overlapped until absolute dates are established back to approximately 3000 years before present in the American southwest, Alaska, Germany, Turkey, Japan, Russia.

f.

This technique is used to date artifacts and fossils in clear association with wooden objects, e.g., g.

beams.

Pine and Sequoia are long-lived trees and can be used with this method.

Dendrochronology Lab University of Arizona (7:14 mins.) Lord of the Tree Rings mins.) Dendrochronology (2:31

1.

Radiocarbon dating: a.

based on measuring the amount of radio carbon 14 (C14) in organic materials such as charcoal, wood, bone, and shells b.

Radiocarbon dating gives absolute dates to approximately 50,000 years before present.

c.

f.

d.

e.

i.

C14 has a half-life of 5730 years ii.

one half of C14 decays to nitrogen 14 in 5730 years.

i.

At an organism's death: 15 beta particles per minute per gram are emitted ii.

5730 years later, 7.5 beta particles per minute per gram are emitted.

The amount of carbon-14 in organic materials is measured to date them.

Basic Principle: The smaller the amount of C14, the older it is.

Radiocarbon Dating at the Rafter Radiocarbon Laboratory (Video 6:58 mins.)

1.

Potassium-Argon Dating: a.

based on measuring the amounts of argon (Ar40; a gas) in volcanic debris.

b.

This method dates fossils and rocks to several billion years.

c.

Used to date hominid fossils to several million years old.

d.

Radioactive potassium (K40; a solid) decays into argon (Ar40) at a constant rate.

e.

The half-life of K40 is 1.3 billion years.

f.

g.

Basic principle: the greater the relative amount of Ar40, the older the rock.

Fossils in association with the dated rock have the same date

Potassium-Argon Dating (Video 10:35 mins; produced by the Khan Academy)