Family-tree relationships can help to narrow down the date when lineages first appeared.
For example, if fossils of B date to X million years ago and the calculated “family tree” says A was an ancestor of B, then A must have evolved earlier.
Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is difficult to match up rock beds that are not directly adjacent.
Sedimentary layers: The layers of sedimentary rock, or strata, can be seen as horizontal bands of differently colored or differently structured materials exposed in this cliff.
How do scientists know how old an object or human remains are?
Carbon-14 dating is a way of determining the age of certain archeological artifacts of a biological origin up to about 50,000 years old.
These standard calibration curves assume that at any given time radiocarbon levels are similar and stable everywhere across each hemisphere. "We went looking to test the assumption behind the whole field of radiocarbon dating," Manning said.
"We know from atmospheric measurements over the last 50 years that radiocarbon levels vary through the year, and we also know that plants typically grow at different times in different parts of the Northern Hemisphere.
At an archaeological dig, a piece of wooden tool is unearthed and the archaeologist finds it to be 5,000 years old.
A child mummy is found high in the Andes and the archaeologist says the child lived more than 2,000 years ago.Stratigraphy is the science of understanding the strata, or layers, that form the sedimentary record.