A child mummy is found high in the Andes and the archaeologist says the child lived more than 2,000 years ago.
How do scientists know how old an object or human remains are?
Familiar to us as the black substance in charred wood, as diamonds, and the graphite in “lead” pencils, carbon comes in several forms, or isotopes.
One rare form has atoms that are 14 times as heavy as hydrogen atoms: carbon-14, or C ratio gets smaller.
Prior to the availability of radiocarbon dates (and when there is no material suitable for a radiocarbon date) scientists used a system of relative dating.
Relative dating establishes the sequence of physical or cultural events in time.
This leaves out aquatic creatures, since their carbon might (for example) come from dissolved carbonate rock.That causes a dating problem with any animal that eats seafood. After about ten half-lives, there's very little C14 left.So, anything more than about 50,000 years old probably can't be dated at all.So, we have a “clock” which starts ticking the moment something dies.Obviously, this works only for things which were once living.