Expert: John Shea - 12/24/2009

Question
I recently watched the NOVA program IN Search of Early Man:  part one.  I just have a short question: in the program they showed an anthropologist finding a 4 million year old skull right out on a rocky plain.  He worked for months or years getting the extraneous rock off, and a hominid skull emerged, ostensibly a direct ancestor of homo sapien.  SO my questions are 1) why can one find a 4 million year old skull lying around on the ground.  2) how can he tell it is 4 million years old?  thanks

Answer
Mike
The skull was embedded in rock that was eroding.  It was buried millions of years ago, and then, over time tectonic activites and erosion exposed the rocks
enclosing it to the surface.  That is where it was found, on an eroding rock surface.
The age of that particular fossil is estimated by two sources.  First, by radioactive isotopes in volcanic tuff contained in sediments above and below the fossil
skull.  (Erosion typically occurs off the horizontal plane, so one can usually trace the fossil level back into a sequence of deposits that preserve overlying sediments.)
Older sediments can be sampled by digging beneath the fossil layer, or by tracing that level to a place where erosion or faulting has exposed older rocks.
The method of dating is called  radiopotassium.  Look it up for technical details.  A second way of dating the fossil is by comparing the other animal species
with which it was found to sequences of radiopotassium-dated fossils from other sites.  A third check on these dating methods is called paleomagnetism, and this tells you
whether the fossils were deposited during periods of "normal" (North-pointing) magnetic polarity or during "reversed" (South-Pointing) periods.  If all three of these dating method
produce consistent findings, paleontologists accept the date.
Cheers
John Shea