Re: How can old footprints (>million years) be dated?

There are two ways to date old footprints.  The easiest way is to simply 
date the mud or ash that the footprint was formed in, but you need the 
cooperation of the owner of the foot.  If the person who made the print 
happened to do so in volcanic ash, or another medium that is directly 
dateable, you're home free.  You can use any of several isotopic 
techniques (such as Uranium-Lead or Argon-Argon) or scintillation 
techniques to date the footprint directly.  There are several assumptions 
which have to be made, but the "true" date can often be determined with 
great accuracy.
The second way is to date rock strata above and below the footprints 
(using the same techniques as above) to get a "date range."  Thus the age 
of the prints has to be at least as old as the rock under them (because 
the rock under the prints had to have been laid down BEFORE the prints 
were made), and the prints have to be older than the rock deposited on top 
of them.  Of course, because the prints are now exposed, finding and 
dating rock strata "above" the prints can be a problem, but usually there 
are remnants of "complete" rock stratigraphy (in the local stratigraphic 
area of interest), so this is usually not a big problem (however, finding 
suitable rock for DATING, either above or below the region of interest, 
can be quite a challenge).  For more information, go to  

http://home.worldnet.fr/clist/Anthro/contents.html 

for a great "anthropological resources" page.  Also,  

http://www.psiaz.com/Schur/azpaleo/paleo.html 

has lots of stuff on paleontology, with information about stratigraphy (and good 
pics of fossils).

Later! (If your mother ever said "Questions are food for the Brain," she 
was ABSOLUTELY RIGHT!)

By the way...the "aquatic ape" theory?  There is NO solid evidence of an 
aquatic phase in the lineage of ANY primate, to my knowledge.  Two years 
or so ago, Dr. Richard Leaky (in a presentation to the public in Los 
Alamos, New Mexico), said the same thing.  The aquatic ape theory is BUNK, 
in my opinion.