A lot of skeletal material used in the debate about human descent is found in the Olduvai gap in Kenya. High age differences between for instance Lucy (an Australopithecus) and modern Man have been suggested, but new findings seem to contradict these differences.
Ald of these skulls (sometimes only fragments) are found in tuff stonreformed out of volcanic ashes). Chemical analysis of the different strata shows the chemial fingerprint of all these strata to be the same, which makes it very likely these strata were formed in a short time as a result of a single (series of) eruption(s) of the same volcano. Chemical fingerprints are unique and different for each eruption.