What fossils teach us about human evolution

Fossils are rare, precious and can tell us the most fascinating things about our ancestors and how they lived

A fossil discovery is improbability squared. First, to become fossilised, human remains must survive the normal process of decay that returns flesh and bone to dust and ashes.

On the African plains, a carcass will be torn to pieces by vultures and hyenas. Perhaps a skull, or thighbone or jaw is dragged apart, perhaps carried into a depression or dry river bed where, white and bleached, it will be carried away by the next flash flood.

Most probably, it will decay but a freak set of circumstances might carry it to peat or quicksand or other anoxic conditions that preserve the bone. Then, over millions of years, as the sands gather above the burial site, minerals in the bone will leak away, to be replaced by rock crystals that precisely copy the bone's shape. Conditions will change; water, glaciers and wind will erode the terrain.

Then comes the next highly unlikely event: just as the bone is exposed, but before it can be eroded, a skilled palæontologist must just happen along, precisely at that point. The surprise is not that there are not more fossil remains, the surprise is that there are any at all.

Sometimes, there are no human fossils, but there may still be other evidence of ancient human presence. At Laetoli in Tanzania, there is a set of hominid footprints preserved in volcanic ash, 3.6m years old. Stone tools found in Happisburgh in Norfolk show that humans must have passed that way more than 800,000 years ago. At Boxgrove near Chichester there is a human shinbone 500,000 years old, but there are also animal bones and flint chippings to show where an axe-maker must have worked.

Animal bones provide clues: if there are hyenas and rhinos in the same stratum we're not talking about the ice age. If the bones of a deer show neat striations, chips or cuts, the conclusion is that a human butcher with a stone axe has used the site. Researchers in Ethiopia have detected evidence of cut marks on fossil animal bones dating back 3.2m years, which pushes the first use of stone tools back almost 800,000 years. Pollen, too, turns up in fossil sites, and provides a guide to bygone temperatures and humidity.

And although DNA degrades with time, human bones can still yield evidence: analysis of 30,000-year-old bones can show whether the owner chose fish from the Palaeolithic menu, or meat.

Contributor

Tim Radford

The GuardianTramp

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